By Dr. Hyder Husain Ph.D.
March 9, 2011
Snubbers are used as restraining devices to control abnormal movement of pipes and equipment due to dynamic events such as earthquakes, turbine trips, safety/relief valve discharge, rapid valve closure, or rupture of pipes. The design of a snubber allows free thermal movement of a component during normal operating conditions, but restrains the component in abnormal conditions.
The snubber restraint forces, as described above, can be generated using either mechanical or hydraulic methods.
Hydraulic snubbers have a piston which is relatively unconstrained during relatively low velocities as would be seen in normal thermal expansion/contraction cycles or slow oscillation of pipes. At high displacement rates, the piston “locks up” and the snubber acts as a rigid restraint.
Hydraulic Snubber with an Overall Stroke of 6″
Currently available hydraulic snubbers use a high viscosity (~ 1000 cStokes) fluid. Typically, the motion of the snubber-fluid shuts off the valve when the piston velocity reaches 8”/min or more. As a result the snubber acts as a rigid component and transfers the shock load to the rigid foundation, saving the upstream components.
Similarly to hydraulic snubbers, mechanical snubbers use a telescoping cylinder to permit free movement of the pipe under normal operating conditions. When the threshold acceleration of 0.2 g’s is exceeded, a internal mechanism of the snubber activates, thereby locking the telescoping cylinder and subsequently producing our restraint force.
Hydraulic vs. Mechanical Snubbers
Because hydraulic snubbers have fewer internal components, they are preferred in outdoor applications or where a corrosive environment is present. Additionally, hydraulic snubbers can be easily designed to accommodate a wide range of pipe displacement.
Mechanical snubbers are optimum solutions for piping used in high radiation areas such as those seen in nuclear power plants because they do not utilize hydraulic fluid that may become degraded in radioactive environments.
Furthermore, mechanical snubbers require less maintenance overall and are considered “solid state” support components in the piping system. Hydraulic snubbers conversely require routine inspection to detect leaking seals or loss of hydraulic fluid.
In regards to dynamic response, mechanical snubbers react consistently regardless of their position during either compression or extension modes. However, hydraulic snubbers may show some variation in their dynamic response depending upon the piston location. Therefore, when choosing a hydraulic snubber for a piping system, one must carefully determine the amount of stroke needed to adequately position the snubber piston orientation for optimum functionality.