Three way manifold valve operation

Three way manifold valve operation

  • The following photograph shows a three-valve manifold bolted to a Honeywell model ST3000 differential pressure transmitter.

  • A bleed valve fitting may be seen inserted into the upper port on the nearest diaphragm capsule flange.

  • In normal operation, the two block valves are left open to allow process fluid pressure to reach the transmitter.

  • The equalizing valve is left tightly shut so no fluid can pass between the “high” and “low” pressure sides.

  • To isolate the transmitter from the process for maintenance, one must close the block valves and open the equalizing valve.

  • The best sequence to follow is to first close the high-pressure block valve, then open the equalizing valve, then close the low-pressure block valve.

  • This sequence ensures the transmitter cannot be exposed to a high differential pressure during the isolation procedure, and that the trapped fluid pressure inside the transmitter will be as low as possible prior to “venting” to atmosphere.

  • Finally, the “bleed” valve is opened at the very last step to relieve pent-up fluid pressure within the manifold and transmitter chambers.


Three valve manifold

A three-valve manifold is a device that is used to ensure that the capsule will not be over-ranged. It also allows isolation of the transmitter from the process loop.

It consists of two block valves-high pressure and low-pressure block valve and an equalizing valve.

During normal operation, the equalizing valve is closed and the two block valves are open.

When the transmitter is put into or removed from service, the valves must be operated in such a manner that very high pressure is never applied to only one side of the DP capsule.

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