Dry Running in Mechanical Seals, Understanding, Diagnosis, and Prevention

Dry Running in Mechanical Seals, Understanding, Diagnosis, and Prevention

Overview

Dry running represents one of the most severe and preventable failure modes in mechanical sealing systems. It occurs when the seal faces operate without sufficient lubrication, typically due to a lack of process fluid, blocked flush lines, or improper commissioning procedures. This leads to rapid temperature rise, face degradation, and ultimately, total seal failure, often within minutes.

Understanding the mechanics, indicators, and preventive strategies related to dry running is essential for reliability engineers, maintenance professionals, and pump system designers aiming to reduce unplanned downtime and operational risk.

The Critical Role of Lubrication

Mechanical seals rely on a controlled fluid film between the stationary and rotating faces to maintain proper operation. This film performs several vital functions:

• Minimises Friction: Reduces mechanical resistance between seal faces.
• Dissipates Heat: Transfers heat away from the contact surfaces.
• Prevents Wear: Reduces face-to-face abrasion.
• Maintains Face Flatness: Supports pressure balance under variable loads.

Consequences of Dry Running

When lubrication is lost, seal faces make direct contact, resulting in:

• Excessive heat generation
• Immediate surface damage or blistering
• Increased torque and shaft load
• Accelerated wear or catastrophic seal failure

Primary Causes of Dry Running

1. Pump Startup Without Priming

The system is energized before the seal chamber is filled with process fluid.

Prevention: Implement interlocks based on flow, level, or pressure sensors to inhibit dry start conditions.

2. Inoperative or Blocked Flush Lines

API Plan 11, 23, or 32 fails to deliver adequate cooling or lubricating fluid.

Prevention: Regularly inspect orifice plates, strainers, and valve positions. Verify continuous flow to the seal.

3. Cavitation or Vapor Lock

Process fluid vaporizes prematurely due to low suction pressure or high temperatures.

Prevention: Adjust suction head or reduce fluid temperature to maintain net positive suction head (NPSH) requirements.

4. Dry Installation of Seal

Mechanical seals, particularly cartridges, are installed without lubrication on elastomers or faces.

Prevention: Always lubricate elastomers and face materials during installation using manufacturer-approved agents.

5. Reverse Flow Conditions

Incorrect flow direction prevents proper fluid circulation across the seal faces.

Prevention: Verify pump rotation and line routing during commissioning.

Symptoms and Diagnostic Indicators

| Symptom | Likely Cause | | --- | --- | | Burnt smell or smoke | Excessive heat at the gland | | Premature leakage | Seal face fracture or distortion | | Startup vibration or noise | Elevated friction and torque | | Face discoloration | Thermal degradation or etching | | Blisters or pits on faces | Localized overheating or dry spots |

Seal Face Damage Profiles

Carbon vs. SiC Damage Characteristics

Carbon Faces: Often show signs of charring, radial cracks, or edge lifting due to rapid thermal expansion.

Silicon Carbide (SiC): While thermally robust, SiC can suffer from face warping or complete fracture under thermal shock.

Elastomers: Elevated gland temperatures can cause hardening, surface cracking, or loss of elasticity, particularly in standard NBR or EPDM compounds.

Dry Running Prevention Strategies

1. Operational Controls

• Enforce startup protocols that ensure seal chambers are filled before energizing the system.
• Integrate PLC-based interlocks triggered by low flow or pressure anomalies.

2. Monitoring and Instrumentation

• Install temperature probes at the gland to detect early thermal rise.
• Use flowmeters on flush plans to confirm constant lubrication.

3. Seal Selection

• Choose seal materials rated for marginal lubrication, such as SiC vs. SiC or diamond-like coatings.
• For intermittent service, consider lift-off or non-contacting gas-lubricated seals.

4. Flush Plan Maintenance

• Periodically check API Plan components, such as orifice plates, filters, and tubing, for blockages or wear.
• Ensure the flush plan provides appropriate differential pressure and temperature regulation.

Best Practices for Installation and Commissioning

| Action | Objective | | --- | --- | | Lubricate faces and elastomers | Avoid friction-induced wear at startup | | Confirm pump priming and fluid level | Prevent initial face contact | | Use compatible barrier fluids | Ensure consistent film integrity | | Monitor system torque and vibration | Detect early signs of seal distress |

Conclusion

Dry running is among the most rapid and destructive failure conditions in mechanical sealing systems, but also one of the most preventable. With proper installation practices, robust monitoring, and disciplined startup procedures, the risk of dry running can be effectively eliminated.

By adopting a proactive approach to seal management, operators can significantly extend seal life, reduce maintenance costs, and ensure safer, more reliable operation of rotating equipment.