A three-way manifold valve 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.
Figure shows a three valve manifold arrangement
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
Operational Sequences of Three way Manifold Valving Transmitter Into Service
- Check all valves closed.
- Open the equalizing valve .this ensures that the same pressure will be applied to both sides of the transmitter, i.e.,zero differential pressure.
- Open the High Pressure block valve slowly, check for leakage from both the high pressure and low-pressure side of the transmitter.
- Close the equalizing valve .this locks the pressure on both sides of the transmitter.
- Open the low-pressure block valve to apply process pressure to the low-pressure side of the transmitter and establish the working differential pressure.
- The transmitter is now in service.
Note it may be necessary to bleed any trapped air from the capsule housing.
Removing Transmitter from Service
Reversal of the above steps allows the DP transmitter to be removed from service.
- Close the low-pressure block valve.
- Open the equalizing valve.
- Close the high-pressure block valve.
The transmitter is now out of service.