Mechanical Seals
What a mechanical seal is, how it works, the types available, how to choose the right faces and elastomers, why seals fail early, and how to get the right one supplied. Written by the engineer who specifies them, not a catalogue.
Supplying the UK, Europe and Asia from Manchester.
In short: a mechanical seal stops process fluid escaping along a rotating pump shaft. Two lapped faces, one rotating and one stationary, run together with a microscopically thin fluid film between them, while O-rings or a bellows seal the static leak paths behind. Choosing one comes down to the fluid, temperature, pressure, solids and how critical the pump is, and most early failures come from that choice rather than the seal itself.
How a mechanical seal works
Every mechanical seal is built from the same six elements. Understanding which one is being overloaded on your pump is usually the fastest route to stopping repeat failures.
Rotating face
Turns with the shaft and is driven by springs, a drive band or a bellows. Usually the softer of the two faces, most often carbon graphite.
Stationary face (seat)
Fixed in the seal chamber and held square to the shaft. Usually the harder face: silicon carbide, tungsten carbide or ceramic.
Secondary seals
The O-rings, wedges or bellows that stop leakage behind the faces. Material choice here is what most chemical failures come down to.
Spring loading
Single spring, multi spring or wave spring, keeping the faces closed at start up and taking up face wear over the life of the seal.
Hardware
Sleeve, gland plate, drive collar and set screws on a cartridge seal, so the seal is set at the factory rather than on the bench.
Flush and support
API plans, quench, barrier fluid and seal support systems that control the environment the faces actually run in.
Types of mechanical seal
Cartridge mechanical seals
Pre assembled seal, sleeve and gland as one unit. Fitted without setting face load on site, so installation error drops sharply. First choice on process pumps.
More detailComponent mechanical seals
Rotating unit and separate seat, set on installation. Lower cost, ideal for general duty water, coolant and light chemical pumps.
More detailEdge welded metal bellows seals
No dynamic elastomer and no sliding secondary seal, so they suit hot oil, heat transfer fluid, steam and cryogenic duties.
More detailDouble and dual seals
Two seal pairs with a barrier or buffer fluid between them, for toxic, flammable, abrasive or hazardous media where zero emission matters.
More detailHygienic seals for food and pharma
FDA, EC1935/2004 and 3A compliant materials, crevice free geometry and clean in place resistance.
More detailSeals for obsolete pumps
Reverse engineered from a sample, drawing or the failed seal itself, or refurbished where the hardware is still sound.
More detailMechanical seal face materials
| Face pairing | Where it suits |
|---|---|
| Carbon vs silicon carbide | General process duty, good dry running tolerance for short periods, the default pairing on most pumps. |
| Silicon carbide vs silicon carbide | Abrasive slurries, solids in suspension, high pressure. Intolerant of dry running. |
| Tungsten carbide vs carbon | Shock loading and thermal cycling where SiC would crack, common on older heavy duty pumps. |
| Carbon vs ceramic | Clean water and low duty applications, low cost, poor thermal shock resistance. |
| Diamond coated faces | Severe abrasive or poor lubrication duties, high cost, long life where downtime is expensive. |
Full detail in the mechanical seal face material guide and the mechanical seal selection guide.
Why mechanical seals fail early
- •Dry running and loss of flush, giving heat checked faces and blistered carbon
- •Chemical attack on the secondary seal, swelling or hardening the O-ring
- •Solids in the seal chamber grooving the faces or clogging the springs
- •Misalignment, pipe strain and shaft deflection opening the faces
- •Cavitation and vibration chipping the seal faces
- •Wrong face pairing for the temperature and pressure of the duty
Specifying a seal for your duty
Fluid and temperature
Chemistry decides the elastomer before anything else. EPDM for steam and caustics, FKM for hydrocarbons, FFKM where both aggressive chemistry and high temperature meet. See FFKM vs FKM.
Pressure and speed
PV limits set the face pairing and whether a balanced seal is needed. High pressure or high surface speed rules out soft pairings and low cost seats.
Criticality
On a pump that stops production, a double seal with a support system costs less over a year than a single seal that fails twice.
Mechanical seals, questions answered
What is a mechanical seal?
A mechanical seal is a device fitted to a rotating shaft, usually on a pump or mixer, that stops process fluid escaping along the shaft. Two flat faces, one rotating with the shaft and one stationary in the housing, run against each other with a microscopic fluid film between them. Springs and system pressure hold the faces together while O-rings or bellows seal the static leak paths behind them.
How does a mechanical seal work?
The two lapped faces are pressed together by spring load and hydraulic pressure. A film of process fluid a fraction of a micron thick separates them, lubricating the interface and cooling it. That film leaks in vapour form, so a healthy mechanical seal leaks a small amount you will normally never see. When the film breaks down the faces run dry, overheat and fail.
What are the main types of mechanical seal?
Component seals, cartridge seals, edge welded metal bellows seals, and single, double or dual arrangements. Component seals are set on installation, cartridge seals arrive pre set, metal bellows seals remove the dynamic elastomer for hot or cryogenic duties, and double seals use a barrier fluid for hazardous media.
How long should a mechanical seal last?
A correctly specified and installed seal on a clean duty should run for several years. Where seals fail in months the cause is almost always specification, installation or the operating environment rather than the seal itself, which is why failure analysis pays for itself quickly.
Why is my mechanical seal leaking?
Steady dripping usually means the faces are damaged or worn: heat checking from dry running, grooving from solids, or chipping from vibration. Leakage that starts immediately after a rebuild is usually installation, pipe strain or an O-ring cut or chemically attacked during fitting.
What is the difference between a cartridge seal and a component seal?
A cartridge seal is supplied as one pre assembled unit with sleeve and gland, so the face load is set at the factory. A component seal has separate rotating and stationary parts that must be positioned correctly on installation. Cartridge seals cost more and fail far less often through fitting error.
Which mechanical seal do I need for my pump?
It depends on the fluid and concentration, temperature, pressure, shaft speed, solids content and how critical the pump is. Our seal selector asks those questions and returns a face pairing, elastomer and arrangement, or send the details and we will specify it directly.
Do you supply mechanical seals to fit other manufacturers?
Yes. We cross reference John Crane, EagleBurgmann, AESSEAL, Flowserve, Roten, Vulcan, Latty and Chesterton seals on dimensions, face materials, elastomer and arrangement. We are independent and not affiliated with those manufacturers.
Can a mechanical seal be repaired?
Often, yes. Where the sleeve, gland and metalwork are serviceable the faces can be relapped or replaced and the elastomers renewed, typically well below the cost of a new OEM seal.
What causes a mechanical seal to fail early?
Dry running, loss of flush, chemical attack on the elastomer, abrasives in the seal chamber, misalignment and pipe strain, thermal shock, and the wrong face pairing for the duty. Over 80 per cent of premature failures trace back to one of these rather than a manufacturing fault.
Ask about a mechanical seal
Send the pump make and model or the seal part number, plus fluid, temperature, pressure and shaft size, and you will get a specification back, including the cheaper option when a cheaper option will do.
Get a quote or technical advice
Send your sizes and duty and Andrew will come back with a price and an honest recommendation, including when a cheaper material will do the job.