Oil leakage from an instrument sensor is one of the most common problems encountered in industrial measurement systems.
When engineers discover oil leaking from a pressure transmitter or differential pressure transmitter, the first reaction is often:
“The sealing ring must be damaged. Just replace it.”
However, after replacing the seal, the leakage may appear again. The problem is not always the sealing gasket — the real cause may be related to the diaphragm, welding structure, filling oil system, installation conditions, or operating environment.
In this article, we will explain the most common causes of pressure transmitter oil leakage and how to troubleshoot the problem effectively.
What Does “Oil Leakage” Mean in Instrument Sensors?
For industrial instruments, “oil leakage” usually refers to leakage of the internal filling liquid used in sensors with oil-filled isolation systems.
Common examples include:
- Differential pressure transmitters with silicone oil filling
- Pressure transmitters with diaphragm seals
- Remote seal systems with capillary tubes
The filling liquid transfers pressure from the process diaphragm to the sensing element. Once the oil leaks out, the measurement accuracy, stability, and long-term reliability of the instrument may be affected.
1. Damaged Isolation Diaphragm
The isolation diaphragm is the first protection barrier between the process medium and the transmitter.
If the diaphragm is damaged, the internal filling oil may leak and the transmitter may fail.
Common causes include:
Corrosion
Aggressive process media such as:
- Strong acids
- Strong alkalis
- Hydrogen sulfide (H₂S)
- Chlorine-containing chemicals
may corrode the diaphragm material and eventually cause cracks or holes.
Selecting the correct diaphragm material (such as 316L stainless steel, Hastelloy, tantalum, or coated diaphragms) is critical for corrosive applications.
Overpressure Shock
A sudden pressure spike beyond the transmitter’s rated pressure range can cause:
- Permanent deformation
- Fatigue damage
- Diaphragm rupture
Common causes include:
- Pump startup impact
- Valve opening too quickly
- Water hammer effect
- Process pressure fluctuation
Mechanical Wear
If the process medium contains solid particles, such as:
- Sand
- Metal particles
- Crystals
long-term erosion may gradually weaken the diaphragm surface and cause leakage.
2. Sealing Structure Failure
A pressure transmitter contains multiple sealing points. Failure at any weak point may result in oil leakage.
O-ring Aging
O-rings may lose elasticity due to:
- High temperature operation
- Chemical attack
- Long-term compression
Over time, they may become:
- Hard
- Cracked
- Deformed
causing leakage between the transmitter body and process connection.
Welding Defects
The diaphragm is usually welded to the transmitter body.
Poor welding quality, manufacturing defects, or long-term vibration fatigue may cause:
- Micro cracks
- Weld leakage
- Internal oil loss
Thread Connection Problems
For threaded process connections, leakage may occur due to:
- Incorrect thread sealing method
- Damaged threads
- Improper installation torque
- Wrong sealing tape or sealant usage
3. Filling Oil Process Problems
The filling oil inside the transmitter must be completely sealed and free of air bubbles.
During manufacturing, improper filling processes may leave residual air bubbles inside the system.
Under conditions such as:
- Temperature changes
- Mechanical vibration
- Pressure fluctuations
air movement may create abnormal pressure changes and increase stress on weak sealing points.
4. Extreme Temperature Conditions
Temperature changes can affect both the filling liquid and the mechanical structure of the transmitter.
At high temperatures:
- Filling oil expands
- Internal stress increases
- Sealing components may experience additional pressure
At extremely low temperatures:
- Oil viscosity increases
- Response time may become slower
- Mechanical stress may increase during temperature cycling
If the transmitter operates outside its designed temperature range for a long time, leakage risks may increase.
5. Mechanical Damage During Transportation or Installation
Many leakage problems are caused before the instrument even starts operation.
Possible causes include:
- Dropping the transmitter during installation
- Hitting the diaphragm surface with tools
- Excessive tightening force
- Incorrect gasket installation
- Misalignment between flanges
Even a small deformation that is invisible to the naked eye may eventually develop into a leakage problem after long-term operation.
6. Remote Seal and Capillary Tube Leakage
For applications requiring remote installation, differential pressure transmitters are often equipped with diaphragm seals and capillary tubes.
Common leakage points include:
- Capillary tube connection joints
- Welded areas
- Damaged protective tubing
Possible causes:
- Excessive bending during installation
- Transportation damage
- Continuous vibration
- External mechanical impact
When leakage occurs in a remote seal system, replacing only the transmitter body usually cannot solve the problem. The complete diaphragm seal assembly should be inspected.
How to Troubleshoot a Leaking Pressure Transmitter?
When oil leakage is found, check the following points:
| Leakage Location | Possible Cause |
|---|---|
| Diaphragm surface | Diaphragm corrosion or rupture |
| Process connection | Seal failure or installation problem |
| Weld area | Manufacturing defect or fatigue crack |
| Capillary tube | Mechanical damage or vibration fatigue |
| Inside transmitter housing | Severe internal failure |
Symptoms After Oil Leakage
A leaking transmitter may show:
- Output signal drift
- Incorrect zero point
- Slow response
- Unstable measurement
- Incorrect pressure or level readings
If these symptoms appear, the transmitter should be inspected immediately.
How to Prevent Pressure Transmitter Oil Leakage?
To reduce leakage risks:
1. Select the correct diaphragm material
Consider:
- Process medium
- Temperature
- Pressure
- Corrosion characteristics
2. Avoid mechanical stress during installation
Always:
- Protect the diaphragm surface
- Use correct torque
- Avoid unnecessary bending of capillary tubes
3. Check operating conditions
Confirm that the transmitter is suitable for:
- Maximum pressure
- Temperature range
- Chemical compatibility
- Vibration environment
Conclusion
Oil leakage from a pressure transmitter is not always caused by a simple sealing problem.
The real cause may come from:
- Diaphragm damage
- Seal failure
- Welding defects
- Filling system problems
- Temperature stress
- Installation damage
Finding the actual failure point is more important than repeatedly replacing seals.
For pressure transmitters and differential pressure transmitters used in demanding industrial applications, proper selection of diaphragm materials, sealing structure, and installation method is the key to achieving long-term reliable measurement.