Three Way Valve Manifold

Valve manifold is used in calibration of pressure or flow instruments. It is commonly used in conjunction with DP transmitter. As the process fluids may be toxic or corrosive, it is necessary to prevent its leakage during calibration. A three-way valve manifold is as shown below.

Figure 1: Three-way Valve manifold

This device incorporates manual valves to isolate and equalize pressure from the process to the transmitter, for maintenance and calibration purposes. A fourth valve called a “bleed” valve used to vent trapped fluid pressure to atmosphere.

Figure 2: Normal operation mode

Figure 3: Maintenance mode

In normal operation, the two block valves are left open to allow process fluid pressure to reach the instrument. 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, the block valves must be closed and the equalizing valve must be open. The recommended 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