Date of Award
Master of Science in Mechanical Engineering and Applied Mechanics
Mechanical, Industrial and Systems Engineering
Hydraulic snubbers are either acceleration or velocity limiting seismic restraints designed to restrict movement of piping or equipment during dynamic events or operational transients. In a piping analysis, snubbers are modeled as linear elastic spring elements governed by Hooke’s law, F = kx. Snubbers are widely used in nuclear power plants, and as such their qualification testing to verify the spring constant k is governed by American Society of Mechanical Engineers (ASME) codes. ASME mandates experimental determination of spring constant k where practical, or a combination of testing and analysis when not practical. This qualification test includes testing at full load under a sinusoidal forcing function at a maximum frequency of 33 Hz.
There remains a practical upper limit to dynamic testing that is imposed by the availability of test equipment. This upper limit is governed by the size of the hydraulic pumps and servo actuators supplying fluid to the actuating test cylinder. At current writing, this limit is approximately 200 kips @ 33 Hz. New reactor designs have applications for snubbers with load capacities up to 1,900 kips. Functional testing can be conducted on these large units to verify the lockup and bleed parameters are correct, as well as the load carrying capacity. However, the dynamic spring rate of the snubber will not be experimentally verified at full rated load.
This study developed an FEA model of a hydraulic snubber that was compared to existing ASME qualification test data performed by Anvil Engineered Pipe Supports (EPS). If an accurate model can be developed for smaller snubber sizes, it can be used to determine the spring rate of units that exceed the capacity of existing test equipment.
The experimental test data shows a spring constant that decreases at an approximately linear rate between 3 Hz and 33 Hz, with a reduction at higher frequencies of approximately 30%. The simulation models linear elastic behavior, and shows up to a 6% reduction between 3 Hz and 33 Hz. Part of the discrepancy can be explained by the load controlled nature of the test negating inertial effects, and increased deflections due to lost motion caused by assembly clearances and manufacturing tolerances. Further testing should be conducted, measuring load through pressure transducers in the cylinder fill ports to overcome these test setup limitations. This testing should be done on at reduced load so that a model can be developed that agrees with both the reduced load testing and rated/emergency load testing.
Palmer, Matt, "FINITE ELEMENT ANALYSIS OF A HYDRAULIC SNUBBER WITH RESPECT TO HISTORICAL TEST DATA AND AMERICAN SOCIETY OF MECHANICAL ENGINEERS REQUIREMENTS" (2015). Open Access Master's Theses. Paper 471.