Impulse Lines – Wet Leg, Dry Leg & Field Failures
Impulse lines connect the process to the pressure transmitter. In real plant conditions, most pressure and DP errors originate in impulse piping — not inside the transmitter.
Types of Impulse Line Installations
Wet Leg
Used mainly in closed vessels for level measurement. Both legs remain filled with liquid to maintain a constant reference.
- Common in steam service
- Reference leg must stay full
- Leak or evaporation causes zero shift
Dry Leg
Used in open tanks or vented systems. One side is referenced to atmosphere.
- Simpler configuration
- Condensation is major issue
- Improper slope creates reading errors
Slope & Routing Rules (Service Dependent)
Gas Service
- Impulse lines should slope toward the transmitter
- Prevents liquid accumulation
- Avoid low points (liquid traps)
Liquid Service
- Impulse lines should slope away from the transmitter
- Prevents vapor pockets
- Maintain full liquid column
Steam Service
- Use condensate pots
- Ensure equal condensate level in both legs
- Insulate both impulse lines equally
Incorrect slope is one of the most common field installation mistakes.
Common Impulse Line Problems
| Symptom | Likely Cause | Field Check |
|---|---|---|
| Slow response | Plugging, wax, debris | Blow down impulse line |
| Zero shift after shutdown | Unequal condensate level | Drain and re-equalize |
| Reading too high | LP leg liquid accumulation | Check slope & traps |
| No reading | Blocked or frozen line | Inspect heat tracing |
Troubleshooting Sequence
- Verify manifold valve positions
- Check impulse line slope visually
- Blow down both legs safely
- Confirm no liquid traps or vapor pockets
- Compare local gauge vs transmitter reading
Field Experience Tips
- Mark High (H) and Low (L) lines clearly
- Insulate both legs equally
- Never open valves rapidly
- Inspect after major shutdowns