Instrument Valves: The Difference Between a Rotating One Piece Stem and a Non-Rotating Two Piece Stem

Process instrument valves are used with devices such as pressure measurement transmitters, gauges, and switches to isolate the device from the process media for replacement, calibration, and maintenance. Instrument valves are combined together and assembled into block and bleed manifolds combing multiple valves into a single body configuration, with each valve having a separate external connection and a common internal connection.

A critical component to the instrument valve is the valve stem. The stem positions the stem point into the orifice and against the valve seat to control flow and to isolate the process.


A rotating one piece valve stem rotates and translates along its axis as it is being driven axially into the orifice. The rotational motion of a one piece stem can produce friction as the stem interacts with the seating area, which could result in galling at the seating surface. This is less of an issue in a soft seat valve or if the media provides lubrication.

A non-rotating two piece stem tip rotates independently from the stem and stops rotating as it is driven axially against the orifice sealing surface while the stem continues its rotational and axial movement.

For more information about process instrumentation valves and valve manifolds, contact Advance Instruments. Call them at (888) 388-6446 or visit their web site at