Stress related technical and execution problems in the design of process plant piping are complex and must be addressed properly. There will be some Piping Designers, Stress Engineers and others who read this and say that they agree. Others may say that they do not agree. Others will just not know one way or the other. This discussion, while not covering solutions to every potential problem, is intended only to highlight some of the most common stress related factors and designer training needs
There are five basic factors that influence piping and therefore piping stress in the process plant. There is temperature, pressure, weight, force and vibration. These factors will come in many forms and at different times. Stress problems become all the more complex because two or more of these will exist at the same time in the same piping system. The main objective of the focus when dealing with problems related to piping systems is not normally the pipe itself. In a very high percentage of the time it is not the pipe that is the weakest link. Note this: the pipe is normally stronger and/or less vulnerable to damage than what the pipe is connected to. Pumps are just one examples of equipment to which pipes are routinely connected. Misalignment problems caused by expansion (or contraction) in a poorly designed system can result in major equipment failure. Equipment failures can lead to the potential for fire, plant shutdown and loss of revenue. At this point it should be emphasized that the success (or failure) of the plant’s operation, years down the road can and will depend on what is done up front by all the members of the design team during the design stage. An important point to remember, “While analysis cannot create a good design, it can confirm a good design” (Improved Pump Load Evaluation,” Hydrocarbon Processing, April 1998, By: David W. Diehl, COADE Engineering Software, Inc
Temperatures in piping systems may range from well over 1000o F (537.8 C) on the high side to below -200 o F (-128.8 C) on the low side. Each extreme on the temperature scale and everything in between brings its own problems. There will also be times when both high and low temperatures can occur in the same piping system. An example of this would be in piping that is installed in an arctic environment. The piping is installed outdoors where it is subjected to -100 o F (-73.3 C) over the arctic winter. Six to nine months later it is finally commissioned started up and may operate at five or six hundred degrees.
The problems that temperature causes is expansion (or contraction) in the piping system. Expansion or contraction in a piping system is an absolute. No matter what the designer or the stress engineer does they cannot prevent the action caused by heat or cold. Expansion or contraction in a piping system it self is not so much a problem. As we all know if a bare pipe was just lying on the ground in the middle of a dry barren desert it will absorb a lot of heat from just solar radiation. In the hot sun piece of pipe can reached 150 o F (65.5 C). The pipe will expand and with both ends loose it would not be a problem. However, when you connect the pipe to something, even if only one end is connected you may begin to have expansion related problems. When the pipe is anchored or connected to something at both ends you absolutely will have expansion induced problems. Expansion induced problems in a piping system is stress. There are a number of ways to handle expansion in piping systems. Flexible routing is the first and by far the cheapest and safest method for handling expansion in piping systems. The other way is the use of higher cost and less reliable flexible elements such as expansion joints.
Stress will exist in every piping system. If not identified and the proper action taken, stress will cause failure to equipment or elements in the piping system itself. Stress results in forces at equipment nozzles and at anchor pipe supports. Two piping configurations with the same pipe size, shape, dimensions, temperature and material but with different wall schedules (sch. 40 vs. sch. 160) will not generate the same stress.
Force in piping systems is not independent of the other factors. Primarily, force (as related to piping systems) is the result of expansion (temperature) and/or pressure acting on a piping configuration that is too stiff. This may cause the failure of a pipe support system or it may cause the damage or failure of a piece of equipment. Force, and the expansion that causes it, is best handled by a more flexible routing of the piping. Some people suggest that force can be reduced by the use of expansion joints. However we must remember that for an expansion joint to work there must be an opposite and equal force at both ends to make the element work. This tends to compound the problem rather than lessen it.
Pressure in piping systems also range from the very high to the very low. Piping systems with pressure as high as 35,000 psi in some plants are not unusual. On the other hand piping systems with pressures approaching full vacuum are also not unusual. The pressure (or lack of) in a piping system effects the wall thickness of the pipe. When you increase the wall thickness of the pipe you do two things. First, you increase the weight of the pipe. Second, you increase the stiffness of the pipe thus the stress intensification affecting forces. Increasing the wall thickness of the pipe is the primary method of compensating for increases in pressure. Other ways, depending on many factors include changing to a different material. With low or vacuum systems there are also other ways to prevent the collapse of the pipe wall. Among these the primary method is the addition of stiffening rings. Stiffing rings may be added internally or externally depending on the commodity type and the conditions.
Weight in a piping system is expressed normally as dead load. The weight of a piping system at any given point is made up of many elements. These include the weight of the pipe, the fittings, the valves, any attachments, and the insulation. There is also the test media (e. g. hydrotest water) or the process commodity whichever has the greater specific gravity. Piping systems are heavy, period. Everybody involved in the project needs to understand this and be aware that this weight exists and it needs to be supported. Ninety-nine times out of a hundred this weight will be supported from a structural pipe support (primary pipe support system) of some kind. However there are times when the piping (weight) is supported from a vessel or other type of equipment.
Vibrations will also occur in piping systems and come in two types. There is the basic mechanical vibration caused by the machines that the piping is connected to. Then, there is acoustic (or harmonic) vibration caused by the characteristics of the system itself. Typically the only place severe vibrations will be found is in piping connected to equipment such as positive displacement reciprocating pumps or high pressure multi-stage reciprocating compressors and where there is very high velocity gas flows.
All of the issues listed above that a piping system is exposed to need to be covered in a company specific or company sponsored piping designer, stress-related training program. This piping designer, stress-related training should be done at the department level, early in the designer’s career and prior to the start of the project. Unfortunately however this is not always the case.
By definition, the role of the piping designer is to design the plant piping systems. This means design all of the system. Design all of the system means that the piping designer shall define the proper routing of each and every pipeline required for the project. This includes each and every inline component (pipe, valves, fittings, flanges, instruments, etc.), every online component (anchors, guides, hangers, etc.). It includes the definition of any attached piece of equipment and the definition of every support point. To do this and do it properly the designer must know about piping stress issues and know what to do about them. The designer is responsible for a lot and so they need to know a lot.
Is there any risk involved to the company or the project in not doing this stress related designer training? Yes! First, a designer who is naïve about the cause and effect of stress related problems would not be able to recognize the symptoms and will burn a lot of budget hours and create bad designs. Second, bad designs are subject to the ‘domino effect’ when the need for corrective action is finally identified and taken then other lines get “pushed” and then modifications to them are required. Third, when the bad design does get to the stress engineer for analysis there is the potential for repeated recycle and a serious delay in the design issue schedule.
What does the piping designer need to know? Piping design is more than just knowing how to turn on the computer, how to find the piping menus and the difference between paper space and model space. So, appropriately, what else does the designer need to know about piping design besides how to connect a piece of pipe to a fitting?
Here is a list of some of the most basic of things that a good piping designer should know. Thinking about every one of these items should be as natural as breathing for a good piping designer.
· Allowable pipe spans – All designer need to know and understand the span capabilities of pipe in the different schedules for a wide variety of common piping materials. When a new project introduces a new material with severely reduced span capabilities; supplemental training may be required.
· Expansion of pipe – All designers must understand that they need to treat a piping system as though it is alive. It has a temperature and that temperature causes it to grow and move. That growth and movement must be allowed for and incorporated in the overall design. Not just of that specific line but for all other lines close by. The process of expansion in a pipe or group of pipes will also exert frictional forces or anchor forces on the pipe supports they come in contact with.
· Routing for flexibility – The piping designer must understand how to route pipe for flexibility. Routing for flexibility can normally be achieved in the most natural routing of the pipeline from its origin to its terminus. Routing for flexibility means (a) do not run a pipe in a straight line from origin to terminus and (b) building flexibility into the pipe routing is far cheaper and more reliable than expansion joints.
· Weight and loads (live loads and dead loads) – The piping designer needs to understand the effects of weight and loading. They need to know and understand that everything has a weight. They need to be able recognize when there is going to be a concentrated load. They need to have access to basic weight tables for all the standard pipe schedules, pipe fittings, flanges, valves for steel pipe. They also need to have the weight tables for other materials or a table of correction factors for these other materials vs. carbon steel. They need to be able to recognize when downward expansion in a piping system is present and is adding live loads to a support or equipment nozzle.
· Equipment piping – The piping designer needs to know the right and the wrong way to pipe up (connect pipe to) different kinds of equipment. This includes pumps, compressors, exchangers, filters or any special equipment to be used on a specific project.
· Vessel piping – The piping designer also needs to understand about the connecting, supporting and guiding of piping attached to vessels (horizontal or vertical) and tanks. They need to know that nozzle loading is important and does have limitations.
· Rack piping – The designer needs to understand that there is a logical approach to the placement of piping in (or on) a pipe rack. It does not matter how wide or how high the rack or what kind of plant, the logic still applies. Starting from one or both outside edges the largest and hottest lines are sequenced in such a manner that allows for the nesting of any required expansion loops. The spacing of the lines must also allow for the bowing effect at the loops caused by the expansion.
· Expansion loops – The designer needs to understand and be able to use simple rules and methods for sizing loops in rack piping. This should include the most common sizes, schedules and materials.
· Cold spring/Pre-spring – Designers should understand the basics rules of cold spring and pre-spring. They need to understand what each one is along with when to and when not to use each.
Any piping designer that has this type of training, this type of knowledge and then consistently applies is indeed a piping designer. He or she will also be a more valuable asset to their company and to themselves in the market place. On the other hand anyone who does not know or does not apply the knowledge about these issues while doing piping work is nothing more than a piping drafter or a CAD operator.
Piping shall be carried on adjustable hangers or properly leveled rigid hangers or supports, and suitable springs...
Hangers used for the support of piping, NPS 2½ (NPS 2 in 1935 edn.) and larger, shall be designed to permit adjustment after erection while supporting the load.
While not quite as explicit, the current ASME B31.3
The layout and design of piping and its supporting elements shall be directed toward preventing... piping stresses in excess of those permitted by in this Code;... unintentional disengagement of piping from its supports;... excessive piping sag in piping requiring drainage slope;...
These paragraph excerpts define standard practice in piping design. That is, during operation, it is neither the intention of the code nor standard practice to allow piping to lift-off. Piping is normally designed to be supported in the operating condition. The means to achieve this is through proper adjustment of the supports during operation. This is important in piping because unadjusted supports will permit the pipe to sag and create locations in steam or condensable gas piping where condensates can collect or concentrate. And it is especially important for piping operating above 800 degF, where unadjusted supports will allow the pipe to permanently deform (creep) over time.
Small gaps are inevitable in actual construction because of fabrication and installation tolerances and would normally be closed by support adjustments. But, so long as the pipe is prevented from significant lateral movement, small gaps below pipe during operation (¼ inch and less in moderate size piping) may be tolerable because the weight analysis is a very simplified and conservative method that the ASME B31 codes use to guard against collapse. Stresses caused by takeup of a small gap below the pipe could even be considered expansion or building settlement type stresses and thus would not need to be considered in the weight analysis. Weight analysis with the intent of designing pipe normally considers all the weight supports perform their intended function. Any significant gaps determined by analysis could either indicate that a support is not required, or that adjacent supports need to be modified, or that an alternate means of support is needed, e.g., a variable or constant spring should be used.
However, if the purpose of an analysis is not to design a new or revamp an old piping system, but to evaluate an as-built and maintained piping system, small gaps may have more significance in as much as they would indicate that the pipe support system may not be acting as designed and maintained. A lack of or improper adjustment of the supports in the operating condition may cause lift-off at rigid supports. Improperly designed or adjusted or maintained or degraded variable or constant spring supports may cause lift-off, too.
The interpretation of the results of the analysis of as-built piping systems need not necessarily conform to the rules of the ASME B31 codes. Remember, the rules in the B31 codes are required for new construction, not the evaluation of existing piping. It is understood that a greater factor of safety is required for the design process because the pipe and its components are not yet available to be measured and materials confirmed, as well as the knowledge of how the piping is to be actually used. The interpretation of the analysis results of as-built piping may be able to take advantage of what the actual piping dimensions and materials are and how the piping has been operated. Competent engineering judgement based on knowledge of the intent of the respective ASME B31 codes must then become part of the evaluation process.
For the reasons noted, it is important to distinguish between the design and analysis of piping. If designing, certain assumptions are normally made with regard to whether the piping is supported in the operating condition. Such assumptions might include tolerating a small gap at a given support but realizing that the installation of the given support will require adjustment. Alternately, a larger gap at the given support may require support relocation to be effective or the selection of a different type of support, most typically a constant or variable spring. If merely analyzing existing piping, no assumptions need be made regarding supports acting and analysis gaps may become important considerations. That said, however, the analyst must realize that the piping analysis model is a very idealized estimation of the as-built piping and for the analysis results to be meaningful, the analyst needs to consider how well the results correlate with the actual performance of the in-situ piping.