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DP Transmitter valve manifolds

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An important accessory to the DP transmitter is the valve manifold. Most DP transmitters come with either a 3-valve manifold or a 5-valve manifold or a single block and bleed valve manifold depending on the application. The valve manifold is used:
  • To isolate the DP transmitter from the process for maintenance and calibration.
  • To ensure that the DP transmitter is not over-ranged
3-Valve Manifold
This device incorporates three manual valves to isolate and equalize pressure from the process to the transmitter, for maintenance and calibration purposes. It consists of two block valves - high pressure and low pressure block valve - and an equalizing valve. The schematic below shows the configuration of a 3-valve manifold:

DPT in the schematic above is the DP transmitter. During normal operation
, the equalizing valve is closed and the two block valves(HP and LP) 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 transmitter capsule.

Operational Sequence for Valving DP Transmitter with a 3-Valve Manifold into Service:
  1. Check all valves HP,LP and the equalizing valve are closed.
  2. Open the equalizing valve. This ensures that the same pressure will be applied to both sides of the transmitter, i.e. zero differential pressure
  3. Open the High Pressure block valve slowly, check for leakage from both the high pressure and low-pressure side of the transmitter.
  4. Close the equalizing valve. This locks the pressure on both sides of the transmitter.
  5. 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. After removing the transmitter from service, it may be necessary to bleed any trapped air from the DP transmitter capsule housing.

Removing The 3-Valve Manifold DP Transmitter from Service:
The DP transmitter can be removed from service using the steps outlined below:
  1. Close the low-pressure block valve.
  2. Open the equalizing valve.
  3. Close the high-pressure block valve.
The transmitter is now out of service. The transmitter capsule housing still contains process pressure; this will require bleeding.

5-Valve Manifold
The 5-valve transmitter manifold is shown in the schematic below:
During normal operation, the HP and LP valves are open while the equalizing valves and the bleed valve are closed. However, It is critically important that the equalizing valve(s) never be open while both block valves are open! If the process fluid is dangerously hot or radioactive, a combination of open equalizing and block valves will let that dangerous fluid reach the transmitter and manifold, possibly causing damage or creating a personal hazard.

Operational Sequence for Valving DP Transmitter with a 5-Valve Manifold into Service:
  1. Check all valves HP,LP, the bleed valve and the equalizing valves are closed.
  2. Open the equalizing valves. This ensures that the same pressure will be applied to both sides of the transmitter, i.e. zero differential pressure.
  3. Open the High Pressure block valve slowly, check for leakage from both the high pressure and low-pressure side of the transmitter.
  4. Close the equalizing valves. This locks the pressure on both sides of the transmitter.
  5. 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.

Removing the 5-Valve Manifold DP Transmitter from Service
The DP transmitter with a 5-Valve manifold can easily be removed from service using the steps outlined below:
  1. Close the HP and LP block valves.
  2. Open the equalizing valves.
  3. Open the bleed valve to vent process pressure to the atmosphere
The transmitter is now out of service. As is always the case, the transmitter capsule housing still contains process pressure; this will require bleeding.
Note that the 3-valve and 5-valve DP transmitter manifolds are usually manufactured as monolithic or single units. The schematics above are just for illustration and explanations.

Single block and bleed valve manifold:
Transmitter valve manifolds also come in single block-and-bleed configurations. In this scenario, the transmitter is being used for gauge pressure applications. Here, the “low” pressure port of the transmitter is vented to atmosphere, with only the “high” pressure port connected to the process medium through impulse lines.
The schematic diagram below shows the single block and bleed valve manifold.

Removing the DP transmitter (DPT in diagram above) is very easy. Simply close the block valve and open the bleed valve to vent process pressure to the atmosphere. The DP transmitter is already out of service.

Putting the transmitter back into operation is also easy. Ensure that both bleed and block valves are initially closed. Then open the block valve and the transmitter is back to service.