Common Causes of Mechanical Seal Overheating

A mechanical seal overheats when its faces generate more heat than the liquid around them carries away, and the most common reason is that the liquid film between the faces has been lost. The other causes either add heat at the faces (flashing, too much face load, a design or face pairing that runs hot, a fluid too thin to lubricate) or stop the heat from leaving (a failed flush or cooler, a pump running at low flow, a stuffing box that holds heat in).

A face that cracked after a sudden temperature change is a thermal shock problem, not steady overheating.

Loss of the Fluid Film Between the Faces

Losing the liquid film is the most common cause of a hot seal.

In a running pump the film is usually lost in one of these ways:

• No prime at start-up: the pump was started before the casing and seal chamber were full, or air was left at the top of the seal chamber.
• Entrained air or gas: a low sump level that lets the suction vortex, or an air leak on a suction line running under vacuum.
• Vapour trapped in the seal chamber: gas collects around the faces because the chamber has no vent or the flush does not sweep it.
• Flow interruption: the suction tank runs empty or a suction valve is closed with the pump still running.

Check venting of the seal chamber, priming before start-up, and the suction side: level, valves, and air leaks. The signs of a dry-running seal confirm whether this is what happened.

Flashing at the Seal Faces

Flashing is the liquid boiling inside the face gap, and it happens when the seal chamber pressure is below the fluid’s vapour pressure at the temperature of the faces. Friction keeps the faces hotter than the bulk liquid, so the liquid in the gap can boil while the process fluid is still below its boiling point.

The usual signs are popping or crackling from the gland, puffs of vapour at the atmospheric side, and leakage that comes and goes.

Check the vapour margin: seal chamber pressure against the fluid’s vapour pressure at seal chamber temperature. API 682, fourth edition, asks for a margin of at least 3.5 bar (50 psi); where that cannot be met, the seal chamber pressure must be at least 1.3 times the vapour pressure.

API 682 is written for hazardous, flammable and toxic services, but the same margin is a sound benchmark for any pump. If a seal falls short of it, start by checking the flush plan pressure.

Flush or Cooling Failure

When the flush or cooling system stops removing heat, the seal overheats even though the seal itself is sound. The common failures are:

• Blocked flush line or orifice: scale, debris or a clogged strainer or cyclone separator stops the flow.
• Closed or throttled valve: a flush or cooling-water valve left shut after maintenance.
• Wrong connection: the flush piped to the wrong gland port, or flowing in the wrong direction.
• Fouled cooler or jacket: a Plan 21 or 23 cooler, or a seal chamber jacket, scaled on the water side. Fouling becomes more common when cooling-water velocity through the coils is below 0.9 m/s (3 ft/s).
• Lost cooling water: the supply is off, or its pressure has dropped.
• Wrong or missing plan: a plan that recirculates hot product on a duty that needs cooling, or no flush at all where the seal needs one.
• Dual-seal reservoir not circulating: a worn pumping ring, low reservoir level, or thermosyphon lines with high points that trap gas.

Check flow and temperature, in and out. A flush line at ambient temperature on a hot pump means no flow, and a cooler whose outlet is as hot as its inlet is not cooling. For plan-by-plan checks, see troubleshooting common flush problems; the plans themselves are on our API plans page.

Low Flow and Off-BEP Operation

A centrifugal pump running far below its best efficiency point heats the liquid it pumps, and the seal chamber heats with it, so the seal runs hot with nothing wrong with the seal.

The usual reasons are a throttled discharge valve, a closed downstream valve that leaves the pump dead-headed, a minimum-flow bypass that is shut, or a pump sized for more flow than the system now takes.

To tell a hot pump from a hot seal, compare temperatures. If the casing and discharge are hotter than the suction, the pump is heating the liquid. If the casing is at process temperature and only the gland is hot, the heat is coming from the seal. Then read discharge pressure against the pump curve to see where the pump is running.

Over-Compression of the Seal Faces

Too much face load raises friction and heat at the faces, and it usually comes from a seal set at the wrong length. The usual ways it happens:

• Wrong working length on a component seal: the setting was measured from the wrong reference, or the drawing dimension was misread.
• Shaft moved after setting: the impeller was adjusted or tightened after the seal collar was locked, which pushes the rotating face into the stationary one.
• Cartridge set screws slipped: the sleeve moved along the shaft in service.
• Cartridge setting clips not used during impeller adjustment: the impeller was moved with the sleeve locked to the shaft, so the seal’s set length moved with it.

Check the set dimension against the stuffing box face and the seal drawing, and on a cartridge seal look for set-screw marks that have shifted on the shaft. Most of these come back to common installation mistakes.

Seal Design and Face Pairing

Some seals run hot because of how they are built: an unbalanced seal at high pressure, two hard faces running together, a face material that conducts heat poorly, or a high sliding speed all raise face temperature before anything has gone wrong.

At higher pressures a balanced seal is the usual fix for a hot unbalanced seal; the trade-offs are in balanced vs unbalanced seals.

Carbon against a hard face generates less heat than a hard/hard pair. Hard/hard belongs where carbon cannot survive: abrasives, very high pressure, or a high-viscosity barrier fluid that would blister a carbon face. More on the choice is in carbon vs silicon carbide seal faces.

Among hard faces, on the makers’ datasheets, sintered silicon carbide conducts about four times as much heat as 99.5 % alumina, and tungsten carbide ranges from about alumina’s level to near silicon carbide’s, depending on grade.

Sliding speed adds to all of this: at the same shaft speed, a larger seal diameter means a higher face speed and more friction heat.

Poor-Lubricity Fluids

Hot water is the poor-lubricity fluid that most often overheats a seal: by about 70 °C (160 °F) its viscosity is below 0.4 cP, too thin to hold a film between two hard faces.

The Fluid Sealing Association’s guidance for hot water service is to avoid hard/hard face pairs and run carbon-graphite as one of the faces wherever possible. Faces with hydropads or hydrodynamic surface features help hold a film, and a Plan 21 or 23 cooler lowers the temperature at the faces. This applies to HVAC pump seals and boiler-circuit pumps alike.

Heat Trapped in the Stuffing Box

A stuffing box that holds heat in around the seal makes a sound seal run hot, so the fault lies with the pump. These are checks on the pump:

• Fouled cooling jacket: a jacketed stuffing box that has scaled up, or has lost its water, no longer cools the seal chamber.
• No throat bushing: without a throat bushing to separate it from the casing, a cool flush mixes straight into the hot product and the seal chamber stays at process temperature.
• Dead-ended box: no flush and no recirculation port, so the liquid around the seal sits still and heats up.
• Tight bore: an old packing bore leaves too little clearance between the seal’s outer diameter and the bore, and too little liquid around the seal to take the heat.
• Heat conducted from the casing: in hot service, heat also reaches the seal through the shaft and the gland.
• Low specific heat product: oils carry less heat per degree than water, so the same face heat raises their temperature more and they need more flush flow.

How to Find the Cause on a Hot Seal

Work from what can be measured on the running pump toward what needs a teardown.

1. Flush flow and temperatures: confirm flow in the flush line and a temperature drop across any cooler. No flow or no drop points to a flush or cooling failure. Gas at the seal chamber vent points to loss of the fluid film.
2. Seal chamber pressure against vapour pressure: a margin below 3.5 bar (50 psi), or a chamber pressure below 1.3 times the vapour pressure, points to flashing at the seal faces.
3. Operating point: compare casing, suction and discharge temperatures, and read discharge pressure against the curve. A hot casing and a pump far left of BEP point to low flow.
4. Set dimension: measure the seal’s setting against the drawing and check the cartridge set screws. A short setting points to over-compression.
5. Pump side: check the jacket, throat bushing, flush or recirculation port, and bore clearance for heat trapped in the stuffing box.
6. Design and fluid: if everything above checks out, the seal is making too much heat for the duty. Look at balance, the face pair, and whether the fluid is hot water or another thin liquid.