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 Houston, TX). On the other hand, proper analysis will identify bad design and potential problems in a piping system design.
Stress Related Design Factors
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.
Author:James O. Pennock is a former Piper with more than 45 years experience covering process plant engineering, design, training, pipe fabrication and construction. He is now retired and lives in Florida, USA.
Original from http://www.pipingdesigners.com/
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