A mechanical seal is one type of shaft seal. “Shaft seal” is the umbrella term for anything that seals a rotating shaft where it passes through a housing, and the mechanical seal is the type that seals between two lapped faces; lip (oil) seals, gland packing and labyrinths are the other types you meet on a pump.
When a supplier sets “shaft seal” against “mechanical seal”, it is using “shaft seal” as shorthand for the lip seal. Pump makers often use the term the other way: Grundfos calls its mechanical seals shaft seals. On a parts list, the seal’s position settles it: where the shaft leaves the pump casing it is a mechanical seal or packing, and on the bearing housing it is a lip seal or a labyrinth.
What Is a Shaft Seal
A shaft seal is any device that stops fluid escaping along a rotating shaft where it passes through a housing, and stops dirt getting in by the same route.
What Is a Mechanical Seal
A mechanical seal is a shaft seal that seals between two flat faces: one turns with the shaft, the other is held stationary in the gland, and springs plus the fluid pressure keep them pressed together. O-rings or other secondary seals seal each face to the shaft and to the gland, so the only leak path left is the gap between the faces.
The Difference Between a Shaft Seal and a Mechanical Seal
A mechanical seal seals across the gap between its own two faces, away from the shaft surface. The other shaft seals seal on the shaft itself or across a clearance around it:
- Lip seal: a flexible elastomer lip is fitted onto the shaft with a designed interference and seals directly on the shaft surface.
- Gland packing: braided rings are squeezed against the shaft or its sleeve by the gland bolts and seal on that surface, cooled and lubricated by a controlled leak.
- Labyrinth: a series of grooves runs around the shaft at a small clearance and throttles the leak without touching it.
Leakage
A healthy mechanical seal shows no visible leakage, while packing is set to drip on purpose.
Tightened until the leak stops, packing overheats and fails. Pump makers usually allow 2 to 60 drops per minute, set at the gland bolts.
Where regulations demand no visible leakage, as in California, 3 drops per minute from a mechanical seal counts as a major leak and the pump must come down for repair.
A lip seal holds oil with very little leakage until the lip wears. A labyrinth accepts a relatively high leak, set by its clearance.
Operating Limits
A lip seal is built for almost no pressure, 0 to 0.3 bar above atmospheric under ISO 6194-1, while a mechanical seal built to API 682 is covered up to 40 barg in its Category 2 and 3 seals.
The API 682 figures bound that standard only. Mechanical seals are built for duties outside them; the standard calls those engineered seals and leaves them outside its requirements.
Packing has no comparable figure. No industry-wide test sets its limits, so the pressure, speed and temperature ratings on a packing data sheet come from each maker’s own method and do not compare across brands.
The Fluid Being Sealed
Only the mechanical seal and packing are meant to hold the pumped fluid, and between the two the choice follows what a drip of that fluid costs you.
Lip seals and labyrinths hold the bearing oil.
Shaft and Sleeve Wear
Packing and lip seals wear the shaft or sleeve they run on; a mechanical seal wears its own faces, and a labyrinth wears nothing because it does not touch.
On a pusher mechanical seal, the O-ring that slides forward as the faces wear can fret a narrow band of the sleeve beneath it.
Cost and Maintenance
Packing and lip seals cost least to buy; a mechanical seal costs more and takes more skill to repair or replace.
Packing needs routine attention: the gland is tightened periodically to hold the leak at an acceptable level, and new rings go in as the old ones wear until the box needs a full repack.
A mechanical seal has no adjustment. It is checked for leakage and replaced when the leak exceeds the maker’s limit. Plants that run programmes to extend the time between repairs average three years or more of seal life.
Lip seals are cheap but replaced more often than non-contacting bearing isolators, with a typical life of about 1,000 to 3,000 hours on pump bearing housings. A properly designed labyrinth needs no lubrication or maintenance.
How to Choose the Right One
Put a mechanical seal on the pumped fluid unless a steady drip of it is acceptable, and a lip seal or a labyrinth on the bearing housing:
- Mechanical seal: the pumped fluid wherever it is hazardous, flammable, toxic, expensive to lose, or must not drip.
- Gland packing: the pumped fluid where a steady drip is acceptable and someone is on hand to adjust the gland; lowest purchase price and simplest repair.
- Lip seal: oil in a bearing housing near atmospheric pressure, accepted as a part you replace regularly.
- Labyrinth: the same bearing-housing job where lip-seal wear, shaft fretting or frequent replacement is the problem.
A lip seal cannot stand in for a mechanical seal at the stuffing box. A packed pump, on the other hand, can usually be upgraded to a mechanical seal once the drip or the gland adjustment is no longer acceptable.