The simple and most commonly used WRC-107 (Welding Research Bulletin #107) Bijlaard methodology for local stress evaluation addresses cylindrical shells and pipes with solid circular, rectangular, and square attachments only. Hollow circular, square, or rectangular tubular shaped attachments on cylindrical shells, though commonly used, are not addressed in WRC-107. ASME Code Case N-392 addresses hollow circular attachments on pipes but is known to be conservative. This paper studies commonly encountered sizes of hollow circular, hollow square, and hollow rectangular attachments of various wall thicknesses on piping utilizing rigorous finite element analysis (FEA) method to obtain the local stresses at the pipe/attachment interface due to mechanical loads. A total of fifty (50) finite element models were analyzed to study the most frequently used configurations. The impact of attachment wall thickness including solid attachment will be addressed. A comparison of finite element results with WRC-107 solid attachment results, when applicable, will be made. Recommendations and guidelines are provided based on the results of the FEA study. The objective is to reduce conservatism, and hence the associated cost in piping and pipe support design by optimizing the round attachment’s wall thickness.
Piping systems are normally used to transport air, gases, steam, water, and other fluids. Piping systems in power plants, petro-chemical, and other industrial facilities that carry non-hazardous fluids like air, water, etc. with no significant pressures at moderate temperatures can be considered as non-critical. Codes such as ASME B31.1 provide suggested dead weight spans for the placement of supports. In this paper, relaxed hanger spans are computed for non-safety related piping systems taking in to consideration the maximum bending stress, maximum deflection or pipe sag, and bearing stresses due to dead weight for pipe sizes ranging from 1” to 42” diameter. For situations in which large diameter pipes are directly sitting on steel with a line contact, bearing stresses are computed using finite element analysis as well as simple formulas. A comparison of relaxed hanger spans with B31.1 suggested spans is presented. Significant benefits in lowering the cost due to reduced number of dead weight supports, and the associated savings due to reduction in materials, fabrication, and installation can be derived by using the relaxed spans. These relaxed hanger spans were utilized over the past nine years on a variety of non-safety related piping systems. Experience with more than a dozen power projects of Pulverized coal (PC), Cogen, Simple cycle, and Combined cycle types indicates that the piping systems, which used relaxed hanger spans, are operating normally and satisfactorily without any problems.
High temperature steam lines in power plant piping systems are often supported by the use of pipe support stanchions welded to the steam pipe. The end of the pipe stanchion has a steel plate welded to it, which typically slides on rack steel. For vertical and guide supports, there could be considerable thermal movement in the lateral unrestrained directions, and could result in significant frictional loads. The associated frictional loads are given due consideration in piping local stress evaluations as well as in the design of pipe support structures. For some situations, it often becomes necessary to utilize a teflon-fluorogold type surface at the stanchion end plate in order to reduce the coefficient of friction and hence the frictional loads. The effectiveness of the teflon-fluorogold surface is dependent on the prevailing temperature at that surface. In situations where the stanchions on very high temperature steam lines arc relatively short, the temperature at the teflon surface of the stanchion plate could be high due to heat transfer from the steam line into the stanchion. This high temperature at the bottom surface of the stanchion plate may interfere with sliding and may eventually lead to unanticipated problems such as sticking, increase in the coefficient of friction, or unpredictable frictional behavior. In this paper, finite element analysis approach is utilized to perform heat transfer analysis and to obtain steady state temperature distribution due to decay or attenuation from the steam line surface along the stanchion. The temperature prevailing at the bottom plate surface of the stanchion is also evaluated and guidelines are provided for practical application of the results.
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