Whether a differential pressure (DP) transmitter delivers an accurate flow, level, or differential pressure reading depends half on the transmitter itself and half on how the three-valve manifold is operated. The manifold looks like nothing more than a metal block with three handles, but turning those handles in the wrong order can at best distort your data and at worst destroy the transmitter's diaphragm capsule. This article explains the correct commissioning and shutdown procedures, and the principles behind them.
The three-valve manifold is mounted between the high- and low-pressure ports of the DP transmitter and contains three manual valves:
Working together, their core task is to keep the two sides of the diaphragm capsule balanced and controllable at all times, so it never experiences a one-sided pressure shock.
After a DP transmitter is installed, with impulse lines connected but no process fluid admitted yet, all three valves are normally closed. Commissioning follows this order:
Commissioning rule: open HP → close equalizer → open LP. The order must never be reversed — and never close the equalizing valve before opening a single-side valve.
Shutdown rule: close LP → open equalizer → close HP. The timing of the equalizing valve is critical — it must be opened before the HP valve is closed.
| Mistake | Consequence |
|---|---|
| Closing the equalizer before opening the HP or LP valve | One-sided pressure on the capsule; output spikes, zero drift, or capsule damage |
| Closing the HP valve on shutdown without opening the equalizer first | LP side stays connected to the process; capsule suffers reverse one-sided pressure |
| Commissioning before impulse lines fill with condensate (steam service) | Steam reaches the transmitter directly; readings fluctuate and the capsule may overheat |
| Opening or closing valves too quickly | Pressure surge saturates the output; water-hammer shock on the capsule |
These errors rarely cause immediate failure. A single one-sided pressure event may not damage the capsule, but after several times the elastic element's stress characteristics change, showing up as zero drift and worse linearity — eventually misdiagnosed as transmitter aging and replaced, when the real problem was the operation.
The steps are simple; doing them by the book every time is the hard part. Remember: commission with "open HP, close equalizer, open LP," and shut down with "close LP, open equalizer, close HP." For steam service, add one rule: if the condensate is not full, do not touch the manifold.
Whether a differential pressure (DP) transmitter delivers an accurate flow, level, or differential pressure reading depends half on the transmitter itself and half on how the three-valve manifold is operated. The manifold looks like nothing more than a metal block with three handles, but turning those handles in the wrong order can at best distort your data and at worst destroy the transmitter's diaphragm capsule. This article explains the correct commissioning and shutdown procedures, and the principles behind them.
The three-valve manifold is mounted between the high- and low-pressure ports of the DP transmitter and contains three manual valves:
Working together, their core task is to keep the two sides of the diaphragm capsule balanced and controllable at all times, so it never experiences a one-sided pressure shock.
After a DP transmitter is installed, with impulse lines connected but no process fluid admitted yet, all three valves are normally closed. Commissioning follows this order:
Commissioning rule: open HP → close equalizer → open LP. The order must never be reversed — and never close the equalizing valve before opening a single-side valve.
Shutdown rule: close LP → open equalizer → close HP. The timing of the equalizing valve is critical — it must be opened before the HP valve is closed.
| Mistake | Consequence |
|---|---|
| Closing the equalizer before opening the HP or LP valve | One-sided pressure on the capsule; output spikes, zero drift, or capsule damage |
| Closing the HP valve on shutdown without opening the equalizer first | LP side stays connected to the process; capsule suffers reverse one-sided pressure |
| Commissioning before impulse lines fill with condensate (steam service) | Steam reaches the transmitter directly; readings fluctuate and the capsule may overheat |
| Opening or closing valves too quickly | Pressure surge saturates the output; water-hammer shock on the capsule |
These errors rarely cause immediate failure. A single one-sided pressure event may not damage the capsule, but after several times the elastic element's stress characteristics change, showing up as zero drift and worse linearity — eventually misdiagnosed as transmitter aging and replaced, when the real problem was the operation.
The steps are simple; doing them by the book every time is the hard part. Remember: commission with "open HP, close equalizer, open LP," and shut down with "close LP, open equalizer, close HP." For steam service, add one rule: if the condensate is not full, do not touch the manifold.
