Mechanical seals are critical components in pumps and other rotating equipment used throughout chemical, petrochemical, pharmaceutical, fertilizer, food-processing, and water-treatment plants. Their primary function is to prevent process fluid from leaking along the rotating shaft while allowing the shaft to rotate continuously.
Despite their relatively compact size, mechanical seals can have a significant impact on plant reliability. A failed seal can result in product leakage, equipment damage, environmental releases, safety hazards, production losses, and unplanned shutdowns.
Understanding the mechanical seal failure causes is therefore essential for maintenance and process engineers. Seal failure is rarely caused by the seal itself alone. Operating conditions, installation practices, pump alignment, lubrication, temperature, pressure, vibration, and process-fluid properties can all influence seal life.
What Is a Mechanical Seal?
A mechanical seal is a sealing arrangement installed between a rotating shaft and a stationary component such as a pump casing.
A typical mechanical seal consists of:
- Rotating seal face
- Stationary seal face
- Secondary seals such as O-rings
- Springs
- Retainer
- Gland
- Shaft or sleeve
- Other supporting components
The two primary seal faces are pressed together to create a controlled sealing interface.
A very thin fluid film between the faces provides lubrication and cooling under appropriate operating conditions.
The objective is not necessarily to create a completely dry interface. Instead, the seal is designed to control leakage within an acceptable range while minimizing wear.
Common Mechanical Seal Failure Causes
Mechanical seal failure can result from several interconnected factors.
1. Dry Running
One of the most common and serious causes of mechanical seal damage is dry running.
The seal faces require an appropriate lubricating film. If the pump operates without sufficient liquid, the faces can rapidly overheat.
Dry running may occur because of:
- Empty suction tank
- Loss of suction
- Pump not properly primed
- Closed suction valve
- Excessive air entrainment
- Low liquid level
- Blocked suction line
- Improper startup procedure
Excessive heat can cause seal-face cracking, distortion, glazing, or rapid wear.
2. Excessive Temperature
Mechanical seals are designed for specific temperature limits.
High process temperature can affect:
- Seal faces
- Elastomers
- Springs
- Secondary seals
- Lubricating film
- Process-fluid properties
An increase in temperature can also cause vaporization near the seal faces, potentially leading to unstable lubrication.
The seal material must therefore be compatible with the actual process temperature, not merely the normal design temperature.
3. Excessive Pressure
Every mechanical seal has a pressure limitation.
Operating above the recommended pressure can increase the hydraulic forces acting on the seal faces.
High pressure may cause:
- Increased leakage
- Face deformation
- Excessive wear
- Secondary-seal problems
- Seal-face separation or instability
Pressure should therefore be checked against the seal manufacturer’s design limits.
4. Shaft Misalignment
Pump and motor alignment has a direct influence on seal performance.
Angular or parallel misalignment can create abnormal movement of the rotating shaft.
This can result in:
- Uneven face loading
- Excessive seal-face wear
- Secondary-seal movement
- Vibration
- Premature failure
Laser alignment or an appropriate precision-alignment method should be used where applicable.
5. Excessive Shaft Runout
A shaft that does not rotate concentrically can cause the mechanical seal faces to move excessively.
Possible causes include:
- Bent shaft
- Damaged shaft sleeve
- Bearing problems
- Incorrect assembly
- Excessive bearing clearance
Shaft runout should be checked when repeated seal failures occur.
6. Excessive Vibration
Vibration is another important mechanical seal failure cause.
Sources of vibration may include:
- Pump cavitation
- Bearing failure
- Misalignment
- Rotor imbalance
- Loose foundation bolts
- Pipe strain
- Coupling problems
- Hydraulic instability
Vibration can damage seal faces and secondary sealing components.
If a mechanical seal repeatedly fails, investigating the source of vibration is often more useful than simply replacing the seal.
7. Cavitation
Cavitation occurs when local liquid pressure falls below the liquid’s vapor pressure, producing vapor bubbles that subsequently collapse.
Pump cavitation can generate significant vibration and hydraulic instability.
Typical causes include:
- Insufficient NPSH available
- Blocked suction line
- Excessive suction losses
- High liquid temperature
- Low suction pressure
- Incorrect pump selection
Cavitation-related vibration can significantly reduce mechanical seal life.
8. Incorrect Seal Material Selection
A mechanical seal must be chemically compatible with the process fluid.
Common seal-face and elastomer materials include combinations involving:
- Carbon
- Silicon carbide
- Tungsten carbide
- Ceramic
- PTFE
- EPDM
- FKM
- Other specialized materials
The correct selection depends on:
- Chemical composition
- Temperature
- Pressure
- Concentration
- Abrasive content
- pH
- Corrosivity
- Solids concentration
A material that performs well in water may fail rapidly in a corrosive or abrasive chemical service.
9. Abrasive Solids
Slurries and fluids containing suspended solids can be particularly difficult for mechanical seals.
Examples include:
- Phosphoric acid slurry
- Gypsum-containing streams
- Mineral slurries
- Catalyst-containing fluids
- Crystallizing solutions
Solids can enter the sealing interface and cause abrasive wear or face damage.
For such applications, appropriate seal-face materials and flushing or barrier arrangements may be required.
10. Improper Installation
Incorrect installation is a common but preventable cause of seal failure.
Potential problems include:
- Damaged O-rings
- Scratched seal faces
- Incorrect spring installation
- Incorrect seal setting dimension
- Excessive tightening
- Contamination
- Improper gland installation
- Incorrect shaft-sleeve dimensions
Mechanical seals should be installed according to the manufacturer’s instructions and relevant equipment specifications.
11. Incorrect Seal Setting
The axial setting of the seal is critical.
If the seal is installed too tightly, excessive face loading can generate heat and accelerate wear.
If it is installed too loosely, the seal may not provide sufficient contact between the faces.
The manufacturer’s specified setting dimension should therefore be maintained carefully.
12. Blocked Flush or Cooling System
Many mechanical seals use flush, quench, cooling, or barrier systems.
If the system becomes blocked or loses flow, heat may accumulate at the seal faces.
Potential causes include:
- Blocked tubing
- Closed valve
- Low flush pressure
- Incorrect piping
- Dirty cooling fluid
- Instrument failure
The seal-support system should therefore be monitored along with the seal itself.
Mechanical Seal Failure Symptoms
Recognizing early warning signs can prevent major equipment damage.
Common symptoms include:
Increased Leakage
A sudden increase in leakage may indicate damaged seal faces, elastomer failure, excessive vibration, or incorrect operating conditions.
Temperature Increase
An unusually high seal temperature can indicate poor lubrication, dry running, excessive face loading, or inadequate cooling.
Abnormal Noise
Grinding, squealing, or unusual sounds may indicate damaged seal faces or mechanical problems.
Vibration
Increasing vibration can indicate shaft, bearing, alignment, hydraulic, or seal-related problems.
Frequent Seal Replacement
Repeated seal failure at short intervals is a strong indication that the root cause has not been corrected.
Simply installing another seal without investigating operating conditions often leads to repeated failures.
Mechanical Seal Troubleshooting Approach
A systematic troubleshooting approach is more effective than immediately replacing the seal.
Step 1: Examine the Failed Seal
Inspect:
- Seal faces
- O-rings
- Springs
- Shaft sleeve
- Retainer
- Gland
The wear pattern can provide important clues.
Step 2: Review Operating Data
Check:
- Pressure
- Temperature
- Flow
- Pump speed
- Suction conditions
- Discharge conditions
- Seal flush pressure
- Seal cooling
Compare actual conditions with the seal’s design limits.
Step 3: Check Pump Condition
Inspect:
- Alignment
- Bearing condition
- Shaft runout
- Coupling
- Foundation
- Pipe strain
- Impeller condition
Step 4: Investigate Process Conditions
Changes in fluid concentration, temperature, solids content, crystallization, or viscosity can alter seal performance.
Step 5: Verify Seal-Support System
Confirm that flushing, cooling, quenching, or barrier systems are operating correctly.
How to Prevent Mechanical Seal Failure
Preventive maintenance can significantly improve mechanical seal reliability.
Important practices include:
- Avoid dry running.
- Maintain correct pump alignment.
- Monitor vibration.
- Maintain proper suction conditions.
- Prevent cavitation.
- Verify seal flush flow.
- Use chemically compatible materials.
- Follow correct installation procedures.
- Maintain shaft and sleeve condition.
- Monitor seal temperature.
- Inspect bearings and couplings.
- Maintain proper operating conditions.
Condition monitoring can also be used to identify developing problems before complete seal failure.
Mechanical Seal Failure in Chemical Plants
Chemical plants often present challenging seal-service conditions.
For example, pumps handling acidic or abrasive process fluids may experience a combination of:
Corrosion + Abrasion + Temperature + Solids + Pressure
In such applications, mechanical seal selection should be based on actual process conditions rather than simply selecting a standard pump seal.
The engineer should consider the complete process envelope, including normal, startup, shutdown, upset, and cleaning conditions.
For highly hazardous or toxic fluids, double mechanical seals with an appropriate barrier or buffer system may be considered depending on the application and engineering requirements.
Conclusion
Mechanical seal failure is usually the result of a combination of mechanical, hydraulic, thermal, chemical, and installation-related factors.
Dry running, excessive temperature, high pressure, misalignment, shaft runout, vibration, cavitation, incorrect material selection, abrasive solids, improper installation, and inadequate seal-support systems are among the major causes.
The most effective approach is not simply to replace a failed seal but to identify why the seal failed.
A detailed inspection of the failed components combined with operating-data review can reveal the underlying problem. By maintaining proper pump alignment, preventing dry running and cavitation, controlling operating conditions, selecting compatible materials, and maintaining seal-support systems, chemical plants can significantly improve mechanical seal reliability.
For maintenance and process engineers, understanding mechanical seal failure mechanisms is an important part of improving pump availability, process safety, maintenance efficiency, and overall plant reliability.