Maintaining a mechanical seal means keeping the pump and its support system inside the conditions the seal was selected for, reading a few indications while the pump runs, and acting on the trend before the faces open. There is no service to perform on the seal itself while it is installed: everything a maintenance programme does for a seal happens around it.
Installation quality belongs to the same programme. A seal set at the wrong working length or fitted to a shaft with excessive runout will be logged as a seal failure months later, so the installation procedure and the checks in the first days after a seal change are maintenance work.
What to Watch on a Running Seal
Four indications are available without stopping the pump, and each one is read against a baseline taken when the seal was new:
- Leakage: a light vapour weepage at the gland is normal operation; a continuous visible drip is a symptom. A seal is not expected to be leak-free; watch for a change in the leakage. Weepage versus a real leak and what counts as acceptable leakage cover where the line falls in practice.
- Seal-area temperature: the gland face, the flush piping and the tubing are the only surfaces you can reach on a running pump. Infrared thermometers are easy to misread, so confirm anything alarming with a contact thermometer before acting on it.
- Vibration: a rising overall level at the pump reaches the faces as fretting, chipping and lost film. Take the reading at the same point and speed each time, since vibration is one of the fastest ways to destroy a seal.
- Flush and barrier flow and pressure: on a supported seal, the piping plan’s setpoint is a reading in its own right. Checking flush plan pressure belongs on the same round as the others.
Record the readings so they can be trended. On critical duties that trending can be instrumented through an API Plan 65 or 75 condition-monitoring arrangement.
Operating Practices That Keep the Face Film Intact
The causes a maintenance programme can actually control are the operating ones — dry running, priming, closed valves, cooling, process upsets and foreign objects — and they respond quickly to procedure. The full list of what kills a seal is set out in 15 reasons for mechanical seal failure.
Never Let the Seal Run Dry
The faces must have liquid between them before the shaft turns. Prime and vent the pump fully before every start, never run against a closed suction valve, and never use the pump to empty a vessel past the point where the seal chamber loses liquid.
On duties where dry running is a live risk — batch transfer, tank stripping, self-priming service — put the protection in the control system with a low-flow or power-draw trip. The early indications are in signs of a dry-running seal.
Run the Pump Near Its Best Efficiency Point
Flow well away from the best efficiency point reaches the seal as vibration, recirculation and cavitation, and the widely quoted 70–120 % band is not your pump’s number. The preferred operating region depends on the pump’s specific speed, so the band that protects your seal comes from the pump curve and the manufacturer.
Two operating points deserve a hard rule. Running at shut-off for more than a few seconds causes mechanical damage, and running out to the far right of the curve brings cavitation and vibration with it, along with flashing when the fluid vaporises in the chamber.
Rapid start-stop cycling has the same effect over time: a pump cycling on level control is a candidate for a control change before it is a candidate for a better seal.
Should You Lubricate a Mechanical Seal
No. The pumped fluid is the lubricant, and the film it forms between the faces lubricates the running seal. Nothing is added to a seal in service.
Grease and oil are used at assembly: a thin film of a compatible lubricant on the elastomers so the O-rings slide onto the shaft or sleeve without rolling or tearing. It never goes on the faces, which must be clean and dry when they meet. Which lubricants suit which elastomers is covered in seal lubrication.
Keep the Flush Plan and Barrier Fluid Working
On a supported seal, one condition must hold at all times: a barrier fluid on a pressurised dual seal stays above seal chamber pressure through commissioning, normal running, upset and shutdown, while a buffer fluid on an unpressurised dual seal stays below it. Set barrier pressure high enough to stay above a chamber pressure that is often not accurately known, and not so high that it exceeds the inner seal’s pressure rating or drives gas into the fluid.
The reservoir level tells you which seal is leaking, and it means opposite things on the two common plans. On a Plan 52, a rising level means liquid leakage past the inner seal, a rising pressure means vapour past the inner seal, and a falling level means leakage past the outer seal. On a Plan 53A, a slowly falling level is normal consumption by both seals; the alarm is a change in refill frequency or a rapid drop, and a falling reservoir pressure points at the nitrogen supply rather than at the seal.
Barrier and buffer fluid is a consumable: it is changed on a schedule rather than topped up indefinitely. No standard publishes that interval; set it from your own condition data and hold to it.
Choose the fluid on the duty. A general-purpose lube oil meets most of the criteria — non-foaming, low process solubility, cheap — but in cold service it thickens and can blister carbon faces. Where a glycol is used, use propylene glycol and water rather than ethylene glycol. The trade-offs are set out in seal water versus barrier fluid.
Two pieces of installed hardware fail without an obvious symptom and should be on the round. The reservoir has to sit above the seal and close to it for venting and thermosyphon circulation, and the cooling water that serves it is rarely monitored at all. On single-seal plans 11, 13 and 21 the orifice is the failure point: it plugs on process particles, and noise at or just downstream of it signals excessive velocity. Plan-by-plan checks are in the flushing system maintenance checklist and the seal pot guide; the arrangements themselves are on the API plans pages.
When to Repair, Re-Lap or Replace a Seal
Re-lap the faces when the wear is flatness and light scoring, replace them when the damage is heat checking, cracking, chipping or wear past the step, and stop repairing when the same seal fails the same way twice.
All three decisions come after the leak has been localised. Gland gaskets, sleeve O-rings, the chamber joint and the drain connection all produce liquid at the gland, and so do bearing and shaft problems that appear as seal symptoms; the leakage paths narrow it down before anything is opened. On a dual seal, start at the reservoir level and pressure, which indicate whether the inner or the outer seal is leaking.
A set seal is not adjusted in the field. Repeated re-tensioning is itself a symptom worth investigating.
Once the seal is out and inspected, the condition of the faces decides it. Lapping needs wear within the lappable depth of the face, sound hardware and springs, and elastomers that can be renewed — the procedure and the acceptance criteria are in lapping seal faces and face flatness. Heat checking, cracks, edge chips and wear past the step are not conditions lapping removes.
A second identical failure on the same pump means the cause sits in the duty, the pump or the support system, and the repair route will keep returning the seal until that is fixed.
How Often to Inspect and Change Seals
The interval comes from your own failure history, because no published figure applies to your pumps.
Plants work to mean time between failure across the fleet. The European Sealing Association’s supplier and end-user data puts 60 months as the benchmark most sites want to reach, 48 months as the figure often accepted in midstream service, and 100 months as the target some majors have set.
Use the benchmark to set a target, then work the list that produces it: survey the fleet, identify the ten to fifteen bad actors, investigate each failure to a cause, and review the data on a fixed cycle. The method is set out in calculating and improving seal MTBF, and it only works on records detailed enough to pinpoint the cause — a failure logged as “seal leaking” gives you nothing to act on.
Scheduled intervention then happens at the pump’s own overhaul rather than on a seal calendar. When the pump is open, renew the elastomers and the gland gasket, re-check gland bolt torque in the correct sequence, and verify shaft runout and sleeve condition before the new seal goes on. Between overhauls, act on the trend readings, and how long mechanical seals last explains why the published lifespan ranges vary too widely to schedule against.
Store Spare Seals So They Are Fit to Install
A spare seal has to come out of the store in the condition it went in, and what ages in the store is the rubber. SAE AS5316, the aerospace standard for storing elastomer seals before assembly, gives a maximum storage life of 15 years for nitrile and chloroprene, while fluorocarbon, silicone and perfluoroelastomer elements are listed as unlimited — treated by many organisations as 25 years.
The storage conditions behind those figures are specific:
- Temperature: below 38 °C, away from boilers, radiators and direct sunlight. If the store runs below 15 °C the elastomers stiffen and must be brought up to roughly 20 °C throughout their mass before being fitted.
- Humidity: below 75 % RH for elastomers not held in sealed moisture-proof bags, and below 65 % RH where polyurethanes are stored.
- Light: original opaque packaging, protected from direct sunlight and from intense artificial light with an ultraviolet content.
- Ozone: no mercury vapour lamps and no sparking electrical equipment in the room.
Age is counted from the cure quarter. Under AS5316, an item cured in January, February or March is not one quarter old until 1 July of that year, so the clock starts at the end of the quarter in which it was cured.
The rest is handling. Leave seals in their packaging until the pump is open, store them free of tension and compression — a large-diameter ring is coiled in three equal superimposed loops, never two — and leave cartridge setting clips in place until the gland is bolted down and the sleeve is locked. Keep the lapped faces covered and never stack anything on them.