In our three previous articles, we looked at pressure design of FRP piping components including pipe, fittings and joints, and flanges. This aspect of piping design is sometimes referred to as “hoop” design of the piping given that the only hoop stress that piping components are normally exposed to is pressure. But who normally carries out that pressure design of components, and what are the next steps in the piping system design process? The answers to these questions are the subject of this article.
The piping system Designer is responsible for ensuring that the selected piping components are suitable for his/her piping system’s design conditions. In the case of metallic piping, the Designer will often carry out pressure design calculations to demonstrate the adequacy of the selected components, particularly non-standard components. But given the specialized nature of FRP piping, the task of pressure design of the piping components is typically left to the manufacturer. For example, RPS has developed a number of piping systems that are suitable for pressures up to 150 psi (10 Bar). These include HPPE P-150, HPPE H-150-200, and HPPE MAXAR™. RPS ensures that all components within those product lines are properly designed for the stated design conditions of pressure and temperature. If none of the RPS standard product lines are suitable for some reason for a specific application, RPS engineers will custom-design piping components that will meet the needs of that application.
Once the piping components have been properly designed for pressure, it is necessary to ensure that the piping system can safely carry the other loads to which it will be exposed. This aspect of piping design is sometimes referred to as “axial” design as the loads of interest usually give rise to just axial stresses.
This step in the piping design process is usually referred to as “pipe stress analysis”. In addition to pressure, the loads that are typically considered in the pipe stress analysis are the operating loads of weight and temperature, and occasional loads such as wind and seismic. The primary purpose of the pipe stress analysis is to ensure that the applied stresses do not exceed the piping allowable stresses. The pipe stress analysis can serve other functions as well including determination of loads on supports, determination of loads on connected equipment, and assessment of piping movements to help avoid potential clashes with other equipment and structures.
There are several levels of analysis that are commonly undertaken for FRP piping systems. These include using rules of thumb, carrying out hand calculations, and comprehensive analyses using purpose-built computer software. The choice of which approach to take depends on several factors including the level of experience of the Designer, the severity of the design conditions of the system, and the criticality of the line.
It is very common to utilize the kind of rule of thumb information as provided in design guides produced by the piping manufacturer for designing piping systems such as vents and drains and other low pressure/temperature systems. For example, documents such as RPS’s Design Manual provide recommended maximum support spans and minimum offset legs to help the designer lay out standard piping systems. If the piping designer takes these recommendations into account as the piping system is laid out, the designer can be assured that the applied stresses will be kept within safe levels.
If there are specific portions of a piping system for which the Designer requires a little more accuracy (e.g. stress at a specific location), or a little more information (e.g. loads on a particular support), he/she can carry out hand calculations using formulas from widely recognized resources such as Roark’s Formulas for Stress & Strain, Kellogg’s Design of Piping Systems, etc.
With the ease of use of today’s pipe stress analysis software, more and more FRP piping systems are being modelled and analyzed using comprehensive analyses. And this is certainly recommended for all systems for which the design pressures and temperatures are high, and also for systems for which the criticality is high, e.g. those systems that contain hazardous or toxic fluids, or those for which unplanned outages must be avoided. Pipe Stress Analysis programs such as CAESAR II are easy to use and provide a very powerful tool for designing and analyzing any piping system, FRP included.
Regardless of the level of analysis of the piping system, the primary goal of the task is to ensure the piping has enough supports to safely carry the weight loads and other “primary” loads, while at the same time providing sufficient flexibility to ensure the “displacement” loads do not become unmanageable. Primary loads are those such as pressure, weight, wind, and seismic loads, and they give rise to “primary stresses” in the piping. Primary stresses are those that are required within the piping to balance the applied primary loads. They are characterized by being able to cause ultimate failure of the piping system should they become too large.
Displacement loads are those caused by constraint of displacement of the piping such as restriction of thermal expansion or by settlement of a support or connected equipment. Displacement loads give rise to displacement stresses. Displacement stresses typically result in excessive distortion should they become too large, but they don’t usually result in ultimate failure. In fact, in ductile piping systems, displacement stresses are permitted to exceed the yield strength of the material, and they are then relieved by local yielding. Due to the fact that the displacement associated with these loads is typically limited to the extent that the piping tries to expand or contract, these stresses are often referred to as “self-limiting”.
Perhaps the most important difference between analysis of metallic piping systems and FRP piping systems is in the way that displacement stresses are handled. In metallic piping systems, these stresses are permitted to be much higher than primary stresses. This is in recognition of the ability of the ductile material to locally yield without inducing ultimate failure. The ASME B31.1 and B31.3 piping codes treat displacement stresses as “Expansion” stresses, and they permit the Expansion stresses (or more correctly, the expansion stress range) to be as much as 2.5 times the allowable stress for primary loads. The expansion stress range is the stress induced in the piping during a change in temperature from the highest temperature to which the piping will be exposed to the lowest temperature.
This same treatment for expansion stresses is not appropriate for FRP piping. FRP does not display significant ductile behavior, so expansion stresses should be treated essentially the same as primary stresses. This fact has given rise to the practice of analyzing FRP piping systems for “Operating” loads, i.e. the combined effects of pressure, weight, and thermal loads, and comparing the resulting stresses to the allowable stress for sustained loads. In this case, the thermal stresses are calculated for the change in temperature from the minimum installation temperature to the highest temperature the system will be exposed to, and in some cases, from the maximum installation temperature to the lowest temperature.
ASME NM.21 permits slightly higher stresses in load cases that include displacement loads, but this is only 10%, much less than is permitted for metallic piping systems.
In our next article we’ll take a look at what information is required to conduct a pipe stress analysis, and in particular, where the allowable stresses for FRP piping come from.
1 ASME NM.2 “Glass-Fiber-Reinforced Thermosetting-Resin Piping Systems
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