In plants that run a seal reliability program, pump mechanical seals average three years or more of service. Any one seal can fall far short of that or run well past it, because its life is set mostly by the pump it sits in, the fluid it seals and the way the pump is run.
At one UK refinery and petrochemical site studied from 1988 to 1991, 48% of mechanical seals failed within 5,000 hours of being installed, while 35% were still running after 24,000 installed hours.
For a whole site, the European Sealing Association gives 60 months as the benchmark most sites want to reach for mean time between seal failures. On an industrial centrifugal pump the shaft seal is usually a mechanical seal, so the same figures answer how long pump shaft seals last.
Design Life vs Service Life
A seal built to API 682 is designed to run continuously for 25,000 operating hours, about three years; the life it gets in a plant is whatever the pump, the fluid and the operation allow, which can be much shorter or much longer.
Most seal makers state that a seal run at one of its published operating limits should last at least two years, or three years for API 682 seal types; those limits assume steady operation and may be overstated under cyclic duty.
When you hold a seal’s life against these figures, count its running hours.
Replace a seal on its condition, not its age: it is due when its leakage becomes unacceptable or its support system alarms on level or pressure.
How Long Each Type of Seal Lasts
No published field data gives a dependable life in years for each seal type. What the data does show is how predictably each type lasts:
- Single spring seals with elastomers: single seals with a sprung face and elastomer secondary seals showed the widest spread of life.
- Metal bellows seals: bellows seals showed a narrow spread of life.
- Double seals: double seals also lasted predictably.
- Cartridge seals: cartridge seals have practically eliminated infant failures.
On any given duty, design and support add life: most seals need a flush plan to carry away the heat the faces generate, and at higher seal-chamber pressure a balanced seal lowers the closing force on the faces, so they run cooler.
How Service Conditions Change Seal Life
The sealed fluid alone can change seal life sixfold: at one UK refinery, in records from 1967 to 1988, average seal life on the best of 17 sealed fluids was six times that on the worst.
The longest lives came on medium hydrocarbons such as kerosene, which lubricate the faces, and on chemically passive fluids such as sweet water. The five worst fluids all combined high viscosity with high sealing temperature, and chemically reactive fluids such as HF acid gave considerably shorter average seal life than sweet water.
Beyond the fluid, steady, continuous service is easy on a seal compared with cyclic service, and every increase in shaft diameter, speed, temperature, pressure or solids content makes the duty harder.
What Shortens a Mechanical Seal’s Life
Most premature mechanical seal failures start in the pump and the way it is run rather than in face wear.
Shaft Runout and Equipment Condition
A bent shaft, a seal chamber out of square with the shaft, and misalignment between pump and driver all shorten seal life, because a rotating-spring seal’s springs and secondary seal have to absorb the movement on every revolution.
Shaft runout at the seal sleeve above 0.002 in (0.05 mm) TIR, measured with a dial indicator while the shaft is turned through 360°, calls for repairing or replacing the shaft. Where the seal maker gives its own runout limit for the seal, use that limit, typically 0.001 to 0.002 in TIR. The seal chamber face should run true to within 0.0005 in per inch of seal chamber bore diameter.
Pipe strain and soft foot pull the pump out of line after it is aligned, and register fits between the bearing housing and seal chamber wear over time.
Running Off the Pump’s Best Efficiency Point
Running a pump well away from its best efficiency point shortens seal life by deflecting the shaft and heating the liquid around the faces.
Cavitation adds shock loads that chip face edges, and vibration loosens drive parts and opens the faces.
Dry Running and Frequent Starts
Dry running shortens seal life because the faces depend on a liquid film for lubrication and cooling; without it they rub, and the heat builds within seconds.
Frequent starts and stops do the same damage: each start runs the faces before a full film forms. A pump started before it is primed and vented, or against a closed suction valve, runs dry from the first turn.
Abrasives in the Pumped Fluid
Abrasive solids shorten seal life by entering the gap between the faces and grinding them, and by packing around the parts that have to move.
Chemical Attack on Faces and Elastomers
Chemical attack shortens seal life when the pumped fluid is not compatible with the elastomers, the faces or the metal parts of the seal.
An incompatible O-ring swells, hardens or cracks and stops sealing on the shaft or in the gland, often well before the faces show wear. Faces are attacked too: strong alkalis and hydrofluoric acid leach the free silicon out of reaction-bonded silicon carbide, and aggressive chemicals can strip the impregnant from some carbon grades.
Temperature speeds the attack, so a material rated for a fluid at ambient temperature can fail in the same fluid hot.
Installation Errors
Installation errors shorten seal life from the first start; in refinery field data, most short-life failures traced back to poor seal and pump installation. The common installation mistakes are:
- Wrong spring compression: on a component seal, too much compression overloads and overheats the faces, and too little lets them open.
- Dirty or damaged faces: dust, fingerprints or a chipped edge on a lapped face holds the faces apart and leaks from the start.
- Cut O-rings: an O-ring dragged over a keyway, thread or sharp shoulder is nicked and leaks along the shaft or sleeve.
- Cartridge setting clips and set screws: setting clips left in place rub on the rotating parts, and drive collar set screws left loose let the seal slip on the shaft and lose its compression.
- Uneven gland bolting: gland nuts tightened unevenly cock the stationary face out of square with the shaft.