1. A device which is used to damp the motion of the valve stem. This is usually accomplished by an oil filled cylinder/piston assembly. The valve stem is attached to the piston and the flow of hydraulic fluid from one side of the piston to the other is restricted. 2. A mechanical or hydraulic device for restraining motion. 3. A device installed between an instrument and the process used to protect the instrument from rapid pressure fluctuations.

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