1. Preparation Before Inspection
Before inspecting a differential pressure (DP) level transmitter equipped with a diaphragm seal and remote seal system, the following preparations should be completed:
- Contact the process operator and confirm the operating conditions.
- Obtain the required work permit according to site safety procedures.
- If the measurement loop is connected to a control valve or automatic control system, switch the loop to manual mode from the control room.
- Wear proper personal protective equipment (PPE).
- Prepare necessary tools, such as:
- Screwdrivers
- Adjustable wrench
- F-type wrench
- Multimeter
- Communication device (HART communicator if required)
2. Initial Field Inspection
Before making any adjustment, first verify that the reference level instrument is reliable.
For example, check whether the local glass level gauge is working properly:
- Confirm that the upper and lower isolation valves are fully open.
- Check that the drain valve is closed.
- Make sure there is no blockage inside the level gauge.
- Confirm there is no false level indication caused by high viscosity media, crystallization, or insufficient heating.
After confirming the glass level gauge is normal, compare:
- Actual vessel level indication
- DP transmitter output
- DCS displayed value
If a significant deviation exists, further troubleshooting is required.
3. Troubleshooting Before Refilling Seal Liquid
A large deviation between the actual level and transmitter indication does not always mean the seal liquid needs to be replaced.
The following items should be checked first:
1) Instrument Configuration
Check:
- Measuring range
- Engineering unit
- Zero suppression / elevation settings
- Density compensation parameters
- Output configuration (4–20 mA)
2) Impulse and Seal System
Check:
- Leakage from diaphragm seal or capillary tube
- Blockage in the pressure connection
- Gas bubbles inside the capillary system
- Damage to the diaphragm seal
Only after confirming that the above conditions are normal should the seal liquid system be considered for refilling.
4. Seal Liquid Refilling Procedure
When the seal liquid needs to be refilled, follow the correct procedure:
Step 1: Isolate the transmitter
- Close the positive and negative side primary valves.
- Close the three-valve manifold.
- Open the vent point at the upper section of the impulse line.
The purpose is to prevent high-pressure seal liquid from causing a sudden pressure impact on the diaphragm.
Step 2: Refill Seal Liquid
- Slowly open the seal liquid filling valve.
- Use a filling pump to inject the correct seal liquid.
- Control the filling speed carefully.
- Allow trapped air to escape through the vent point.
During filling:
- Avoid generating air bubbles inside the capillary system.
- Never allow the diaphragm seal to be exposed to pressure from only one side.
A single-sided pressure impact may cause:
- Permanent diaphragm deformation
- Zero point shift
- Measurement instability
5. Correct Three-Valve Manifold Operation
After completing seal liquid filling, the transmitter can be returned to service.
The correct operation sequence is:
Open positive pressure valve
↓
Close equalizing valve
↓
Open negative pressure valve
This sequence prevents sudden differential pressure impact on the transmitter sensing element.
Incorrect operation may cause:
- Overpressure on one side of the diaphragm
- Zero drift
- Damage to the measuring cell
6. Zero Adjustment and Verification
After the transmitter is placed into operation, verify the measurement value.
Important:
A DP level transmitter will not always display zero after the valves are opened.
The transmitter output depends on the actual liquid level inside the vessel.
For example:
- If the vessel level is 50%, the transmitter should output a corresponding differential pressure signal.
- The transmitter displays zero only when the actual liquid level corresponds to the zero reference condition.
If the transmitter is at zero-level conditions and the output is still incorrect, perform zero adjustment according to the instrument manual.
7. Final Verification
After adjustment:
- Slowly open the primary isolation valves.
- Check transmitter output stability.
- Compare the transmitter reading again with the local level indicator.
- Verify the 4–20 mA output and DCS indication.
If the indication is still incorrect under stable process conditions, with:
- Correct medium density
- Correct operating temperature
- Normal impulse system condition
the transmitter should be removed and sent for calibration.
8. Completion of Work
After finishing the maintenance work:
- Stop the seal liquid filling equipment.
- Remove tools and clean the working area.
- Confirm the instrument status with the process operator.
- Return the control loop to normal automatic operation if required.
Key Practical Notes
1. Three-valve manifold sequence
Correct:
Positive pressure valve → Close equalizing valve → Negative pressure valve
This prevents one-side pressure loading of the transmitter diaphragm.
2. Seal liquid filling precautions
Before refilling:
- The three-valve manifold must be closed.
- The transmitter must be isolated.
This prevents high-pressure seal liquid from directly impacting the diaphragm.
3. Zero point misunderstanding
A DP level transmitter does not normally show zero after commissioning.
The output value depends on the actual hydrostatic pressure generated by the liquid column.
Only under the defined zero-level condition should the transmitter output correspond to zero.