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Problem with piping "Liftoff"
Contemporary commercial piping analysis programs deal differently with the problem of apparent lift-off of an operating pipe at a rod hanger or a one-way vertical support, such as a pipe on a support rack. A few programs provide error messages; others show a vertical movement with a possible increase in sustained (weight) stress (see NOTE below for CAEPIPE). A proper understanding of the standard piping design practice is the key to correct interpretation of these results from different programs. Such standard piping design practice was generally understood when the sustained and flexibility analysis rules were introduced in the 1955 Edition of the ASME B31 Code for Pressure Piping.
The problem with lift-off is compounded by the intention of the piping analysis being performed - whether the intent is to design new or revamp existing piping or the intent is to analyze as-built. The intention of the various sections of ASME B31 Code (B31.1, B31.3, etc.) is to provide guidance for new construction. Note, since the publication of the 1935 Edition of ASME B31.1 (which included the predecessor of B31.3 as a chapter, Paras. 101.6 and 121.4 and their predecessor paras.) state:
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 Para. 321.1.1 states:
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.
NOTE: In case of lift-off, CAEPIPE will show a gap and possibly increased sustained stresses. The user must interpret the gaps according to whether the user is designing new or revamping existing piping or is analyzing an existing condition.
Author: Mr. Ron Haupt, P. E., of Pressure Piping Engineering (www.ppea.net) is a member of several piping code committees (B31, B31.1, B31.3, BPTCS, and others). He consults with us in the capacity of Nuclear QA Manager.