The common mechanical seal installation mistakes fall into the three stages of the installation procedure: pump checks skipped before the seal goes on, errors made while it is fitted, and a pump started before it is ready for the seal.
In the European Sealing Association’s 2025 review of 3,500 pump failures, maintenance causes, the category that holds assembly, seal setting, alignment and runout, accounted for 28% of premature seal failures.
Mistakes Before Fitting the Seal
Before the seal goes on, the mistake is fitting it to a pump nobody has measured. Correct the pump before fitting if a reading is outside the limit. The limits below are the common figures; where your seal’s instructions give a different one, use theirs.
Not Measuring Shaft Runout
A new seal on a shaft with too much runout has to follow the shaft’s wobble on every revolution: it leaks more, wears out early, and at high pressure and speed can fail quickly.
The common limit is 0.05 mm (0.002 in) TIR. A reading past the limit means a bent shaft or worn bearings to fix before any seal goes on.
Measure runout on the shaft or sleeve where the seal will sit, not at the coupling end.
Not Checking Shaft and Bore Dimensions
A seal fitted to a shaft or seal chamber that is off its drawing dimensions sits off-center or with the wrong squeeze on its O-rings, and it leaks or rubs from the first start.
Check the shaft or sleeve OD, the seal chamber bore and depth, the gland pilot, the stud size and bolt pattern, and the distance to the first obstruction against the seal assembly drawing. If parts do not go together, measure again rather than force them.
The bore should be concentric to the shaft within 0.125 mm (0.005 in) TIR at most, and tighter on small shafts.
Leaving Burrs and Sharp Edges on the Shaft
A burr or sharp edge cuts or nicks an O-ring as it slides over, and a cut sleeve O-ring leaks between shaft and sleeve from the first start.
Break every sharp corner an O-ring must pass over: shaft steps, threads, keyways, reliefs and snap-ring grooves. The old seal’s set screws leave marks in the shaft, and the raised metal around them has to be filed smooth before the new seal goes on. Where an edge cannot be removed, cover it with thin tape or plastic film while the seal slides past.
A sleeve that is pitted or scratched where an O-ring seals should be replaced.
Not Checking the Seal Chamber Face for Squareness
If the seal chamber face is not square to the shaft, the gland and stationary seat sit tilted, so leakage rises and the springs fatigue.
The common limit is 0.0005 in per inch of seal chamber bore diameter (0.0005 mm/mm) TIR. Hold the shaft axially while taking the reading, or its end play shows up as face runout.
Mistakes While Fitting the Seal
While the seal goes on, the mistakes are dirtying or damaging its parts and assembling or setting them wrongly. Fitting mistakes are more likely on a component seal, where the fitter assembles and sets every part by hand.
Contaminating the Seal Faces
Dirt or a fingerprint on a seal face holds the faces apart, so the seal leaks from the first start.
Work on a clean bench with clean hands and keep fingers off the lapped faces. Wipe the faces just before fitting with a lint-free cloth and a quick-drying solvent such as alcohol; once clean, they should not be touched again.
Setting the Wrong Working Length
A component seal set too short compresses its springs too hard and wears its faces fast. Set too long, the springs cannot hold the faces together at start-up and standby, and the seal leaks.
Take the working length from the seal drawing and set the compression with the shaft in its running position, measuring from the seal chamber face.
Set the impeller clearance before the seal. On pumps with semi-open impellers, moving the impeller moves the shaft axially and changes the seal’s compression.
Not Tightening the Drive Set Screws
Drive set screws left loose let the seal’s rotating parts slip on the shaft, so they lose their drive and can move axially off their setting.
When installing a cartridge seal, tighten the set screws before taking out the setting clips. Tighten them alternately to the torque in the seal’s instructions, on clean, undamaged shaft.
Missing or Misassembled Parts
The parts that get left out or fitted wrongly are the small loose pieces of a component seal. Check each against the drawing:
- O-rings: one fitted in the wrong groove extrudes and leaks when pressure comes on. Where the static and dynamic O-rings are different materials, a mix-up puts the wrong compound in each place.
- Springs: on a multi-spring seal, a spring knocked out of its pocket during assembly loads the face unevenly.
- Mating ring: seated crooked in the gland, it cannot make full contact with the rotating face, and the seal leaks.
- Spring-energized PTFE seals: they usually seal in one direction only, so one fitted the wrong way round leaks.
- Centering shims: a component seal’s stationary seat is centered with the shims supplied; without them it can sit off-center, and the rotating face can run off its edge.
- Gaskets: flat gaskets are single-use, so fit new ones with every seal.
Leaving the Setting Clips in Place
A cartridge seal started with its setting clips still fitted is damaged at start-up.
Take the clips off completely once the set screws are tight, and turn the shaft by hand to feel for binding before the first start. Clips that are only swung or slid aside on the gland can drop onto the turning sleeve or drive collar.
Not every cartridge seal uses removable clips: some use an internal spacer that stays in place in service, and the seal’s instructions say which.
Overtightening or Unevenly Tightening the Gland Bolts
Gland bolts pulled down too hard or unevenly distort the stationary face or tilt the mating ring, and the seal leaks across its faces.
Tighten the nuts alternately across the gland, in steps, to the torque in the seal’s instructions, and never extend the wrench for extra leverage. If the new seal leaks at start-up, tightening the gland further adds to any distortion.
Using the Wrong Lubricant on Elastomers
The wrong lubricant makes O-rings swell, and on a rubber bellows seal any grease or oil lets the bellows slip on the shaft, so the rotating face loses its drive.
EPDM swells in petroleum oils and greases, so petroleum jelly and mineral oil should never go on EPDM O-rings. Silicone grease suits most O-rings, but the safe rule is to use only the lubricant supplied with the seal.
Fit a rubber bellows with water or the fitting liquid its maker specifies.
Keep lubricant off the faces; they are not oiled unless the seal drawing calls for it.
Rough Handling of Seal Parts
Rough handling chips or cracks the seal faces and breaks graphite secondary rings, and a damaged face leaks from the first start. The damage comes from dropping, forcing or hammering parts, or from a face’s bore knocking against the shaft or a keyway as it slides on.
A seal that has been dropped should not be fitted even if it looks undamaged; have the maker check it.
Mistakes After Fitting the Seal
Once the seal is fitted, the remaining mistakes are made getting the pump ready to run.
Skipping Pump and Driver Alignment
A pump run out of alignment with its driver wears its seal faster, and the vibration it sets up reaches the seal faces and springs. Taking the pump apart to fit the seal can disturb the alignment, so check it again with the coupling back on, to the coupling or pump maker’s tolerance, before the first start.
Bolting the pump down and connecting its piping can also push the shaft off-center in the seal housing, so watch it there with a dial indicator while doing both. A change of more than 0.05 mm (0.002 in) as a hold-down bolt is tightened means a soft foot, and any change as a pipe is connected means pipe strain; correct either before the start.
Alignment set cold also shifts as the pump heats up, so recheck it at operating temperature and pressure. On hot services, align with the pump full of hot liquid.
Not Connecting the Seal Support System
A seal started before its flush, quench or barrier system is connected and working runs without the cooling, clean liquid or pressure it was specified with, and it fails early.
Check that each line goes to the right gland port and that its valves are open. Loop plans such as Plans 23, 52 and 53A also have to be vented when the pump is commissioned.
On a double seal, fill the barrier system and bring it up to pressure before the pump is pressurized. Keep it above seal chamber pressure, by the margin on the seal’s data sheet, at all times.
Starting the Pump Dry
Started before the seal chamber is full of liquid, the seal runs dry: with no liquid film between them, the faces overheat and score, often with an audible squeal.
Before the first start, open the valves to flood the pump with liquid, then vent air and vapor from the casing and the seal chamber. On a vertical pump the seal chamber is the high point and has to be vented on its own.
