What is three valve manifold? | how a 3 valve manifold works?

Field Instrumentation Valves
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3 valve manifold

Differential pressure transmitters are used in level measurement and flow measurement. For ensuring the protection of pressure transmitter or for isolating the transmitter from the process loop, manifolds are used.

Three valve manifolds system used for differential pressure transmitter is shown here

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A three-valve manifold system consists of an equalising valve and two blocking valves. One blocking valve at the high-pressure side other at the low-pressure side.

The two block valves are opened and the equalizing valve is closed during normal operation.

When the transmitter is removed from the system, the manifold must be operated such that the high pressure is never applied to the single side of the transmitter.

Operating sequence for the Three-valve manifold:

  • Initially close all the three valves

  • Open the equalizing valve alone, Now the differential pressure should be zero, check it on the transmitter readings. This means that same pressure is at the LP & HP side.

  • Slowly open the high-pressure block valve and after then close the equalizing valve. Opening the HP block provide the leakage check on either side. Only after doing so, close the equalizing valve, lock the pressure on both sides.

  • Now allow the low-pressure flow into the DP by opening the LP block valve. The transmitter will start showing the differential pressure in the readings.

  • Now the transmitter is ready for service.

For removing the transmitter close the LP valve, then equalizing valve, then HP valve in sequence

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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

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To return the transmitter to live service, simply reverse these steps: close the bleed valve, open the low-pressure block valve, close the equalizing valve, and finally open the high-pressure block valve.