From Control Valves

Hydraulic Snubbers

Snubbers are used where dynamic instability causes the valve stem to jump or oscillate. Installation of a hydraulic snubber between the valve yoke and the diaphragm casing is an effective method of suppressing the vertical oscillation.

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Hydraulic snubbers are used on severe control valve applications to alleviate problems in a valve plug by damping vertical instability. Very few applications require snubbers. Most cases of dynamic instability are handled by using a large actuator, a higher loading pressure with a heavier spring or by reversing the valve in the line so that flow impinges on the bowl of the body.

The following categories of valve may require a hydraulic snubber:

  1. Flow-to-close single seated valves except angle valves or valves under 1 inch size.
  2. Double seated valves in gas or vapor service, if the inlet pressure rating multiplied by the valve size is greater than 1800. A 4" valve with 600 psig inlet pressure is a good example.
  3. Double seated valves in liquid service where the inlet velocity exceeds 15 ft/sec.

The snubber consists of a hydraulic cylinder attached between the valve bonnet and the actuator stem, and/or is added between the diaphragm case and spring housing. The extended actuator rod, passing through the cylinder and sealed at both penetrations, has a piston mounted on it. It is filled with oil so that a wide clearance piston carried by the stem will damp the valve movement. As the piston moves, it displaces oil from one side of the piston, through adjustable snubbed orifices back to the other side. The snubber is self-contained and requires no external power.

Another style of snubber uses needle valves between the chambers, above and below the hydraulic piston. The closure of these needle valves is adjustable to obtain the desired damping action.

© 1998 Instrument Society of America

Products & Services
Hydraulic Valves
Hydraulic valves contain and transfer the flow and pressure of hydraulic fluid in hydraulic power systems. They range from simple shutoff valves to precision control valves.
Valve Actuators
Valve actuators mount on valves and, in response to a signal, move a valve to a desired position using an outside power source.
Manual Valve Actuators
Manual valve actuators do not require an outside power source. They use a handwheel or lever to drive a series of gears whose ratio results in a higher output torque compared to the input (manual) torque.
Pneumatic Valve Actuators
Pneumatic valve actuators adjust valve position by converting air pressure into linear or rotary motion.

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