This article should only be used as a guide. It's intended purpose is to help the piping designer who is responsible for placement of one specific item in a typical refinery, chemical or petrochemical process plant or someone who may need help in developing a total plot plan for a complex unit.
The guidelines given here are based on my many years of experience with one of the world's largest engineering, design and construction companies along with the U. S. OSHA Part 1910 and the NFPA (National Fire Protection Association) Code No. 30.
The latest editions of these codes and any other applicable national, regional and local codes should be referred to and used because they may be more stringent.
The subjects covered in this article have been arranged in alphabetical order in the hope it will make them easier to locate.
Access (See Maintenance)
Columns (See Vertical Vessels)
Compressors, Centrifugal
Locate centrifugal compressor as close as possible the suction source. Top suction and discharge lines either should be routed to provide clearance for overhead maintenance requirements, or should be made up with removable spool pieces.
Support piping so as to minimize dead load on compressor nozzles; the load should be within the recommended allowance of the compressor manufacturer.
Centrifugal compressors should have full platforming at operating level. Heavy parts such as upper or inner casing and rotor should be accessible to mobile equipment. Review the equipment arrangement for access and operation.
Locate lube and seal oil consoles adjacent to and as close as possible to the compressor. Oil return lines from the compressor and driver should have a minimum slope of 1/2 inch per foot to the inlet connection of seal traps, degassing tanks, and oil reservoir. Pipe the reservoir, compressor bearing, and seal oil vents to a safe location at least 6 feet above operator head level.
Compressors, Reciprocating
Locate reciprocating compressors so suction and discharge lines that are subject to vibration (mechanical and acoustical) may be routed at grade and held down at points established by a stress and analog study of the system.
Accessibility and maintenance for large lifts such as cylinder, motor rotor, and piston removal should be by mobile equipment if the installation is outdoors or by traveling overhead crane if the installation is indoors (or covered).
Horizontal, straight line, reciprocating compressors should have access to cylinder valves. Access should be from grade or platform if required.
Depending on unit size and installation height, horizontal-opposed and gas engine driven reciprocating compressors may require full platforming at the operating level.
Control Valves
Locate control valve stations accessible from grade or on a platform. In general, the (flow, level, pressure, temperature) instruments or indicators showing the process variables should be visible from the control valve.
Cooling Towers
Locate cooling towers downwind of buildings and equipment to keep spray from falling on them. Orient the short side of the tower into the prevailing summer wind for maximum efficiency. This means that the air flow (wind) will travel up the long sides and be drawn in to both sides of the cooling tower equally. When the wind is allowed to blow directly into one long side it tends to blow straight through and results in lower efficiency. Locate cooling towers a minimum of 100 feet (30m) from process units, utility units, fired equipment, and process equipment.
Cradles (See Insulation Shoes and Cradles)
Equipment Arrangement (General)
Arrange equipment, structures, and piping to permit maintenance and service by means of mobile equipment. Provide permanent facilities where maintenance by mobile equipment is impractical.
Group offsite equipment, pumps, and exchangers to permit economical pipe routing. Locate this equipment outside of diked storage areas.
Exchanger, Air Cooler (Fin Fans)
Air Coolers are in typically used in the cooling of the overhead vapor from tall vertical vessels or towers such as Crude Fractionators and Stripper Columns. The natural flow tends to follow gravity, where the tower overhead is the
Exchanger, "G" Fin (Double Pipe)
These exchangers can be mounted almost anywhere any they can be mounted (with process engineer approval) in the vertical when required. A G-Fin Exchanger is recognizable by its shape. One segment looks like two long pieces of pipe with a 180 degree return bend at the far end. It is one finned pipe inside of another pipe with two movable supports. This type of exchanger can be joined together very simply to form multiples in series, in parallel or in a combination of series/parallel to meet the requirements of the process. This exchanger is not normally used in a service where there is a large flow rate or where high heat transfer is required. The key feature with this exchanger is the maintenance. The piping is disconnected from the tube side (inner pipe). On the return bend end of this exchanger there is a removable cover. When the cover is removed this allows for the tube (inside pipe) to be pulled out. This exchanger is normally installed with the piping connections toward the pipe rack.
Exchangers, Reboiler (Kettle Reboiler)
Locate kettle reboilers at grade and as close as possible to the vessel they serve. This type of reboiler is identifiable by its unique shape. It has one end much like a normal Shell and Tube exchanger then a very large eccentric, bottom flat transition to what looks like a normal horizontal vessel. You could also call it a "Fat" exchanger. The flow characteristics on the process side of a kettle reboiler are the reason for the requirement for the close relationship to the related vessel.
Reboilers normally have a removable tube bundle and should have maintenance clearance equal to the bundle length plus 5 feet (1.5m) measured from the tube sheet.
Exchangers, Shell and Tube
Shell and tube exchangers should be grouped together wherever possible. Stacked shell and tube exchangers should be limited to four shells high in similar service; however, the top exchanger should not exceed a centerline elevation of 18 feet (5.5m) above
Exchangers with removable tube bundles should have maintenance clearance equal to the bundle length plus 5 feet (1.5m) measured from the tube sheet to allow for the tube bundle and the tube puller.
Maintenance space between flanges of exchangers or other equipment arranged in pairs should be 1'- 6" (0.5m) (min.). Exchanger maintenance space from a structural member or pipe should not be less than 1'- 0" (300mm) (min.).
Furnaces (Fired Equipment)
Locate fired equipment, if practical, so that flammable gases from hydrocarbon and other processing areas cannot be blown into the open flames by prevailing winds.
Horizontal clearance from hydrocarbon equipment (shell to shell) 50'- 0" (15m) Exception: Reactors or equipment in alloy systems should be located for economical piping arrangement.
Provide sufficient access and clearance at fired equipment for removal of tubes, soot blowers, air preheater baskets, burners, fans, and other related serviceable equipment.
Clearance from edge of roads to shell 10'- 0"(3m)
Pressure relief doors and tube access doors should be free from obstructions. Orient pressure relief doors so as not to blow into adjacent equipment.
The elevation of the bottom of the heater above the
Furnace Piping
Locate snuffing steam manifolds and fuel gas shutoff valves a minimum of 50 feet (15m) horizontally from the heaters they protect.
Burner Valving for a Floor Fired Furnaces: Combination oil and gas firing valves should be operable from burner observation door platform. For those fired by gas only, the valves should be near the burner and should be operable from grade.
Burner Valving for a Side Fired Furnaces: Locate firing valves so they can be operated while the flame is viewed from the observation door.
Flare Stacks
Locate the flare stack upwind of process units, with a minimum distance of 200 feet (60m) from process equipment, tanks, and cooling towers. If the stack height is less than 75 feet (25m), increase this distance to a minimum of 300 feet (90m). These minimum distances should be verified by Company Process Engineering.
Future Provisions
Space for future equipment, pipe, or units should not be provided unless required by the client or for specific process considerations. When applicable this requirement should be indicated on the plot plan and P&IDs.
Insulation Shoes and Cradles
Locate Insulation shoes anywhere a line crosses a support for hot insulated piping when the piping is 3 inch (80mm) and larger carbon and alloy steel lines with design temperatures over 650 degrees F (350C).
Large diameter lines (20 inches (500mm) and over), stainless steel lines where galvanic corrosion may exist, lines with wall thickness less than standard weight, and vacuum lines should be analyzed to determine if shoes or wear plates are needed.
Provide cradles at supports for insulated lines in cold service and for acoustical applications.
Ladders & Cages
Maximum height of a ladder without a cage should not exceed 15'-0" (4.5m)
Maximum vertical distance between platforms 30'- 0" (9m)
Cages on ladders over 15'-0" (4.5m) high shall start at 8'-0" (2.5m) above grade.
Minimum toe clearance behind a ladder 0'- 7" (200mm)
Minimum handrail clearance 0'- 3" (80mm)
Level Instruments
Locate liquid level controllers and level glasses so as to be accessible from grade, platform, or permanent ladder. The level glass should be readable from grade wherever practical.
Wherever possible, orient level instruments on the side toward the operating aisle.
Loading Racks
Locate loading and unloading facilities that handle flammable commodities a minimum of 200 feet (60m) from away from process equipment, and 250 feet (75m) from tankage.
Maintenance Aisles (at grade)
Equipment maintenance aisle for hydraulic crane (12T capacity) should have a horizontal clearance width of 10'- 0" (3m) (min.) and a vertical clearance of 12'- 0" (3.5m) (min.). Where a fork lift and similar equipment (5000 lbs / 230kg capability) is to be used the horizontal clearance should be 6'- 0" (2m) (min.) and the vertical clearance should be 8'- 0" (2.5m) (min.).
Where maintenance by portable manual equipment (A-frames, hand trucks, dollies, portable ladders or similar equipment) is required the horizontal clearance should be 3'- 0" (1m) (min.) and the vertical clearance 8'- 0" (2.5m) (min.).
Operating Aisle (at grade)
Minimum width 2'- 6" (800mm)
Headroom 7'- 0" (2.1m)
Orifice Runs and Flanges
Locate Orifice runs in the horizontal. Vertical orifice runs may only be used with the approval of Company Control Systems Engineering. Orifice flanges with a centerline elevation over 15 feet (4.5m) above the
Locate orifice taps as follows:
Air and Gas
-Top vertical centerline (preferred)
-45 degrees above horizontal centerline (alternate)]
Liquid and Steam
-Horizontal centerline (preferred)
-45 degrees below horizontal centerline (alternate]
(Note: The piping isometrics should show the required tap orientations)
Personnel Protection
Locate eye wash and emergency showers in all areas where operating personnel are subject to hazardous sprays or spills, such as acid.
Personnel protection should be provided at uninsulated lines and for equipment operating above 140 degrees F (60 C) when they constitute a hazard to the operators during the normal operating routine. Lines that are infrequently used, such as snuffing steam and relief valve discharges, may not require protective shields or coverings.
Pipe
Clearance between the outside diameter of flange and the outside diameter of pipe to the insulation should not be less than 0'- 1"* (25mm)
Clearance between the outside diameter of pipe, flange, or insulation and structural any member should not be less than 0'- 2"* (50mm)
*With full consideration of thermal movements
Platforms
Minimum width for ladder to ladder travel: 2'- 6" (800mm)
Headroom: 7'- 0" (2.1m)
Headroom from stairwell treads: 7'- 0" (2.1m)
Minimum clearance around any obstruction on dead end platforms: 1'- 6" (500mm)
Pressure Instruments
Locate all local pressure indicators so they are visible from grade, permanent ladder, or platform. Those located less than 15 feet (4.5m) above
Process Units
The relation of units, location of equipment, and routing of pipe should be based on economics, safety, and ease of maintenance, operation, and construction requirements. The alignment of equipment and routing of pipe should offer an organized appearance.
Process Unit Piping
Locate all pipe lines in major process units on overhead pipeways. In certain instances, pipes may be buried, providing they are adequately protected. Lines that must be run below grade, and must be periodically inspected or replaced, should be identified on the P&IDs and placed in covered concrete trenches.
Cooling water lines normally may be run above or below ground, based on economics.
Domestic or potable water and fire water lines should be run underground.
Pumps
Locate pumps close to the equipment from which they take suction. Normally, locate pumps in process units under pipeways.
Design piping to provide clearance for pump or driver removal. Similarly, on end suction pumps, piping should permit removing suction cover and pump impeller while the suction and discharge valves are in place.
Arrange suction lines to minimize offsets. The suction lines should be short and as direct as possible, and should step down from the equipment to the pump. Suction lines routed on sleeperways may rise to pump suction nozzle elevation.
Orient valve handwheels or handles so they will not interfere with pump maintenance or motor removal. Valve handwheels or handles should be readily operable from grade.
Maintenance and operating aisles with a minimum width of 2'-6" (800mm) should be provided on three sides of all pumps.
Pump Strainers
Provide temporary conical type strainers in 2 inch (50mm) and larger butt weld pump suction lines for use during startup. Arrange piping to facilitate removal.
Use permanent Y-type strainers on 2 inch (50mm) and smaller screwed or socket weld pump suction piping.
Railroads
Headroom over through-railroads (from top rail) 22'- 6"** (7m)
Clearance from track centerline to obstruction 10'- 0"** (3m)
(** Verify conformance with local regulations)
Relief Valves (Pressure, Safety and Thermal)
Locate all relief valves so they are accessible. Wherever feasible, locate them at platforms that are designed for other purposes. Relief valves with a centerline elevation over 15 feet (4.5m) above
Pressure relief valves that discharge to a closed system should be installed higher than the collection header. There should be no pockets in the discharge line.
Safety relief valves (in services such as steam, etc.) that discharge to the atmosphere should have tail pipes extended to a minimum of 8 feet (2.5m)above the nearest operating platform that is within a radius of 25 feet (7.5m). This requirement may be waived, provided a review of the proposed arrangement indicates that it does not present a hazard. Review all pressure and safety relief valves discharging flammable vapors to the atmosphere within 100 feet (30m) of fired equipment for vapor dissipation.
Pressure and Safety relief valves, 1-1/2 inch (40mm) and larger, should only be installed with the stem and body vertical position.
Thermal relief valves, 1 inch (25mm) and smaller, may be installed with the stem and body in a horizontal position when it is impractical to install it in the vertical position.
Roads
Major process plants normally have three classes of roads. They might be called Primary roads, Secondary roads and Maintenance access ways.
Clearance or distance required
|
Road type |
Vertical |
Width |
Shoulder |
Side or off road |
|
Primary |
21'-0" (6.5m) |
20'-0" (6m) |
5'-0" (1.5m) |
20'-0" (6m) |
|
Secondary (*) |
12'-0" (3.7m) |
12'-0" 3.7m) |
3'-0" (1m) |
10'-0" (3m) |
|
Maintenance access |
10'-0" (3m) |
10'-0" (3m) |
(not req'd) |
5'-0" (1.5m) |
(*) Normally secondary plant roads may be used as tube pull areas.
Safety Access
Provide a primary means of egress (continuous and unobstructed way of exit travel) from any point in any building, elevated equipment, or structure. A secondary means of escape should be provided where the travel distance from the furthest point on a platform to an exit exceeds 75 feet (25m).
Access to elevated platforms should be by permanent ladder. Safety cages should be provided on all ladders over 15'-0" (4.5m)
The need for stairways should be determined by platform elevation, number of items requiring attention, observation and adjustment, and the frequency of items.
Ladder safety devices such as cable reel safety belts and harnesses, may be investigated for use on boiler, flare stack, water tank, and chimney ladders over 20 feet (6m) in unbroken lengths in lieu of cage protection and landing platforms.
Sample Connections
Locate all sample connections so they are readily accessible from grade or platform.
In general, where liquid samples are taken in a bottle, locate the sample outlet above a drain funnel to permit free running of the liquid before sampling.
Hot samples should be provided with a cooler.
Sleeper Pipe Supports
Normally, route piping in offsite areas on sleepers. Stagger the sleeper elevations to permit ease of crossing or change of direction at intersections. Flat turns may be used when entire sleeper ways change direction.
Spectacle Blinds
Locate spectacle blinds to be accessible from grade or platform. Blinds located in a pipeway are considered accessible. Blinds that weigh over 100 lbs (45kg) should be accessible by mobile equipment. Where this is not possible, provide davits or hitching points.
Closely grouped flanges with blinds should be staggered.
Steam Traps
Locate all steam traps at all pocketed low points and at dead ends of steam headers. Also, provide traps periodically on excessively long runs of steam piping, for sufficient condensate removal, and to ensure dry quality steam at destination. Steam traps should be accessible from grade or a platform. Steam traps located in pipeways should be considered accessible by portable ladder.
Tankage
Locate any tankage containing hydrocarbon or other combustible fluids or gasses a minimum distance of 250'-0" (115m) from any process unit, rail loading facility or truck loading facility.
The minimum spacing of offsite storage tanks and dike requirements should be in accordance with the latest edition of the National Fire Protection Association, Code No. 30, and OSHA part 1910.106 (b), where applicable.
Temperature Instruments
Locate temperature test wells, temperature Indicators and thermocouples to be accessible from grade or a portable ladder. Those located in a pipeway should be considered accessible by a portable ladder. Those located over 15 feet (7m) above
Locate all local temperature indicators (TI) should be visible from grade, ladder, or platform.
Towers (See Vertical Vessel)
Utility Stations
Provide and locate utility stations with water, steam, or air as indicated below:
All areas should be reachable with a single 50 foot (20m) length of hose from the station.
Provide water outlets at grade level only, in pump areas, and near equipment that should be water washed during maintenance.
Provide steam outlets at grade level only in areas subject to product spills, and near equipment that requires steaming out during maintenance.
Provide air outlets in areas where air-driven tools are used such as at exchangers, both ends of heaters, compressor area, top platform of reactors, and on columns at each manway.
Hose, hose rack, and hose connections should be provided by the client or be purchased to match the clients existing hardware.
Valve Handwheel Clearance
Clearance between the outside of hand wheel and any obstruction (knuckle clearance) should be 0'- 3" (80mm)
Valve Operation
Locate operating valves requiring attention, observation, or adjustment during normal plant operation (noted on the P&IDs) so they may be within easy reach from grade, platform, or permanent ladder as follows:
- 2" (50mm) and smaller may be located reachable from a ladder.
- 3" (80mm) and larger must be reachable and operable on a platform
Operating valves with the bottom of handwheel is over 7 feet (2.1m)above high point of finished surface or operating platform may be chain-operated.
The centerline of handwheel or handles on block valves used for shutdown only, located less than 15 feet (4.5m) above
The centerline of handwheel or handles on block valves used for shutdown only and located over 15 feet (4.5m) above high point of finished surface, except those located in pipeways, should be operable from permanent ladder or platform.
In general, keep valve handwheels, handles, and stems out of operating aisles. Where this is not practical, elevate the valve to 6'- 6" (plus or minus 3 inches) clear from
Vents and Drains
The P&IDs should indicate, locate and size all vents, drains, and bleeds required for process reasons and plant operation.
Provide plugged hydrostatic vents and drains without valves at the high and low points of piping.
Provide valved bleeds at control valve stations, level switches, level controllers, and gage glasses per job standard.
Vertical Vessel (Column) Piping and Platforms
Locate vertical vessels in the equipment rows on each side of the pipeway in a logical order based on the process and cost. The largest vessel in each equipment row should be used to set the centerline location of all vertical vessels in that equipment row. This largest vertical vessel should be set back from the pipe rack a distance that allows for; any pumps, the pump piping, an operation aisle between the pump piping and any piping in front of the vessel, the edge of the vessel foundation and half the diameter of this the largest vessel. Set all other vertical vessels in this same equipment row on the same centerline.
Provide a clear access area at grade for vessels with removable internals or for vessels requiring loading and unloading of catalyst or packing.
Provide vessel davits for handling items such as internals and relief valves on vessels exceeding a height of 30 feet (9m) above the
Walkways
Walkways should have a 2'-6' (1m) horizontal clearance (not necessarily in a straight
line) and headroom of 7'- 0" (2.1m)
The question on many minds may be "Why does Piping do Vessel Orientation?" We can answer that question two ways. The first answer would be, because of the traditional role of Piper and the content of the vessel orientation activity itself. The traditional role of the Piper has always been the bringing together of multi-discipline information to create the plant layout and piping plans. The activity of vessel orientation has the same multi-discipline focus.
The second way to answer the question is to ask "If not the Piper, then who?" Civil? Structural? Electrical? Instrumentation? No, they are not logical candidates. Structural? The structural engineer does engineer the support for some vessels but they do not truly design the support. Process? While the process engineer does have a great deal of interest and input in the workings of a vessel, their interest is more from a function and performance focus. Vessels? Why doesn't the vessel engineer do the vessel orientation? Or better yet, why doesn't the Vendor do the vessel orientation? The response to that is in all of the non-vessel factors that influence the vessel orientation activity. What are non-vessel factors?
Non-vessel factors include:
A. Site -- Vessel orientation is influenced by where the vessel is located on the site
B. Relationship to related equipment -- Proper vessel orientation must consider the location and method of connection to related equipment
C. Support -- Vessel orientation of many vessels includes the method of support
D. P&ID interpretation -- The person responsible for vessel orientation must be very proficient in reading and understanding a P&ID
E. Internals to external object relationships -- Internals effect the nozzle locations that in turn connect to the piping. The piping is subject to thermal expansion, and must be supported. The piping must meet all the process requirements from the P&ID, and must be in compliance with the Plant Layout Design Specification. The piping must also be supported, and must meet the all the applicable Code criteria, etc.
F. Operations and Maintenance -- Vessel orientation must be compatible with the requirements of the operators and the people who must maintain the vessels.
This brings us back to answer number one. Vessel orientation requires the bringing together of and the coordination of data and requirements from many disciplines. Piping in their Plant Layout role is already functioning in this mode. Most major engineering and design firms (in our Industry) have found that Piping Design is the most logical and most efficient group for developing complex vessel orientations.
The ideal scenario for the development of a vessel orientation is like a chain. The links of the chain are like the steps required completing the finished design. With the ideal scenario you would not start step two until step one is completed and so on. The ideal circumstances means that the Plot Plan has been firmed up and approved, the P&IDs have been developed, reviewed, and issued approved for design (AFD). It means that the unit piping transposition has been developed. It means that Process has completed their input to the vessel datasheet and Vessels has completed their preliminary work.
Occasionally, the piping designer has been required to initiate a vessel orientation under other than the most ideal of circumstances. In some cases the vessel orientation has been started before the P&IDs were ready for the first Client P&ID review. Starting Vessel orientation before the source documents are ready will expose the job to risks, errors, recycle and increased costs.
As much as we try to avoid this situation, it can still happen. Premature starts in vessel orientation are due to the requirement for early purchase of vessels identified as long delivery. The Construction schedule of any project is based on the delivery of key equipment and materials. The construction schedule in turn will impact the start-up schedule. Once the Client has awarded the project, they are anxious to get their plant "on-stream" as soon as possible. The sooner they get on-stream, the sooner they can recover the capitol investment and see the expected profits.
The delivery time for vessels such as: alloy reactors, heavy wall high pressure vessels, or crude vacuum columns often take more than a year from
Under normal circumstances a Vessel fabricator will not normally do any rolling and cutting of plate until the order has reached a certain milestone. They will need the final checked, corrected and approved vessel drawings. This includes all the nozzles, pipe supports, pipe guides, ladders, platforms, etc. The Vendor's fabrication and delivery performance clock does not start ticking until they get the drawings back approved.
A project with a fast track schedule or pre-purchased vessels will put a lot of pressure on the piping design group. Piping should normally have time to properly develop the Plot Plan, the P&ID transposition, the other related piping layouts, in order to come up with the best vessel orientations for economics, operability, and maintenance.
As piping designers you owe it to the Client, your company, as well as to yourself to do the best job you know how. This philosophy is true when doing vessel orientations as with any other piping design activity. You should check into all aspects of the vessel piping and the orientation. You need to start by collecting, verifying, and using the proper information.
During Plot Plan development, the piping designer must take into consideration many items that can also have a bearing on the vessel other than the orientation itself.
Such items include:
Lay-down space -- Prior to erection, tall columns require space for final assembly
Erection equipment -- The cranes (or other lifting devices) planned to lift and set the vessels require vast amounts of space
Plant road limitations; Rack heights, shoulder clearances, logistics
Special vessels such as Reactors have several factors, which should be kept in mind. The most important one, of course, is to keep the alloy piping as short as possible by locating the Reactors near the Heaters. Catalyst handling facilities is another important consideration. This is true whether the catalyst is to be loaded by crane or by vessel mounted monorail. The removal of spent catalyst, usually by tote bin, truck, or conveyor, is another space consideration.
We all need to remember space is money to the Client. Wise use of plot space can save the Client money by reducing installation costs and operating costs.
Vessel Configurations
Vessels come in a wide variety of configurations. The variety is expressed in their sizes, shape, and function. They also will have a wide range of pressure, temperature and metallurgy. This list is only intended to highlight the main examples.
Vertical Vessels with no internals
(A.k.a.: Tanks, Drums, and Pots)
Example: Mix Tank, Air Receiver, Volume Bottle, Flash Drums, Fuel Gas K. O. Pot, Feed Surge Drum, and Dump Tank
Discussion: This type of vessel will normally be small (< 24" diameter x 3' - 0" T-T) to medium sized (24"dia to 48" diameter x < 10" - 0" T-T). They may be mounted to the support surface (grade, floor, or platform) via a traditional vessel skirt, attached legs, or lugs. When located at grade this vessel may be mounted directly on the concrete paving or floor depending on vessel weight and soil conditions.
Vertical Vessels with simple Internals
Simple internals such as Demister Pads
Example: Feed Knockout Drum, Separator Drum, Filter, and Coalescer Drum
Discussion: This type of vessel will normally be medium (24"dia to 48" diameter x < 10" - 0" T-T) to large sized (Over 48" diameter and over 10' - 0" T-T). They may be mounted to the support surface (grade or platforms) via a traditional straight vessel skirt, a flared skirt, attached legs, or lugs. When located at grade this vessel will normally be mounted on an octagon foundation.
Vertical Trayed Vessels with straight sides
Example: Fractionator, Contactor, and Stripper
Discussion: This type of vessel can be as small as two or three feet in diameter or may be very large at 20' - 0" or more in diameter. The diameter, height, number of trays, type of trays along with the other related items depends on the function. These vessels will normally be supported at grade via a traditional vessel skirt. This vessel will normally be supported on the traditional 9" to 1' - 0" high octagon concrete foundation.
Vertical Trayed Vessels - Coke Bottle (two diameters w/ transition)
Example: Splitter, Stabilizer, Lean Oil Still, and Absorber Column
Discussion: This type of vessel will have two diameters. The Coke Bottle Vessel is a multi purpose vessel. The larger section will have different internals and function differently than the smaller section. The bottom of the Column will normally be the larger diameter with a conical transition piece to join the two. This type vessel will normally be mounted at grade via a traditional vessel skirt and be supported on an octagon foundation.
Variation: A variation of this type vessel is the Inverted Coke Bottle. The Inverted Coke Bottle Vessel will normally have a short skirt at the transition point and be mounted on an elevated platform in a structure. The smaller (lower) section will hang down inside the structure.
Example: Dryers, Feed Purifiers,
Discussion: these types of vessel will normally be medium sized. Packing may be a manufactured mesh or a granulated natural material. The location and orientation of this type of vessel must consider the loading and removal of the packing. These vessels may operate at ambient, temperatures, the lower normal process temperatures, or at high temperatures. These vessels may be mounted to the support surface (grade or platforms) via a traditional vessel skirt, attached legs, or lugs. When located at grade this vessel will normally be mounted on an octagon foundation.
Vertical (Refinery Type) Reactor Vessels
Example: Reactor, Converters
Discussion: This type of vessel will normally be medium to large sized, high pressure (> 500 psig) and high temperature (> 600o F). These vessels will be filled with one or more layers or beds of various materials that will act as a catalyst. The sidewalls and heads on this type of Reactor may be five to seven inches thick. Refinery Reactors may be mounted to the support surface on a short vessel skirt, on lugs, or on legs. The bottom head and nozzle must be elevated to allow for removal of the catalyst. The location and orientation of this type of vessel must consider the loading and removal of the catalyst. These vessels will normally operate at very high process temperatures and will be located in close proximity to fired heaters.
Vertical (PharmBio & Fine Chemical Type) Reactor Vessels
Example: Reactor, Mix Tank, and Cook Tank
Discussion: This type of vessel will normally have a diameter and height of similar dimensions. The ratio of nozzles to vessel size will be very high. These vessels will have added complexities with the requirements for mixers and jacketing. These vessels will normally be mounted to the support surface on lugs, a collar, or on legs. These vessels are normally located on an upper level of an enclosed structure or building. The bottom head and nozzle must be elevated to allow for operator access, gravity flow to other equipment, or critical pump NPSH requirements.
Vertical Vessels - Bins and Silos
Example: Agricultural Product Storage, Dry Chemical Storage
Discussion: Bins and Silos are used for dry material storage. These vessels are normally thin walled, operate at atmospheric pressure, and made of materials other than carbon steel. These vessels will normally have a cone bottom. The configuration of the cone is based on the angle of repose of the commodity to be stored. These vessels may be supported via skirt, legs, or lug mounted in an elevated structure. These vessels may have flat, cone, or dome roofs.
Horizontal Vessels at grade
Example: Condensate Collection Drum, Separator, and Settler Drum
Discussion: This type of vessel will normally be small to medium sized. They may be mounted to the support surface (grade or platforms) on extended vessel saddles. The extended saddle allows for clearance for bottom connections at a lower cost. When located at grade this vessel may be mounted on a foundation or the paving (depending on vessel weight and soil conditions).
Horizontal Vessels - Elevated without Boots
Example: Steam Drum, and Feed Surge Drum
Discussion: these types of vessel will normally be medium to large sized. They will be mounted to the support surface (foundation or platforms) on traditional vessel saddles. When located near grade this vessel will normally be mounted on an elevated foundation. The NPSH requirements of the related pumps are critical to setting of the support elevation.
Horizontal Vessels - Elevated with Boots
Example: Stripper Receiver, Accumulator, Interstage K. O. Drum, and Flare K. O. Drum
Discussion: these types of vessel will normally be medium to large sized. They will be mounted to the support surface (foundation or platforms) on traditional vessel saddles. When located near grade this vessel will normally be mounted on an elevated foundation. Access is normally required for the Boot operating valves and instruments. The NPSH requirements of the related pumps are critical to setting of the support elevation.
Horizontal - Underground or Pit Vessels
Example: Dump Tank, Kill Tank, and Hazardous Material Storage Tank
Discussion: This type of vessel may be small, medium, or large in size. They will be mounted to the support surface on traditional vessel saddles. When located at grade this vessel will normally be mounted on a low foundation. When located in a pit, the pit size must allow for safety, operation, and maintenance. Pit mounted installations may also require sumps and drainage pumps. Underground (buried) installations may require double wall tanks with leak detection provisions.
API Storage Tanks
Example: Feed Storage, Intermediate Product Storage, Off-Spec Product Storage, Finished Product Storage, Batch Storage, Fire (or other) Water Storage
Discussion: These are the traditional Tank Farm tanks. There are a number of sub-types, which include Cone Roof Atmospheric; Cone Roof with captured venting, Open Floating Roof, Enclosed Floating Roof, and Double Wall LNG Storage Tanks. These tanks have specific location, support, piping connection, safety, and access criteria based on the commodity to be stored.
Special
Example: Spheres, Spheroids, and Bullets
Discussion: These vessel types have special location and orientation criteria and should be handled on an Ad Hoc basis.
Vessel Supports
There is a wide variety in the methods used to support vessels.
There include:
a. Skirts
b. Saddles
c. Ring Girders
d. Lugs
e. Legs
f. Portables on Casters
g. Pads
h. Direct Bury
Each of these support types may also have variations
Vertical Vessel Components
The pressure containment elements of the vessel are based of the process requirements for pressure, temperature, commodity, corrosion rate, plant life criteria, and the applicable Codes.
The Pressure containment components include the following:
a) Shell
b) Heads
c) Boot
d) Transitions (Coke Bottle Vessels)
e) Nozzles
The other components include the following:
a) Trays
b) Internal piping
c) Support
d) Load Handling Devices
e) Pipe supports and Guides
f) Platforms, Ladders, and Cages
g) Code Name Plate
Vertical Vessel Terminology
Normally vessel components are described using common terms such as shell, head, nozzle, and support. Some vessels will also have special terms based on function.
Typical special terms include the following:
a) Flash Section -- The area or zone of the fractionation vessel where the primary feed enters the vessel.
b) Fractionation Section -- The portion of the vessel that includes the trays.
c) Stripping Section -- A place in the vessel that includes the introduction of supplementary heat such as high temperature steam
d) Surge Section -- The bottom portion of the vessel that normally includes the main outlet nozzle which is connected to the bottoms pumps.
Shell
The shell of the vertical trayed vessel will have many variables including the following:
a) Wall thickness
b) Metallurgy (May have different material at top vs. bottom)
c) Layers (single layer vs. multiple layer or cladding)
d) PWHT (Post weld heat treat) requirements for all or part
e) Vacuum reinforcement rings
f) Insulation support rings
Heads -- Top and Bottom
Heads for vessels will include the following shapes:
a) Dished -- The Dished head is a flatter version of the Semi-Elliptical
b) Semi-Elliptical -- The traditional type used on process plant pressure vessels (2:1 SE Head)
c) Spherical -- This head is sometimes referred to as a round head or Hemispherical-head
The top head and the bottom head may be the same shape but they will have some differences.
The differences for the top head include:
a) Same material as top of Shell
b) May be thicker material for reinforcing
c) May be thinner material
The differences for the bottom head include:
a) Same material as bottom of Shell
b) May be thicker material for reinforcing
c) May be thinner material
Transitions
The cone or transition piece for regular and inverted Coke Bottle vessels may come in the following shapes:
Flat side -- The cone is cut from flat plate and formed to a simple cone. There is no knuckle radius at the top or bottom of the cone. The connection to the straight shell of the vessel is an angled weld. Usually there is a reinforcing ring on the shell very close to the shell/cone junction.
Shaped side -- The cone is cut from flat plate and rolled to a shaped cone. There is a knuckle radius at the top and bottom of the cone. The cone has a straight tangent at the top and bottom to match the shells. The connection to the straight shell of the vessel is a common butt weld.
Nozzles
Overhead Vapor Outlet Nozzles
The overhead vapor outlet nozzles on a vertical vessel can have some latitude when it comes to attachment location. The attachment connection can be direct to the top head of the vessel or may be from the side. When the connection is from the side there will normally be a pipe inside the vessel angled up to the top head area. Small vapor outlet nozzles from small diameter vessels can be located out the side of the vessel and still be cost effective. Large diameter vapor outlet nozzles on large diameter vessels will be more cost effective if attached to the top head. The line is then looped over to the selected pipe drop position to go down the vessel.
Heater/Vessel Feed Transfer (Feed Inlet) Nozzles
All vertical fractionation vessels will have a feed inlet nozzle. This feed nozzle is special and critical on some vessels. Refinery Crude columns and Vacuum columns are examples that have this type of nozzle. This nozzle installation is characterized by the following:
a) Attached line originated at a fired heater
b) High temperature
c) High velocity
d) Mixed phase flow
e) May require internals such as a distributor pipe or impingement plate
A Feed Transfer nozzle will normally be the "Key" (Genesis) nozzle for any large fractionation vessel. Normally any side inlet orientation is possible but in most cases this will then dictate the tray orientation.
Liquid (secondary) Inlet Nozzles
A normal liquid feed nozzle will not have the same complexities as the Feed Transfer type. This nozzle installation is characterized by the following:
a) Attached line originated at an exchanger
b) Hot but not overly high on the temperature scale
c) Some may have potential for mixed phase flow
d) Normal line velocity
e) May require vessel internals such as a distributor or inlet pipe
f) Watch Instrument connections in relationship to Inlets and reboiler returns.
Reflux Nozzles
A normal reflux nozzle will not have the same complexities as other nozzles.
This nozzle installation is characterized by the following:
a) Attached line originated at a pump
b) Low on the temperature scale
c) All liquid flow
d) Normal line velocity
e) May require internals such as a distributor or inlet pipe. Multiple pass trays will require a more complex distributor or inlet pipe than a single pass.
Draw-Off Nozzles
The purpose of this nozzle is to draw-off or remove the primary product. They are also used to Draw-off a secondary product to side stream stripper. May be installed with a sump to remove unwanted water in the process stream.
This nozzle installation is characterized by the following:
a) Located in the downcomer area of the column
b) May be in a sump
c) May be a larger size than the normal attached line size (Some of the initial vertical drop will be the larger size)
d) All liquid flow
e) Normal line velocity May require internals if multiple pass trays
Bottom Reboiler Feed Nozzles
The liquid outlet nozzle will normally be in the center of the bottom vessel head.
This nozzle installation is characterized by the following:
a) Located in the bottom of the surge section of the column
b) May be a very large size and all liquid flow
c) Normally very low line velocity
Side Reboiler Feed Nozzles
This is also a potential Key Nozzle. The liquid outlet nozzle must be oriented in the same quadrant as the bottom downcomer.
This nozzle installation is characterized by the following:
d) Located in the downcomer area of the column
e) Will be in a sump
f) May be a larger size than the normal attached line size (Some of the initial vertical drop will be the larger size)
g) All liquid flow
h) Normal line velocity
i) Relationship to elevation of associated Reboiler is critical to nozzle elevation and internals
Side Reboiler Vapor Return Nozzles
One of the primary issues with this nozzle is the orientation relative to the other internal items and nozzles. If not placed in the right place the velocity of the return can blow liquid out of a seal pan or can affect the readings of any instruments attached to the far wall.
This nozzle installation is characterized by the following:
a) Attached line originated at a thermo-siphon or kettle type reboiler
b) High temperature
c) Moderately high velocity
d) All vapor flow
e) May require internals such as a pipe or impingement plate
f) Relationship to elevation of associated Reboiler is critical to nozzle elevation and internals
Bottoms Out and Drain Nozzles
The bottoms-out nozzle is normally a pump suction source. The standard type is located in the bottom head then piped through the skirt with a drain nozzle off the bottom out line nozzle. This would be a combination nozzle. A variation of the bottoms nozzle is the siphon or winter type. This type may be used (with process approval) when bottom clearance is a problem.
Note: It is common industry practice to avoid locating any flanged connections inside the vessel support skirt. All flanges are subject to leaks, and vessel skirts are classified as a confined space.
Level Instrument Nozzles
Extreme care must be used when locating level instrument nozzles. There are access and clearances problems that must be considered on the outside of the vessel. There are sensing location and turbulence problems associated with the inside of the vessel.
These nozzle installations are characterized by the following:
a) Must be attached in the same pressure volume of the vessel
b) Lower nozzle in liquid of the surge section, upper nozzle in vapor space
c) Located in static area (or with stilling well)
d) Requires external access for operation and maintenance
Pressure Instrument Nozzles
Pressure readings are normally taken in the vapor area of a vessel. Pressure connections shall be located in the top head area, 3" to 6" under a tray, or well above any liquid level in bottom section.
These nozzle installations are characterized by the following:
a) Located in a vapor space of the vessel
b) Requires external access for operation and maintenance
Temperature Instrument Nozzles
Temperature readings are normally taken in the liquid area of a vessel. Temperature connections shall be located 2" to 3" above the top surface of a tray, in the downcomer, or well below any liquid level in bottom section.
These nozzle installations are characterized by the following:
a) Located in liquid in the downcomer area
b) Requires external access for operation and maintenance
c) Interference with internals
Vapor temperature readings may be required for some situations. When required the preferred location is in the downcomer area half way between the two trays.
Tangential or
Steam-Out Nozzles
Process plant vessels that contain hydrocarbon or other volatile fluids or vapors will normally have a Steam-Out Nozzle. This nozzle has a number of options such as:
a) A simple blind flanged valve on the nozzle -- After the plant is shut down by Operations, the maintenance group would remove the blind flange from the valve. They then attach a temporary flange fitted with a hose coupling and proceed to steam out the vessel by connecting a hose from a utility station.
b) A blind flanged valve and hard piped steam line configured with a steam block valve and a swing ell.
c) A fully hard piped connection from a steam source. This method would have double block valves, a bleed, and a spec blind for positive shutoff.
The vessel steam-out nozzle should be located near the surge section (bottom) Manhole on vertical vessels.
Manholes
Manholes are also considered a nozzle. They just do not have any pipe attached to them. They are however, a very complex piece of the vessel orientation puzzle. The types of manholes normally relate to the method of cover handling provided.
Manholes come in the following types:
a) With Hinge -- A Manhole may be hinged for side mount, for top mount, or for bottom mount
b) With Davit -- A Manhole may have davits for side mount or top mount only
c) Plain -- A Plain Manhole may be for side mount, for top mount, or for bottom mount
The manhole orientation in top or non-trayed section of a vertical vessel is somewhat flexible. Normally any orientation is possible; however, the orientation of the manhole should be checked to insure that the entry path is not blocked by any internals.
The Manhole may be located in the top head on large diameter vessels if there is a platform that is required for other items. Top Manholes on large diameter vessels have their built in good points and bad points. The good point is that during shutdown the open manhole provides for better venting. It also allows for a straight method for removal and reinstallation of the trays. The bad point is that ladder access must be provided down to the top tray, and the manhole is competing with the other nozzles for the space on the vessel head.
Orientation for manholes that are located in the trayed section of the vessel is more complicated. The location of between the tray manholes has a number of restrictions. These restrictions include the type of trays and the tray spacing. The first choice for the location of a manhole is between the down comers. The last choice is in the downcomer space, but behind the downcomer. The downcomer would be fitted with a removable panel to allow further access into the vessel. The location to be avoided is above a downcomer where there is the potential for falling down in the downcomer space and injury. It would be better to seek approval to move the manhole up or down one tray than placement over a downcomer.
Manhole orientation in the surge section of a vessel is not as restrictive. The surge section of a vessel is the bottom portion that, during operation will contain a large volume of liquid. Any orientation is possible for a manhole in this section. However, the location of all manholes should be in the back half of the vessel away from the pipeway. The surge section may have a large baffle plate bisecting the diameter of the vessel and extending vertically many feet. A removable plate or hatch may be installed in this baffle (by vessels) to allow access to the far side. The vessel orientation of the manhole should not hit the baffle or be located so close to the baffle that entrance is obstructed.
Trays
The type of trays, the number of trays, and the number of passes are not the specific responsibility of the piping layout designer. However, there is the need to know factor. A common understanding of terminology will improve communications and prevent errors. The common tray parts are:
a) Tray (support) Ring -- The tray support ring (or Tray ledge) is technically not a part of the tray itself. The tray support ring is only there to support the tray. If there are no trays, then there is no need for tray support rings, therefore tray rings are linked to the trays. Tray support rings are normally a simple donut shaped strip welded to the inside of the vessel. They could also be in the shape of an inverted "L" welded to the vessel wall. Problems arise when the Designer does not allow for the tray support device.
b) Trays (or Tray Deck) -- One or more sections, consisting of plates, forming a horizontal obstruction throughout all or part of the vessel cross section. The trays will normally be constructed to form flow patterns (one or more) called passes. The purpose of tray deck is to provide a flow path for the process commodity and contain the fractionation or separation device.
c) Weir -- A low dam (on a tray) to maintain a liquid level on the tray
d) Downcomer -- The primary liquid passage area from one (higher) tray to another (lower) tray
e) Valves -- Tray hardware device
f) Bubble Caps -- Tray hardware device
g) Draw off - A way to remove liquid from the vessel
h) Trough - A way to collect and move liquid from one point to another
i) Riser - A device to channel vapor from one lower point to a higher point
j) Seal Pans - A device (with a liquid seal) that prevents vapors from passing
k) Beams & Trestles - Devices that support trays (or other types of internals) in very large diameter vessels
l) Baffles - A separation device inside a vessel
m) Chimneys - (See Riser)
The trays and the related down comers can be arranged in a wide verity of patterns.
Typical Tray arrangements are:
a) Cross Flow,
b) Cross-Flow,
c) Reverse Flow,
d) Radial Flow -- (Rare)
e) Circumferential Flow -- (Rare)
f) Cascade Flow -- (Rare)
The single pass tray will have a single downcomer. The 2, 3, or 4-pass tray will have the same number of down comers as passes. The number of passes (number of down comers) will have a big effect on the orientation. Some towers may have more than one Tray pass configuration. They may have single pass in the top Trays and two-pass Trays in the bottom. The change from one pass configuration to another is chance for error. The alignment of the single pass tray will normally be perpendicular to the two pass trays.
Tray Types
There is what would be considered "Standard" Trays, and there are also "High efficiency Trays".
a) "Standard" Trays -- This tray will have an open downcomer with no separation occurring in the downcomer area. This tray is the old stand-by and has been used for many, many years.
b) "High efficiency Trays" -- This tray will have a sealed downcomer with separation occurring in the downcomer. This tray type is fairly new. It will most likely be used on most new vessels in the future. It is also the type of tray that is favored on revamp projects to get more out of an existing tower.
Tray hardware devices
The normal trays inside the typical vertical vessel will contain openings (or holes) and may be fitted with a fractionation or separation device. This device is what will accomplish the purpose of the vessel. If these devices are not present or do not function properly then the product is not made.
The common tray devices are:
a) Bubble Cap (Used mostly on revamps) -- Simple, and common method to facilitate the separation process. The Bubble Cap will normally be a round (cup shaped) cap inverted over a short and smaller diameter chimney. The skirt area of the inverted cap may be plain or have (open or closed) slots.
b) Box Cap -- This cap is very much like the common Bubble Cap except it is square.
c) Tunnel Cap -- This will be a long narrow rectangular shape
d) Uniflux Tray -- This is a series of overlapping and interlocking plates. In cross section the Uniflux tray will have the shape of a reclining squared off "S".
e) Valve (Most common) -- The valve tray will have small flat metal plates fitted over the holes in the trays. The plate is loose to move up and down, but is retained in position by a clip type device. Vapor pressure under the "Valve" plate causes it to rise and gravity brings it back down.
f) Sieve (2nd most common) -- The Sieve tray will have holes and nothing else. The hole size is calculated to provide a fragile balance between the liquid head above the tray and the vapor pressure under the tray.
Weirs
There may be a number of places where weirs are used. The simple weir to provide proper tray flooding will normally not cause any design problems. There are also some special purpose weirs that may effect the location of nozzles. In most cases the existence of special purpose weirs will not be known at the start of the Vessel orientation activity. It is however, a good idea to ask the question anyway.
Down comers
Down comers can come in a verity of shapes also. They straight across in the horizontal direction, or they can be bent. They can be straight up and down in the vertical direction, they can be sloped or slanted (tapered), or they can be a combination. These variations will all impact the orientation to some extent. The major impact, by the downcomer on the orientation is the geometry or location of the vertical plane itself. The orientation of the down comers will have a direct relationship to the orientation of certain nozzles and manholes.
Other Tray Terms
Some other terms that will be found relating to trays.
a) Sump -- This is a sealed downcomer type area that is designed to provide a retention volume for some purpose.
b) Seal Pans -- This is a portion of a tray that is set deeper than the rest of the tray to form a seal for the downcomer from the tray above.
c) Side Draw Tray -- A tray arrangement that allows the removal of a specific liquid product
d) Chimney Tray -- A full circumference tray fitted with long open pipes to allow vapor to pass from below the tray to the space above.
e) Baffles -- Plates installed in the vessel for a specific purpose
f) Impingement Plates -- Somewhat like a baffle but normally a plate installed in the vessel at the inlet to prevent blowout to devices located on the opposite side of the vessel.
g) Tray manholes -- Most, if not all, trays will have a removable panel (somewhere in the tray) to allow inspection passage without dismantling the total tray
Vessel Support
The method of vessel support depends on various factors. These factors include process function, operation access, maintenance clearances, ease of constructability, and cost. Meeting the positive criteria for all or the majority of these factors will drive the support method.
The primary methods of support are:
a) Tall Skirt on foundation at grade (Most common)
b) Short Skirt on elevated pier foundation, table support, or structure
c) Legs on foundation at grade
d) Lugs on elevated pier foundation, table support, or structure
Each of these vessel support methods has their own good points and bad points. The Tall Skirt is the most common because it meets more of the "preferred criteria" than the others do.
Skirt Vessel Support
The minimum height of the skirt is normally set by process based on the NPSH requirements of the pumps or for the reboiler hydraulic requirements. The designer may need to increase the skirt height due to:
a) Vertical distance required by pump suction line geometry
b) Vertical distance required by reboiler line geometry
c) Operator aisle headroom clearance
d) Suction line entering the pipe rack without pockets
The approval of the Process engineer, Project Manager, and the Client will be required for any increase to the skirt height.
The skirt will have one or more access openings and will have skirt vents.
Skirts of vessels in refineries or other plants processing flammable commodities will normally be fireproofed. The fireproofing is normally a two-inch (2") thick layer of a concrete type material applied to the outside of the skirt. Check for the specific type. Some materials may require up to 6" to obtain the required fire rating.
Load Handling Devices
Load handling devices are required for Vertical Vessels if:
a) The vessel is over thirty feet (30') tall
b) The vessel has removable trays and internals
c) The vessel has components that require frequent removal for routine maintenance (PSV, control valves)
d) The components weigh 100 pounds or more
Methods of load handling include:
a) Davit -- A small somewhat inexpensive device used for lifting and supporting heavy objects up and down from elevated platforms. Limited to a fixed reach.
b) Monorail -- A more expensive method
c) Crane -- A far more expensive method and is dependent on availability
If a davit or monorail is not installed then a crane with the required reach and load rating must be rented or an alternate method must be jury-rigged. Any jury-rig method will have a high potential for accident and injury.
When a Davit is to be included the following must be determined and furnished to Vessels:
a) The location
b) The swing
c) The clearance height (including lifting device)
d) The reach - the removal items (e.g... PSV, Control Valve, Block Valve, Blinds, etc.) and the drop zone
e) The maximum load of external items (Vessels will determine weight of internals)
When a Monorail is to be included the following must be determined and furnished to the Vessels engineer:
a) The platform, and monorail support configuration
b) The clearance height (including lifting device)
c) The reach to the drop zone
d) The maximum load of external items (Vessels will determine weight of internals)
Pipe Supports and Pipe Guides
The Pipe Supports and Pipe Guides (PS & PG) for the piping that is attached to the vessel is the responsibility of the Piping Group. You're the Piper, that's pipe, and you need to make sure it is properly supported and guided. The rule is (or should be) that all lines shall be properly supported and guided. One key element of the PS & PG is the "L" dimension. The "L" dimension is the distance from the O. D. of the back side of the pipe to the O. D. of the vessel. This dimension should be as small as possible but not less than required for maintenance. The rule of thumb for the "L" dimension is 12" minimum and 20" maximum. Dimensions of under the 12" and over the 20" are sometimes allowed. For example, if fitting make up results in an "L" dimension of 11 13/16" do not add a spool piece and extra weld.
Lines should be supported as close to the nozzle as possible. The type of support is based on the weight of what is being supported. It may be just a straight pipe dropping down the side of the vessel. Or, it may be much more.
The requirements for pipe supports attached to a vessel must be evaluated for the following:
a) The shell thickness
b) Orientation
c) Elevation
d) The "L" dimension
e) The weight of the basic pipe and fittings (based on size and wall schedule)
f) The weight of the water during hydro test
g) The weight of the insulation (if any)
h) The weight of any added components (block valves, control valve stations, relief valves, etc.)
i) The clearance to other objects (Seams, Stiffener rings, Nozzles, Clips, Pipe Lines, Platforms)
The requirements for pipe guides attached to a vessel must be evaluated for the following:
a) The shell thickness
b) Orientation
c) Elevation
d) The "L" dimension
e) The size of the line at the point of guiding
f) The distance above the horizontal turn out (allow 25 pipe diameters +/-)
g) The maximum allowable span between guides
h) The clearance to other objects (Seams, Stiffener rings, Nozzles, Clips, Pipe Lines, Platforms)
Pipe supports and guides should be staggered vertically for clearance from supports or guides on other lines running parallel.
Platforms, Ladders, and Cages
Platforms with access ladders must be provided as required for access to manholes, operating valves, and instruments as defined in the project criteria. Normally objects below 15' - 0" from grade will not require permanent platforms and ladders. These objects are judged assessable by portable means (Check the Project design requirements).
Platform spacing shall be even foot increments when multiple platforms are serviced from a single ladder. The platforms shall be arranged to allow the following:
a) Minimum 7' - 0" headroom to underside of any obstruction
b) Minimum 2' - 6" radial width for primary egress path (I. D. of platform to O. D. of platform)
c) Minimum 2' - 6" clear distance between ladders
d) No obstructions in path between primary egress ladders
e) Maximum 30' - 0" vertical travel length of ladder between platforms
f) Side step off at all platforms (Step through ladders are considered dangerous and therefore should be avoided). This requirement should have been reviewed with the Client and defined in the Design Criteria.
g) Combining with platforms on other vessels when potential for improved operations or maintenance exists
h) Flanges of top head nozzles shall be extended to provide access to bolts
i) Minimum 1' - 6" clearance around objects if for maintenance access only
Code Name Plate
Every vessel will have a Code Name Plate. On a vertical vessel the code name plate must be on the (pressure containment) part of the shell. It cannot be attached to the skirt. The best place for the code name plate on a vertical vessel is 2' - 6" above the horizontal centerline of the surge section manhole. Make sure the location selected is accessible on grade or on a platform.
Common problems with vertical vessels
a. Schedule crunch - Vessels scheduled for purchase too early requiring firm orientations with very little backup information.
- Approved and Issued for Design P&IDs
- Exchanger type and location
- Flare header and PSV location
b. Thin wall vessels not able to support load on pipe supports
c. High wind presence requiring extra guides
d. Late changes to PSV sizing prompting changes to pipe support and guides on line to flare
e. Late change to control valve location criteria (Flashing service now required to be located to elevated platform on vessel with line downstream of valve self drain to vessel)
f. Reboilers requiring spring mounted supports due to tight piping and differential growth
g. High steam-out temperature requiring extra flexibility in the piping
h. Extra heavy object removal in excess of Davit load capabilities
Vertical Vessel Orientation
Recommendations
Uniformity
a. The ladder approach at grade should be free of obstructions and easily accessible (Verify preferred location with Project requirements).
b. The Manhole orientation should be oriented in the back half of the vessel toward the access way. The manholes should be arranged with consideration to the type of load handling device (One centerline if monorail, one or two centerlines if davit, no specific restriction if crane).
c. Load drop area should be located on the main access side
d. Level instruments should be located on or near the front half of the vessel and visible from the main operating aisle
e. The piping risers to and from the vessel should be located to the front half of the vessel for easy routing to the pipeway and equipment
Manholes
a. Manholes will influence the entire vessel orientation to a certain degree. The location of the manholes must be compatible with the location of the tray down comers. The down comers in turn influence the location of the process and instrument nozzles.
b. The preferred elevation of manholes above the platform is 2' - 6" from the centerline. The limits are; 6" minimum from the top of the platform to the bottom of the flange, or 4' - 0" maximum from the top of the platform to the bottom of the flange (Verify preferred location with Project requirements).
c. Platforms may not be required for manholes that are 15' - 0" or less above grade, unless a platform is required for another reason such as an instrument (Verify preferred location with Project requirements).
d. Space and clearances are important around manholes. Check flange swing and tray lay down space.
Ladders and Platforms
a. Check to see that the approach to the ladder at grade is clear of all obstructions and hazards.
b. Check to see that the entry onto each platform is clear and not blocked by level or other instruments.
c. Check to see that the entry onto each platform is clear and not blocked by an open manhole flange.
d. Check to see that there is a clear path from one (down) ladder to the next (down) ladder for unobstructed travel during emergencies.
e. Platforms may need to be added or extended for access to operating valves, spec blinds, or instruments.
f. Special platforms are often required at the channel end of a thermo-siphon reboiler or other equipment that is mounted directly into (or onto) the vessel.
g. Investigate lining up and connecting platforms servicing equipment (Reboilers or Accumulators) located in adjacent structures but related to the vessel.
h. Maintenance criteria at Reactors often require platforms large enough and strong enough for large flange or head lay down in addition to catalyst storage and handling.
i. Check the location and size of the pipe penetration holes through platforms. The opening is to be one inch larger (in diameter) than the flange or pipe plus insulation, which ever is greater (Verify preferred location with Project requirements).
j. Provide proper routing and support for all lines regardless of size. Do not route small lines vertically behind the ladders. Do not route small lines vertically between the vessel shell and the inside radius of the platforms. Do not route small lines vertically up the outside of the platforms in line with or close to the manholes.
k. Ladder access openings must be fitted with a safety gate. Check for proper clearance for gate swing.
l. Some processes are subject to periods of hazardous operations. Ladders and ladder cages may need to be designed for operators with self-contained suits and air packs (SCBA).
Skirts
a. The minimum skirt height is set by Process and indicated on the P&ID.
b. The skirt height is normally based on the minimum NPSH of the bottom pumps.
c. The skirt height may be influenced by the physical requirement of a thermo-siphon reboiler.
d. The final skirt height needs to consider and be adjusted for; physical configuration of the bottoms nozzle, any headroom clearance required over operating aisles, vertical fitting geometry of the piping configuration, and the pump suction nozzle location.
e. As a general rule no flanged connections are allowed inside the skirt of a vessel. This area is considered a confined space in most plants and flanges will tend to leak over time.
f. Increasing the Skirt height may be considered when adjacent vessels warrant lining up and connecting platforms.
Reboilers
a. Reboilers will be one of the following; Fired (Heater Type), Thermosiphon (vertical or horizontal shell & tube), or Kettle type (horizontal shell & tube).
b. Fired Reboilers shall be located a minimum of fifty feet from the vessel.
c. Piping to and from any type of reboiler will be hot, and have sensitive flow conditions.
d. The Kettle or Thermosiphon Reboiler elevation is set by Process and indicated on the P&ID.
Pipe Supports and Guides
a. Piping is responsible for locating the pipe supports and guides on vessels
b. Piping is responsible for defining the size and loads on the pipe supports on vessels
Piping Flexibility
a. Piping must determine the operating thermal growth of the vessel. The vessel will have a series of temperature zones from the bottom to the top.
b. The differential expansion between the piping risers and the vessel must be checked to prevent over stressing the piping or the vessel shell.
c. The routing of cooler reflux lines must consider the total growth of the hotter vessel.
d. Potential for differential settlement needs to be investigated
e. Each piping system or line needs to be considered individually
Instrumentation
a. The HLL, NLL, and LLL need to be carefully considered because they will set the elevations of the level instruments
b. Orientation of level instrument connections needs to consider the internals
c. All instruments shall be accessible
d. Watch out for space requirements for gage glass illuminators.
e. TI and TW connections will require removal space
Electrical
a. Space shall be allocated for conduit runs up the vessel. These conduits will carry power to platform lights, gage glass illuminators, and in some cases electrical tracing.
b. Conduits are also required for controls (instrumentation)
Piping Valves
a. Valves are meant to be operated and to be operated they must be accessible.
b. Small valves (2" & smaller) may be considered accessible from a platform or ladder. Large valves (3" & larger) shall be accessible on a platform.
Misc. Piping issues
a. Lines to and from vessels may be subject to conditions such as 2-phase flow or vacuum.
b. Some PSV relieving to atmosphere will require snuffing steam. The steam pressure (in the line) must be adequate to reach the top of the vessel.
c. Large overhead lines vs. PSV location require special attention for function and support.
d. Vertical vessel piping needs to be checked for heat tracing requirements. A tracer supply manifold may need to be added at the top of the vessel.
Constructability
All vertical vessels shall be reviewed for constructability. This review needs to consider receiving logistics lay down orientation, lifting plan, pre-lift assembly items (piping, platforms, ladders, internals, etc.)
- Pre-lift assembly items may include the following:
a. Piping
b. Platforms
c. Ladders
d. Internals
e. Paint
f. Insulation
Fire Protection
a. Some vessels may require special insulation for fire protection.
b. Some vessels may require fire monitor coverage
c. Some vessels may require sprinkler systems
Misc.
Some vessels will be lined. Linings may be metallic, plastic, or glass. Welding to the vessel shell after initial fabrication is not allowed.
Some vessels will have flanged connections that are larger than 24". These connections will occur at connections for piping, reboilers, or other equipment. Flanged connections over 24" do not have a single standard and need to be defined for specific type (API or MSS).
you must take everything in to consideration. Everything is important! Someone may tell you that some things do not matter but this is not true, everything matters.
You need to consider the following:
a) Timing: Vessel orientation is normally the only equipment related layout activity that can be done without specific input from a vendor. All of the information required for vessel orientation is generated on the project in the form of P&ID data and project standards. It is also one of the few activities that will feed one or more other downstream groups whose work is critical to the project schedule. With this in mind this activity can and should be started as soon as the P&ID reaches "Approved-For-Design" (AFD) status. Te vessel orientation activity can be started manually or on basic 2D CAD before the 3D PDS data base is fully loaded and checked. There is some logic to doing this activity manually or in 2D CAD because of the amount of trial and error required to finally achieve an acceptable and approved orientation. Once the orientation is approved and the PDS data base is ready the 3D model can be built with no recycle.
b) The Plot Plan (Note 1): The plot plan is required to identify the location of the vessel and its related equipment. The related equipment includes the equipment that feeds the vessel (is up-stream) and also the equipment that the vessel feeds (is down-stream). It shows and locates adjacent, non-related equipment. It also shows adjacent structures that may support the related up-stream or down-stream equipment. It also indicates the plant features such as pipe racks, operating aisles, maintenance access areas and the direction of Plant North.
c) The project foundation criteria: Vertical vessels normally sit on an octagon pad foundation with the top of grout at EL101' - 0" (
d) The P&ID's (Note 1): The P&ID's are required to show the process streams that connect to the
e) The project Line List (Note 1): The line List is required to give you specific and critical key data about the lines such as the Line Number, line class, maximum operating temperature and insulation requirements,
f) The project Piping Material Specifications (Note 1): The Piping Material Specifications are required to give you the data about metallurgy and any specifics about fittings, flanges, valves or requirements for PWHT (post weld heat treatment).
g) The Vessel Drawing (Note 2): The vessel drawing at this time will most likely be marked "Preliminary." It will give you; the inside diameter (I.D.), the tangent-to-tangent shell length, the shape of the top and bottom heads and the skirt height. This drawing should also have a table showing all the nozzles with the basic information such as: identification, quantity, and size, flange rating, the elevation above (or below) the bottom tangent line for each nozzle, the purpose for the nozzle and any special instructions. The vessel drawing needs to also indicate where the internals start and end inside the vessel.
h) The Internals (Note 2) (Trays or Packing) A tower can have a number of different types and configurations of internals. It may be Trays or it may be some form of Packing.
- Trays: If you have Trays then you need to know: the number of trays, the spacing of the trays, the number of passes for the trays (1-pass, 2-pass, 3-pass etc.). You also need to know if there are any "draw sumps," baffles or other special features.
- Packing: You need to know the number of "Beds," the depth of the beds and the method of installing and removing the packing material. You also need to know and understand about the type of feed distributor(s) to be used. You need to know about the packing discharge nozzles.
For the purpose of this article we will assume we have 35 single pass trays.
i) The Thermosiphon Reboiler data sheet (Note 1): This will give you the preliminary size and type information. The P&ID indicates that this vessel has a vertical Thermosiphon reboiler fitted to it. Some discussion should normally take place to determine the optimum tube length and the proper support elevation and support method.
j) The project Vessel Platform Standards (Note 1): This will give you the required information about the minimum vertical spacing between platforms. It will also give you specific details about platform supports and how to make the openings where pipes must pass through a platform. This drawing will (or should) also give you specifics about handrails.
k) The project Vessel Ladder Standards (Note 1): This drawing will give you all the required information about ladder construction and more important the limits for the maximum vertical run for a single ladder.
l) The project Vessel Nozzle Standards (Note 1): This will give you all the normal options for un-reinforced and reinforced nozzles. It may also show you some options for internal nozzle piping.
m) The project Vessel Davit Standards (Note 1): A davit is a small device permanently mounted on the vessel that acts as a crane for lifting heavy objects such as tray sections.
n) The project Vessel Pipe Support and Guide Standards (Note 1): These are devices attached to a vessel that support and/or guide the vertical runs of pipe. This drawing also defines the minimum distance from the outside of a vessel shell to the back of an adjacent pipe. Where I came from this was called the "L" dimension. The "L" dimension was normally 12" (adjusted as required for insulation) The maximum was 20" without a special design. The key was to have a minimum of 7" clear between two co-existing insulations. These supports and guides also require a wider than normal line spacing in the vertical plane as the lines go up or down a vessel. This is mainly due to the configuration of the Trunnion (Note 3) support attached to the pipe and the pipe clamp used for the guide.
o) The project Piping and Vessel Insulation Specification (Note 1): From this document you will get the thickness of the insulation needed for the pipes and vessel at the operating temperature.
(Note 1): These items are normally created by your company for the project and should be "Approved for Design" (AFD) quality documents. This means that they have been through all of the proper in-house reviews and checks and have then been approved by the Company and the Client for use in the design of the work.
(Note 2): These documents will initially come from the project Vessel Engineer. They will normally be marked "Preliminary" until they receive and process your orientation drawings. Later you may receive the vessel fabricator's detail drawings for "Squad Check" (review and approval).
(Note 3) For more information about a Trunnion support see www.pipingdesigners.com look under Training and Secondary Pipe Supports
There may be other documents that are required due to a specific company's method of operation.
The next things you need to consider is; functionality, safety, operation, maintenance and constructability.
Functionality: No matter what, this vessel must do its job. You must know and understand what that intended job is. You do not need to be a process engineer but you should be involved in the review of the P&ID for this specific vessel. You need to hear what the critical issues are relating to this vessel and the connected piping. If your company does not include piping in the formal review of the P&ID's then you need to seek out the process engineer and ask him or her to explain the function, key points and any critical issues relating to this vessel.
Safety: This is the other important issue relating to vessel orientation. The operation must be able to be done in a safe manner. The same must be said for both maintenance and constructability. To achieve this goal the locations of nozzles relative to the placement and arrangement of the ladders and platforms must be carefully considered. The travel path (access and egress) must be arranged so the main travel path cannot be blocked by open manholes, scaffolding, tools, tray parts, valves or piping. The basic rule here; a: ladder #1 comes up with a side step-off (right or left) on to platform #1. Then b: there is a minimum rest space equal to one ladder width. Then c: the next ladder (#2) continues up to the next platform. Platform #1 can continue beyond ladder #2 around the vessel to provide access to nozzles and manholes. This arrangement does not impede or obstruct the clear path for rapid escape from the vessel for anyone from a higher elevation. Other safety issues include one or more skirt access openings located near grade which should be located with clear access. There will also be four or more skirt vents located high near the skirt-to-vessel attachment which also should not be blocked.
Operation: Process plants need to be operated. Most operation is concentrated around valves and instruments. These items must be accessible. Accessible means reachable. This reachable is conditional. Nozzles with a nominal size of 2' (NPS) and smaller can be reachable from a ladder or from a platform. Nozzles 3" (NPS) and larger shall be reachable on a platform. In this context the from means that the object is not more than 18" (one arms length) from the ladder or platform and the on means the object must be fully inside the platform. There is normally only one exception to this rule. That is for valves or nozzles that are located less than 20 feet from grade and can be accessed with scaffolding or a "Man-Lift".
Maintenance: All the accessibility issues that apply for operations also apply for maintenance. In addition don't block access to manholes with control valve assemblies or other piping. Make sure the Electrical and Instrument people don't locate a panel or a transmitter assembly in the operations or maintenance access ways.
Constructability: This vessel needs to be erected and therefore it will need Lifting Lugs. These are normally very large steel shapes with "eyes" welded to the top head. They will normally not interfere with your orientation, however you should check to make sure.
Your second question: "What are the key steps in the process for doing a column nozzle orientation?"
The key steps in the process are:
(You may choose for some reason to do something in a different order, but this is how I think I would do it. It should be noted that I like to be able to have all things numbered from the bottom up. This includes trays, nozzles, ladders, platforms, etc. However, sometimes due to company preference or the tray manufacturer standards the trays are numbered from the top down.)
1. Data collection - Collect a copy of all the drawings listed above. Make a folder file (or a stick file) to keep them in. Mark all the drawings "Stripper Tower Orientation Master" (STOM). This STOM file is your justification for everything you do or did. If anyone has reason to question why you did what you did then you have a file of the source material you based the work on. It is your responsibility to use the proper information and to properly file and incorporate changes from all new revisions when received.
2. P&ID conditioning - Take your STOM P&ID and pick-up any marks from the Project Master copy. From time to time as you work, go back and recheck the Project Master P&ID for any new marks (i.e.: line size changes, additions, deletions, etc.). Study the
3. Plot Plan conditioning - Take the STOM Plot Plan and with a yellow high-lighter identify the
4. Prepare preliminary elevation - Manually or by CAD, create a scale drawing of the vessel elevation (side view) Locate the bottom tangent line and in phantom (dotted line) the bottom head. Accurately locate the top tangent line from the bottom tangent line and draw in the top head. We will assume that this vessel is a skirt supported vessel and that the skirt is 20 ft high. (If not skirt supported then Leg or Lug supported will require optional considerations that we can discuss if applicable.) At the bottom accurately create the skirt (vessel support). Check with the Structural department and find out how high the foundation is for this vessel. Make sure they give you the top of grout (TOG) not just top of concrete. They are not the same. I will assume that the TOG is EL. 101' - 0." Now indicate the
5. Prepare preliminary plans - Manually or by CAD, create a scale drawing of a number of plan views. The plan views will be where you will do most of your work so make one for each ten feet +/- (3 to 4 meters) of vertical elevation ending with one above and showing the very top platform. These starter plans should have crossed center lines and the actual I.D. of the vessel. (We are using 8' - 0" for this article). Mark the location of Plant North on each mini-plan. Normally plant north is the same as 0 degrees on the vessel shell. East is 90 degrees, South is 180 degrees and all additional orientation is clockwise from north and 0 degrees. Don't worry about the O.D. or the wall thickness. Now, look at the platform drawing and get the clearance from the vessel shell and the inside edge of a platform. Draw a very light circle (different color and/or layer) on each mini-plan to indicate where the inside edge of a platform might be. Now draw another very light circle 3'-0" (1meter +/_) more in diameter to indicate where the outside of a platform might be. These are not real platforms yet they are just guide lines to remind you of platforms as you do other work. Now mark the "Front" (pipeway side) of the vessel and the "Back" (maintenance side) of the vessel.
6. Thermosiphon Reboiler: The Reboiler for our sample vessel has a 42" shell, 24 ft fixed tube (vertical mount) shell and tube exchanger. The shell side is high temperature steam. The tube side is the process fluid from the bottom of the tower which enters at the bottom end of the reboiler. The process vapor exits the top end of the reboiler and returns to the tower below tray #1. The placement and support of the Thermosiphon Reboiler is the next thing we should cover. Because of the plot plan placement of our
7. Bottoms section baffle - Because of the way this vessel works there is a baffle dividing the bottom section of the tower. The baffle can not be on the centerline of the vessel because the reboiler feed nozzle is centered on the bottom head. Therefore the baffle must be offset to miss that nozzle connection. The height of the baffle is the same as the "High Liquid Level." All of the liquid that comes off the downcomer from tray #1 goes into the "large" side of the bottom section. It then goes through the reboiler and returns to the vessel as vapor. Excess liquid from the "large" side overflows the baffle and becomes the "Bottoms" and is drawn off by the bottoms pumps. The connection for the bottoms nozzle "B" is on the "small" side of the baffle.
8. Check for nozzle continuity - Look at the STOM P&ID and the table of nozzles on the vessel drawing. They should match in number and size. In pencil mark each line connecting to the P&ID vessel with the nozzle number from the vessel nozzle table. Do they match in number? Do they match is size? If not, go see the Process Engineer and ask for clarification.
(Sample)
The bottom tangent line elevation = 121' - 0"
The top tangent line elevation = 232' - 8"
|
# |
Name or Function |
Size (NPT) |
Rating |
Dimension (from tangent line) |
Elevation (plant datum) |
Comments |
|
V1 |
Vapor Out |
14" |
300# RF |
113' - 6" |
234' - 6" |
|
|
V2 |
PSV |
6" |
300# RF |
113' - 6" |
234' - 6" |
|
|
V3 |
Vent |
4" |
300# RF |
113' - 6" |
234' - 6" |
|
|
R |
Reflux |
6" |
300# RF |
106' - 6" |
227' - 6" |
w/internal pipe |
|
F |
Feed |
8" |
300# RF |
73' - 0" |
194' - 0" |
w/internal pipe |
|
B |
Bottoms |
10" |
300# RF |
7' - 0" |
117' - 3" |
|
|
D1 |
Drain |
6" |
300# RF |
8' - 0" |
116' - 2" |
nozzle on nozzle B |
|
D2 |
Drain |
6" |
300# RF |
8' - 2" |
115' - 9" |
nozzle on nozzle N1 |
|
N1 |
Reboiler Feed |
14" |
300# RF |
7' - 0" |
116' - 9" |
|
|
N2 |
Reboiler Return |
16" |
300# RF |
29' - 3" |
150' - 3" |
|
|
M1 |
Manhole #1 |
24" |
300# RF |
2' - 0" |
123' - 0" |
|
|
M2 |
Manhole #2 |
24" |
300# RF |
73' - 0" |
194' - 0" |
|
|
M3 |
Manhole #3 |
24" |
300# RF |
107' - 0" |
228' - 0" |
|
|
S1 |
Steam Out |
2" |
300# RF |
0' - 6" |
121' - 6" |
|
|
S2 |
Steam Out |
2" |
300# RF |
71' - 6" |
192' - 6" |
|
|
S3 |
Steam Out |
2" |
300# RF |
105' - 6" |
226' - 6" |
|
|
L1 & L2 |
Level Gage Bridle |
2" |
300# RF |
0' - 6" 25' - 0" |
121' - 6" 146' - 0" |
|
|
L3 & L4 |
Level Transmitter |
2" |
300# RF |
0' - 6" 25' - 0" |
121' - 6" 146' - 0" |
|
|
T1 |
Temperature Element |
1" |
300# RF |
30' - 0" |
151' - 0" |
|
|
T2 |
(Ditto) |
1" |
300# RF |
72' - 0" |
193' - 0" |
|
|
T3 |
(Ditto) |
1" |
300# RF |
107' - 0" |
228' - 0" |
|
|
P1 |
Pressure Element |
1" |
300# RF |
28' - 0" |
149' - 0" |
|
|
P2 |
(Ditto) |
1" |
300# RF |
74' - 0" |
195' - 0" |
|
|
P3 |
(Ditto) |
1" |
300# RF |
108' - 0" |
229' - 0" |
|
9. Check for nozzle temperature - You now have all the nozzles connected or identified to its specific line. Now look at the line list and fine the maximum operating temperature for each of the flowing lines (feed and main outlet lines). Don't worry about vents and drain. In pencil, mark these temperatures onto the STOM P&ID at the point where the line connects to the vessel. You now have the vessel identified, the line from somewhere connecting to the vessel, you have the connection point identified with a nozzle number and you have a temperature at that nozzle.
10. Locate nozzle elevations - Based on the elevation for each nozzle (given in the Nozzle Table on the Vessel Drawing) locate all the nozzles on the scale vertical view (side view) of the vessel. Most of these flowing lines will be above the bottom tangent line. What this means is that all things connected to the nozzles above the bottom tangent line will grow up when the vessel is hot and in full operation. Only four of the nozzles are located below the bottom tangent line and these nozzles (and their attached piping) below the bottom tangent line will grow down when the vessel is hot and in full operation.
11. Establish temperature zones - The next step is to calculate the incremental and total vertical growth of the vessel. The incremental growth means the growth for a specific section of the vessel. Trayed vessels do not have the same operating temperature from bottom to the top. They have a graduated temperature. You may be asking what temperature you use for this operation. DO NOT USE THE VESSEL DESIGN TEMPERATURE. The vessel design temperature may be something like 500 degrees F. If you use this number along with the height of the vessel and the coefficient of expansion for the vessel metallurgy you would end up with a total expansion that would be incorrect. You look at the temperatures you marked for each of the Flowing lines. You take two adjacent Flowing nozzles that have a temperature. Let's say we take the Feed nozzle and the Bottoms Out nozzles. (I am assuming there are no other flowing nozzles between these two nozzles. If there are then make the appropriate adjustment). These two nozzles and their temperatures form a zone. You add their two temperatures together and divide the answer by 2 to get an average temperature for the zone (example: (475 degrees F and 395 degrees F)/2 = 435 degrees F). You use this 435 degrees F figure for the maximum operating temperature along with the zone length and the coefficient for the vessel shell material for the calculation of the incremental expansion. Do the same for each set of flowing nozzles and calculate the incremental expansion for each zone. The overhead vapor line temperature may be as low as 180 degrees F. Somewhere lower down the vessel there is another flowing nozzle with its operating temperature. This forms the top zone in the group. For talking purposes let's say we have five zones. Let's say that Zone one expands a total of 1", Zone two expands ¾". Zone three expands ½", Zone four expands ½" and Zone five expands ¼" for a total of 3". You need to mark each of the incremental expansions at the appropriate place. Now take each of the incremental expansions and add them together as you progress up the vessel. Part of Zone one is below the support point so some of the expansion grows up and some of it grows down. Because of this let's say that the top of Zone one only grows up 5/8" during operation. The top of Zone two grows up a total of 1-3/8". Zone three grows up a total of 1-7/8". The top of Zone four grows up a total of 2-3/8'. And the top of Zone five grows up a total of 2-5/8". You also need to mark each of the accumulated expansions at the appropriate place. You now have a basis for the preliminary pipe flexibility work you will do later.
12. Locate manholes - We have three manholes and they are only used during maintenance. These manholes will be the hinged type and for our situation they will all open to the right. They are identified as M#1 (bottom section) through M#3 (top section). They are not used or needed during operations. So Manholes should normally be located on the "back" side of the vessel. This is logical and it works 90% of the time. One of the times it does not hold true is for the lower shell manhole when there is a vertical Thermosiphon reboiler attached to the back of a vessel. So you can start with all of our Manholes on the back centerline of the vessel. This may not be the final location but it is a starting point. From the bottom of the vessel M#1 is in what is called the "surge" section. There are (normally) no internals in this section. So if we need to we can locate M#1 at any orientation. M#3 is in the very top section above the top tray so it also has few limits to its orientation. Manhole M#2 is located between trays at a maximum spacing of (say) twenty trays. In our case M#2 is on tray #19. The side manholes need to enter on a tray, not behind the downcomer.
13. Steam out nozzles: Along with each manhole there will also be a steam out nozzle. This nozzle will be fitted with a valve which will be blind flanged. During shut-down the blind flange is removed and a flanged spool with a steam coupling will be installed. Prior to any entry into the vessel the steam will be turned on for 12 to 24 hours to remove (steam-out) hydrocarbons. The steam-out nozzle will be located in close proximity to the manhole. The recommended placement for the steam-out connections on our vessel will be to the right and 1' - 6" below the manhole center line.
14. Set tray orientation - As we said above, we have 35 single pass trays. Tray #1 is 35' - 10" above the bottom tangent line of the tower and tray #35 is 104' - 10" above the bottom tangent line. Since we have trays that have only a single pass (downcomer) then we have almost 270 degrees of orientation with which we can place the manholes. However that 270 degrees of orientation needs to be in the right quadrant. If the excluded part of that circle is centered on 0 degrees (North) then we need to ask if that manhole can move up one tray or down one tray. If we have trays that are two pass or three pass then we need to find ways to orient the manholes, nozzles and trays so they co-exist. We have located all our manholes on the maintenance (north) side centerline at 0 degrees. We will then place the orientation of the trays on an East/West center line. We then insure that we adjust the vertical location of the manholes (up or down one tray) to enter on to a tray.
Up to this point you have doing the very important background work that is required before you can do the actually vessel orientation. Next you need to locate the nozzles, determine where the pipes will travel up or down the vessel and establish the support and guide points for each line. As you do that you also need to establish the ladder and platform requirements to provide proper access for operation and maintenance.
So let's move on to the next task.
15. Nozzle placement - As we stated before large nozzles need to be accessible "on" a platform. So keep that in mind as you proceed. Start with the nozzles at the top of the vessel and work down. Here is a key to remember, the line (up-or-down the vessel) and the nozzle do not need to be at the same bearing point. By this I mean that the line up-or-down the vessel can be at one point, say 196 degrees, and then wrap around the vessel to where the nozzle is on the other side of the vessel say at 315 degrees. The line would rise up the vessel and then turn horizontal to go around the vessel. It would then turn vertical again, go through the platform required for nozzle access and then enter the nozzle. This allows the nozzle to be "on" a platform but the line does not penetrate all the other platforms. Nozzles "F" and "R" on this vessel might be done using this method. The other lines from the "V1' nozzle and the PSV can simply drop down the vessel at the most convenient point. The lines to and from the Thermosiphon Reboiler will connect almost fitting to fitting with no valves. The bottoms line to the pumps is also a simple routing and might exit the vessel skirt at the 90 or 180 degree point depending on where the pumps are located. Instrument connections need to be placed so they perform their function and so they are accessible from a ladder or a platform. They do not normally extend far from the vessel shell thus do not cause an obstruction so with care they may be positioned on the vessel in the space between two ladders.
16. Pipe Supports - Each line that travels up or down the vessel will need one or more pipe supports. Lines that travel up-or-down the vessel at the same bearing point as the nozzle only need one pipe support. For side mounted nozzles this support will be located a short distance below the top elbow. For top mounted nozzles the support will be located a short distance below the vessel top weld seam. Lines that travel up-or-down the vessel at a different bearing point as the nozzle need to be considered for two supports. One below the nozzle elbow and a second support below the elbow where the line drops down the vessel.
17. Pipe Guides - Each line that travels up or down the vessel will need to be considered for pipe guides. The two factors in determining the number of guides a line requires is the wind force at the jobsite and the length of vertical travel. Some lines require only one guide and others require more than one pipe guide. Each line that travels up-or-down the vessel normally turns (elbows) horizontal at some lower elevation. The bottom guide should not be placed closer than 50 pipe diameters above this elbow. Other guides for a line may be spaced by taking the elevation of the support (at the top of the line drop) and then deduct the elevation of the bottom guide. The space remaining is then considered for one or more additional guides. Guides should be spaced every 20 to 30 feet.
18. Ladder placement - All of the ladders should be placed in the same general quadrant of the vessel. It is simple to work out the minimum spacing from one ladder to another. As stated before the minimum space between two ladders should be equal to one ladder (measured at the center of the cage). So if the ladder (with cage) is 2'-6" +/- wide then the space between two ladders is also 2'-6"+/-. This makes the center to center between two ladders 5'-0"+/-. Most of the ladders on this vessel can be in the quadrant from 45 degrees to 135 degrees. For a vessel 8' - 0" in diameter this would mean:
- Ladder #1 would be at 135 degrees
- Ladder #2 would be at 90 degrees
- Ladder # 3 would be at 45 degrees.
- Ladder #4 is back at 135 degrees.
- Ladder #5 is at 90 degrees and
- Ladder #6 is at 45 degrees.
- There will be a ladder #7 on this vessel which we will discuss when we talk about platforms.
19. Platforms - Platforms are the next thing to be defined.
|
Platform # |
Dimension from tangent line (in feet) |
Project Elevation (in feet) |
|
#1 |
1' - 0" |
120' - 0" |
|
#2 |
24 - 0" |
145' - 0" |
|
#3 |
45' - 0" |
166' - 0" |
|
#4 |
70 - 0" |
191'- 0" |
|
#5 |
90 - 0" |
211'- 0" |
|
#6 |
103 - 0" |
224'- 0" |
|
#7 |
113 - 0" |
234'- 0" |
|
#2a |
19 - 0" |
140' - 0" |
|
#2b |
27' - 0" |
148' - 0" |
Platform #1 would start at the step-off from ladder #1 (135 degrees) and wrap around the vessel (counter clock wise) to about the 350 degree point, beyond Manhole #1.
Platform #2 would start at the step-off from ladder #2 (90 degrees) and wrap around the vessel (counter clock wise) to ladder # 7 located at 315 degrees. Ladder #7 goes both up and down to provide access to two auxiliary platforms #2a and #2b. These small maintenance platforms provide access to the head flange of the reboiler and to nozzle N2. They must be sized to meet the criteria that the nozzle and head flange is "on" the platform.
Platform #3 would start at the step-off from ladder #3 and wrap around the vessel (clock wise) to and under ladder #4 at 135 degrees.
Platform #4 would start at the step-off from ladder #4 and wrap around the vessel (counter clock wise) to about the 315 degree point for access to Manhole #2 and to provide maintenance access for nozzle "F".
Platform #5 would start at the step-off from ladder #5 and provide a minimum platform (counter clock wise) for access to ladder #6
Platform #6 would start with a side step-off from mid way up ladder # 6 and wrap around the vessel (counter clock wise) to about the 315 degree point for access to Manhole #3 and to provide maintenance access to nozzle "R".
Platform #7 is a "Top" platform supported from the vessel head. This platform must be sized to allow space for the piping off the vessel head, access to the Davit and room for maintenance people to work during turn-around.
The imaginary vessel we have been discussing above is really a very simple vessel. After you read all of this you may think that vertical vessel orientation is very complex. You are right! However, I think vessel orientation is also the most fun there is in all of piping design.
For those of you who may want to try this vessel as a trial run I say give it a shot. Please feel free to E-mail me at (jopennock@netscape.net) when you start and maybe I can offer some suggestions.