Snubbers also called shock arrestors are restraining devices used to control the movement of pipe and equipment during abnormal dynamic conditions such as seismic disturbances, turbine trips, relief valve discharges and water hammers. The design of a Snubber allows slow movement due to thermal expansion of a component during normal operation conditions, but restrains the component from sudden movement in dynamic conditions.
Types of Snubbers
There are two types of snubbers:
- Hydraulic Snubbers
- Mechanical Snubbers
Snubbers are available in various types of designs. However, the function of any design is to protect the downstream structure from abnormal shocks. Snubbers are designed for various load ratings depending upon the magnitude of seismic activities and the criticality of fluid induced shocks.
Hydraulic Snubbers
Courtesy: Anvil International
Most hydraulic snubbers have a piston which is relatively unconstrained in motion at low displacement rates. At high displacement rates the piston "locks up" and acts as a rigid restraint.
The design usually comprises of either two concentric cylinders or two parallel cylinders with their respective moving pistons. Both the main cylinder and the compensating cylinders are filled with fluid. The main and the compensating cylinders are connected to velocity limiting valves and a main piston which works in either a push or pull mode. Under normal operating conditions, the valves remain open and allow the piston to move freely under thermal expansion or contraction of the supported component. When the threshold velocity is reached, the valve activates by closing the flow through the valve and the flow through the system stops momentarily. At this point, the main piston that takes the shock load, stops moving and the load is transmitted to the supporting structure, thereby avoiding any damage to the structure downstream of the snubber. As soon as the shock wave passes, the snubber resumes normal operation.
Mechanical Snubbers
A mechanical snubber uses gears and springs to produce the restraint force when activated. Mechanical Snubber operates on the principle of limiting the acceleration of any pipe movement to a threshold level of .02 g’s. This is the maximum acceleration that the snubber will permit to be imposed on the piping system. At the same time, thermal expansion, being a gradual movement, is not restricted. Following are the types of Mechanical Snubbers manufactured by Piping Technology and Products.
MSA Mechanical Snubber
With this type of snubber, the linear movement of the rod connected to the piping component is converted to rotary motion. When the centrifugal acceleration exceeds a certain threshold acceleration (typically 0.02g), a centrifugal type clutch flares out and locks at the peripheral slot of the cylinder and restricts linear motion.
Anchor-Darling Mechanical Snubber
With this type of snubber, the linear motion of the central rod that is connected to the structural component is converted to oscillatory motion via a verge mechanism. This oscillatory motion is in turn converted to rotary motion via a set of gears. As the linear velocity increases, the inertia force generated in the oscillating verge and the train of rotating gears increases. The extent of this increase depends upon the amount of inertial mass and gear train’s angular velocities thereby limiting the velocity of the piping components within the safe limit.
Hydraulic vs Mechanical Snubbers
Mechanical Snubbers were thought to be a better alternative to Hydraulic Snubbers because they eliminate the issue of hydraulic leaks. However, when a Mechanical Snubber fails, it has a broken part which is not visible and will get unnoticed. The failed part can fully lock a Snubber making the support rigid and restrain even thermal expansion. This is one of the major disadvantages of Mechanical Snubber and hence Hydraulic Snubbers is the preferred choice.