Manifold Valves – Isolation & Equalization Logic
Manifold valves allow safe isolation, equalization, venting, and calibration of pressure transmitters. Incorrect valve sequencing is one of the most common causes of zero shift and sensor damage.
Types of Manifold Valves
3-Valve Manifold
- High-pressure (HP) isolation
- Low-pressure (LP) isolation
- Equalizing valve
Common for DP transmitters.
5-Valve Manifold
- HP & LP isolation
- Equalizing valve
- HP vent / drain
- LP vent / drain
Used for frequent calibration or steam service.
Normal Operating Condition
| Valve | Status |
|---|---|
| High Pressure Isolation | Open |
| Low Pressure Isolation | Open |
| Equalizing Valve | Closed |
Equalizing valve must always remain closed during normal operation.
Correct Zero / Equalization Procedure
- Close High Pressure isolation valve
- Close Low Pressure isolation valve
- Open Equalizing valve slowly
- Confirm transmitter reads zero
- Close Equalizing valve
- Open Low Pressure valve first
- Open High Pressure valve slowly
Opening HP first during restart can cause sensor shock and span shift.
Common Field Mistakes
- Equalizing valve partially open during operation
- Opening HP rapidly after maintenance
- Leaving one isolation valve slightly closed
- Internal manifold leakage
- Vent valves not fully tightened
Failure Symptoms
- DP always near zero
- Unstable pressure reading
- Zero drift after calibration
- Flow reading inconsistent with process
Many transmitter replacements are unnecessary — the problem is often manifold handling.
Field Experience Tips
- Always tag HP, LP, and EQ clearly
- Operate valves slowly
- Equalize before zero calibration
- Verify both legs pressurized before restart