6.6 Mechanical Seals: Components, Types, and Installation
Key Takeaways
- A mechanical seal's primary seal is the rubbing interface between a rotating face and a stationary mating ring, held together by springs and hydraulic pressure.
- A balanced seal reduces the net closing force on the faces, allowing higher pressure and lower heat generation than an unbalanced design.
- A pusher seal uses a dynamic secondary elastomer that slides along the shaft; a non-pusher bellows seal has no sliding secondary seal and resists hang-up on scaled shafts.
- A mechanical seal must never be run dry, because the face film that lubricates and cools the interface disappears in seconds.
- Setting clips or spacers on a cartridge seal must be removed after the gland is bolted and the shaft is fixed, and never before.
How a mechanical seal works
A mechanical seal replaces packing on a rotating shaft. Instead of squeezing a stationary material against a moving shaft, it puts two extremely flat, lapped faces together — one rotating with the shaft, one held stationary in the gland — and lets a microscopic film of process liquid separate them.
That film is the whole design. It is thin enough to leak only vapor, and thick enough to lubricate and carry away the heat generated at the interface. Run the seal dry and the film is gone in seconds; the faces then run metal-to-ceramic at high speed, heat-check, and crack.
Components
| Component | Function |
|---|---|
| Primary ring (rotating face) | Turns with the shaft; often carbon |
| Mating ring (stationary seat) | Fixed in the gland; silicon carbide, tungsten carbide, or ceramic |
| Secondary seals | O-rings, wedges, or bellows that seal the primary ring to the shaft and the mating ring to the gland |
| Spring or springs | Provide initial closing force before pressure builds; single spring or multiple small springs |
| Drive mechanism | Set screws, dents, or a drive band that turns the rotating face with the shaft |
| Gland plate | Bolts to the pump, holds the stationary element, and carries flush, quench, drain, and vent ports |
Face material pairs are chosen for the service: carbon against silicon carbide is the general-purpose combination; silicon carbide against silicon carbide handles abrasives at the cost of needing better lubrication.
Balanced and unbalanced
In an unbalanced seal, full stuffing box pressure acts to close the faces. It is simple and inexpensive, but above a moderate pressure the closing force generates too much face heat.
In a balanced seal, the shaft or sleeve is stepped so that part of the hydraulic closing force is relieved. The net closing force is lower, so the seal runs cooler and handles higher pressure and higher speed. Most refinery and chemical service uses balanced seals.
Pusher and non-pusher
- A pusher seal uses a dynamic secondary O-ring or wedge that must slide along the shaft or sleeve as the faces wear and as the shaft moves axially. It is simple and widely used, but the sliding elastomer can hang up on a shaft that has scaled, fretted, or corroded — and a hung-up secondary seal means the springs can no longer keep the faces closed.
- A non-pusher seal uses a metal or elastomeric bellows instead. There is no sliding secondary seal, so it tolerates scale and fretting well and is common in hot and crystallizing services.
Single, double, and cartridge configurations
| Configuration | Description | Use |
|---|---|---|
| Single | One set of faces | General service where a small vapor emission is acceptable |
| Dual, pressurized (double) | Two seals with a barrier fluid at higher pressure between them | Hazardous fluids; barrier leaks inward, so no process escapes |
| Dual, unpressurized (tandem) | Two seals with a buffer fluid at lower pressure between them | Containment and leak detection |
| Cartridge | Complete pre-assembled unit with gland, sleeve, and setting clips | Reduces installation error; the practical default |
Barrier versus buffer is a testable distinction: a barrier fluid is at a pressure above the process, so any leakage moves into the process; a buffer fluid is below process pressure and acts as a containment and detection medium.
Flush plans
The flush keeps the seal chamber cool, clean, and above the vapor pressure of the fluid. Common API-designated plans a mechanic will see stamped on the gland:
| Plan | Arrangement |
|---|---|
| Plan 11 | Recirculation from the pump discharge through an orifice into the seal chamber — the most common single-seal plan |
| Plan 13 | Recirculation from the seal chamber back to pump suction, used on vertical pumps |
| Plan 21 / 23 | Recirculation through a cooler to lower seal chamber temperature |
| Plan 32 | Clean flush injected from an external source, used in dirty or abrasive service |
| Plan 52 | Unpressurized buffer fluid circulated to a reservoir on a dual seal |
| Plan 53 | Pressurized barrier fluid circulated to a reservoir on a dual seal |
| Plan 62 | External quench such as steam or water on the atmospheric side |
Plugging the Plan 11 orifice — with weld slag, gasket material, or scale — is a classic cause of premature seal failure, because the seal chamber then heats up and the fluid flashes between the faces.
Installation procedure
- Verify the shaft or sleeve for runout, and check the seal chamber bore and face for squareness and for the correct dimensions. Excessive shaft runout will destroy any seal.
- Deburr and polish the shaft, especially at keyways, shoulders, and set screw marks. A burr will cut the secondary O-ring on the way to its seat.
- Work clean. Seal faces are lapped to within a few light bands of flat. Fingerprints, grit, and a dropped face are all scrap conditions. Handle faces by the outside diameter and leave the protective covering on until the last moment.
- Lubricate elastomers with a compatible lubricant — never a petroleum product on EPDM, and never a silicone lubricant where the manufacturer prohibits it.
- Set the working length exactly as specified. Too little spring compression and the faces open; too much and the faces overheat.
- Bolt the gland evenly in a crisscross pattern so the stationary face is not cocked.
- On a cartridge seal, remove the setting clips only after the gland is fully bolted and the impeller clearance is set, so the sleeve is locked to the shaft in the correct position. Removing them early loses the setting; leaving them in prevents the seal from functioning and will destroy it at startup.
- Vent the seal chamber before starting, especially on a vertical pump, so the faces are not asked to run in vapor.
- Never start the pump dry and never run it against a closed discharge for more than the manufacturer's permitted time.
Reading a failed seal
| Evidence | Cause |
|---|---|
| Faces heat-checked, cracked, or blistered | Dry running, flush loss, or the fluid flashing in the seal chamber |
| Carbon face worn heavily but evenly | Abrasives in the fluid; consider Plan 32 and harder faces |
| Secondary O-ring fretted a groove in the sleeve | Pusher seal hanging up, excessive vibration, or misalignment |
| Faces polished bright with no wear but leaking | Face distortion from gland bolting or piping strain |
| Springs packed with solids | Wrong flush plan for a crystallizing or solids-laden service |
Why does a pusher seal hang up on a scaled or fretted shaft while a metal bellows seal does not?
On a dual seal arrangement, what distinguishes a barrier fluid from a buffer fluid?
When should the setting clips on a cartridge mechanical seal be removed?