A mechanical seal works by pressing two lapped flat faces together — one rotating, one stationary — with a microscopic film of process liquid between them. That film both lubricates and seals: too thick and it leaks, too thin and the faces touch, overheat and crack.
Whether the film survives is decided by the force balance: closing forces (hydraulic + spring) versus the opening force of the film. The seal's balance ratio tunes this — balanced seals cut the closing force for high-pressure duties; unbalanced ones are simpler but only suit low pressure. The product P×V (face pressure × speed) must stay under the faces' limit.
API 682 standardises the rest: single vs dual seals, and the flush plans (Plan 11, 23, 32, 52, 53…) that keep the faces clean, cool and lubricated for the service.
1 · Why a mechanical seal
Wherever a pump shaft passes through the casing, there is a gap the process fluid wants to escape through. The old answer was gland packing — rings of braided rope compressed around the shaft. Packing always leaks a little by design (it needs the drip to stay cool), wears the shaft, and needs constant adjustment. For anything hazardous, hot, or environmentally regulated, that controlled leak is unacceptable.
The mechanical seal replaced it. Instead of sealing against the rotating shaft's circumference, it seals against a flat face perpendicular to the shaft. One face rotates with the shaft; the other is fixed to the casing; springs hold them together. The sealing happens across the thin gap between them — and because that gap is measured in microns, the leakage is essentially vapour.
2 · Anatomy of a seal
Strip away the housing and every mechanical seal has the same handful of parts:
- Rotating face — turns with the shaft (via the sleeve).
- Stationary face — fixed in the gland. One face is usually hard (silicon carbide, tungsten carbide), the other often carbon.
- Springs — provide the initial closing force that keeps the faces together at start-up and zero pressure.
- Secondary seals (O-rings) — seal the small leak paths around the faces and shaft.
- Sleeve & gland — the cartridge hardware that carries it all and bolts to the pump.
3 · How the faces seal — the film and the balance
The faces are lapped flat to within a few light bands (well under a micron). Pressed together and spun, they don't run dry — process fluid is dragged into the gap and forms a hydrodynamic / hydrostatic film a fraction of a micron thick. That film carries the load, lubricates the faces, and conducts away the friction heat. The entire art of sealing is keeping that film in a narrow happy band:
- Film too thick → the faces lift apart → the seal leaks.
- Film too thin → asperities touch → friction, heat, and the liquid in the gap flashes to vapour → the faces overheat, crack and fail in seconds.
What sets the film thickness is the force balance across the faces. Pushing them together: the spring force, plus the process pressure acting on the seal's hydraulic area. Pushing them apart: the pressure of the film itself, spread across the face. The net is a face contact pressure, and the classic seal balance equation captures it:
Then the heat-and-wear duty is the product of that pressure and the sliding speed — the PV value:
Interactive 1 — Face balance & PV
Live modelPush the process pressure up on an unbalanced seal (balance ratio > 1) and watch PV blow through the limit. Then drag the balance ratio down to ~0.75 — a balanced seal — and watch the same duty come back into the safe zone.
PV vs process pressure
At the faces
Pface = Δp(B−K) + Pspring with K=0.5, Pspring=2 bar; V = π·0.06·n; PV limit set to an indicative 70 bar·m/s. Real PV limits depend on the face pair (carbon / SiC / tungsten carbide) and are manufacturer data — use them for design.4 · Balanced vs unbalanced
The interactive shows the single most useful seal-design idea. An unbalanced seal (B > 1) exposes the full face to closing pressure: simple, cheap, fine at low pressure — but the closing force, and so the PV, climbs steeply with process pressure. A balanced seal steps the sleeve so only part of the area sees the closing pressure (B ≈ 0.65–0.75), cutting the net closing force so the faces run cooler at high pressure.
| Unbalanced (B > 1) | Balanced (B < 1) | |
|---|---|---|
| Closing force | High — full pressure on faces | Reduced by the balance step |
| Pressure range | Low (typ. < ~10–14 bar) | High (the standard for duty service) |
| Cost / complexity | Lower | Higher (stepped sleeve) |
| Heat at the faces | Rises fast with pressure | Stays manageable |
5 · Single, dual, and the API 682 flush plans
Getting the faces right is only half the job. The faces also need to be kept clean, cool and lubricated — and for hazardous fluids, any leakage has to be contained. API 682 (identical to ISO 21049) is the petroleum-industry standard that codifies this into seal arrangements and piping plans.
Arrangements
- Arrangement 1 — single seal. One pair of faces. The default for clean, benign services.
- Arrangement 2 — dual, unpressurised. Two seals with a buffer fluid between them at low pressure; the outer seal is a backup and lets you detect inner-seal leakage. For nastier but not extreme fluids.
- Arrangement 3 — dual, pressurised. A barrier fluid between the seals held above process pressure, so any leak is clean barrier fluid inward — zero process emission. For toxic, flammable or environmentally critical service.
The flush plans
The piping plans route fluid to the seal to do a specific job. The common ones:
| Plan | What it does | For |
|---|---|---|
| Plan 11 | Recirculates from pump discharge through an orifice into the seal chamber | The clean-service default — flushes and pressurises the faces |
| Plan 13 / 14 | Recirculates from the seal chamber back to suction (or both) | Vertical pumps; venting |
| Plan 21 / 23 | Recirculated flush passed through a cooler (23 = closed loop, far more efficient) | Hot fluids near their vapour point |
| Plan 32 | Clean, cool fluid injected from an external source | Dirty / abrasive / slurry service |
| Plan 52 | Unpressurised buffer fluid in a reservoir (Arrangement 2) | Hazardous, non-flashing fluids; leak detection |
| Plan 53A/B/C | Pressurised barrier fluid above process pressure (Arrangement 3) | Toxic / flammable / zero-emission service |
| Plan 62 | External quench (steam/water) on the atmosphere side | Fluids that crystallise, coke or freeze on leakage |
Choosing among them follows the fluid. The selector encodes the usual logic:
Interactive 2 — Seal arrangement & flush-plan selector
Decision tool6 · How seals fail (and what it tells you)
Because the film is so unforgiving, most seal deaths trace to a handful of causes — and a seal that comes out tells a story worth feeding into your root cause analysis and FMECA:
- Dry running — loss of flush or running a pump dry/dead-headed (see Part 2): the film vanishes, faces flash-heat and crack. Heat-checked, cracked faces.
- Off-BEP operation & vibration — running far from the best efficiency point (Part 3) drives shaft deflection and chatters the faces. Chipped edges, fretting.
- Contamination — solids in the film score and wear the faces. Grooved faces → wrong/absent flush plan.
- Chemical attack / wrong materials — swollen or hardened O-rings, corroded faces.
- Thermal — flashing across the faces on hot duty → needed a Plan 23.
The reliability point: a mechanical seal almost never fails on its own — it reports a problem elsewhere. Dry-running, off-BEP operation, a blocked flush, the wrong plan. Fix the cause the seal is pointing at, and seal life multiplies. That is exactly the chain the Bluestream toolbox is built to follow.
Key takeaways
- A mechanical seal seals on a micron-thin film between two lapped faces — too thick leaks, too thin cooks.
- The balance equation
Pface = Δp(B−K)+Pspringsets the face load; keep PV under the face limit. - Balanced seals for high pressure, unbalanced only for low pressure.
- API 682 arrangements & plans keep the faces clean, cool, lubricated and contained — match the plan to the fluid.
- Seal failures point upstream — to dry running, off-BEP duty, or the wrong flush. Treat them as symptoms.