Properly designed piping systems keep stresses in the pipe wall itself within established allowable limits, and ensure that end point reaction loads do not overload connected equipment nozzles. While avoiding excessive equipment nozzle loads is important in all cases, this is especially true for piping systems connected to pumps, compressors and steam turbines. Excessive piping loads in these cases can cause high machine vibration, shaft misalignment, and coupling failures. These problems result in increased maintenance costs, and can lead to equipment and plant shutdowns. This series of articles will discuss suggestions for the design and installation of such machinery piping systems which will reduce the likelihood of nozzle overloads.
Before we can discuss specific means for keeping nozzle loads within acceptable limits, those limits must first be established. In addition, achieving an acceptable machinery piping system design and installation requires a cooperative effort among the plant owner, piping system designer, and equipment manufacturer. Designing such a system in a vacuum without an adequate degree of communication among these three key players is the surest way to have problems.
The ASME B31 standards require consideration of the effects that piping system displacements have on connected equipment. However, these standards give wide latitude regarding the specific analysis to be done and no guidance at all regarding acceptable nozzle loads.
API Standard 610, “Piping Load Limitations for Centrifugal Pumps,” provides an allowable load basis for pumps purchased to this standard, and covers nozzle sizes 2" through 16" inclusively. For nozzle sizes outside this range, the pump allowable load basis must be mutually agreed to among the owner, pump manufacturer, and piping designer prior to the start of engineering. NEMA SM-23 for steam turbines and API 617 for centrifugal compressors contain allowable load limitations for equipment purchased to those standards. Individual owner companies often have their own nozzle load criteria which supplement or replace those contained in these industry standards.
At this point, suffice to say that a consistent rotating equipment allowable nozzle load basis must be decided on before engineering work is begun in order for this to proceed in a cost-effective manner. This basis must be understood and agreed to by all concerned parties, and then followed during piping system design.