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Mechanical seal

Written and reviewed by Paul Foster, Founder·Last updated 3 August 2026

A mechanical seal is the shaft seal on a pump: two flat faces, one turning with the shaft and one fixed in the housing, pressed together with a very thin film of the pumped liquid between them. That film lubricates the faces and does the sealing. Lose it and the seal fails.

In more detail

It replaced gland packing on most duties. Packing is braided rope compressed round the shaft by a follower, and it seals by leaking deliberately and being adjusted as it wears. A mechanical seal seals end face to end face instead, leaks a fraction as much, and needs no routine adjustment.

What it will not tolerate is running without liquid. The seal is the weak point in almost any pump, and dry running is how most of them go.

The liquid film between the faces is the whole mechanism

The two faces are lapped flat to optical tolerances and held together by a spring, a bellows or the pressure in the seal chamber itself. In normal running they never quite touch. A very thin film of the pumped liquid sits between them, carries the load, lubricates the rubbing surfaces and takes the heat away. That film is the bearing and the seal at the same time. You cannot pick a seal without knowing what the pump is moving.

A mechanical seal needs liquid to lubricate it. Take the liquid away and the faces run dry against each other: friction rises, the faces heat, the elastomer behind them hardens or extrudes, and the seal is gone. That is why running dry kills centrifugal and rotary lobe pumps. The wet end often survives it and the seal does not.

The design cause is suction conditions that were never right in the first place. The everyday cause is somebody shutting a valve and forgetting it is shut, then starting the pump. No design fault involved, same result. What happens when a submersible pump runs dry works through that on a wet well duty.

Single, flushed, double and cartridge, and what each one buys

The trade names four things, and only three of them are arrangements. API Standard 682, Pumps — Shaft Sealing Systems for Centrifugal and Rotary Pumps, is where the vocabulary comes from, and it names three: a single seal, a dual unpressurised seal and a dual pressurised seal.

Arrangement	What it is	What it buys	What it costs
Single (Arrangement 1)	One pair of faces, lubricated by the process liquid	The simplest and cheapest build, nothing to plumb	Only suits a medium that lubricates well and a small leak to atmosphere you can live with
Single with a flush	The same seal with liquid piped into the seal chamber. Plan 11 takes it from the pump discharge; Plan 32 injects clean liquid from an external supply	Keeps the faces cool and clean where the medium is abrasive, polymerising or a poor lubricant	Plan 32 dilutes the process with whatever you inject, and you have to supply and control it
Dual unpressurised (Arrangement 2)	Two pairs of faces with a buffer fluid between them, held below seal chamber pressure	The outer seal contains what the inner one passes, and the buffer reservoir shows you the inner seal has gone before it becomes a leak	Not a zero emission answer. Process liquid still reaches the buffer
Dual pressurised (Arrangement 3)	Two pairs of faces with a barrier fluid held above seal chamber pressure	Nothing escapes to atmosphere, because leakage runs inward instead. The containment answer for a toxic or hazardous medium	A barrier system to build, fill, pressurise and maintain, and barrier fluid ends up in your product

Cartridge is not a fourth arrangement. It describes how the seal arrives: faces, springs, elastomers, sleeve and gland assembled and set at the factory as one unit, so nothing has to be measured or set on site. A component seal is fitted piece by piece, and setting the working length wrong is a common way to wreck a new seal on its first start. Any of the arrangements above can be supplied as a cartridge, and on the pumps we quote it usually is. The Grundfos CR ships with a maintenance-free cartridge seal that is changed as a unit, and the Grundfos SE carries a double mechanical cartridge seal whose two seals lift out together, so it cannot be fitted misaligned.

How mechanical seals actually fail

A seal rarely fails on its own account. It fails because something upstream of it changed, and the seal is where the change shows up first.

It ran dry. The fastest of them. A seal that would have lasted years does not survive a long run against an empty suction.
The elastomer was wrong for the medium. The O-ring swells, hardens or perishes long before the faces show anything. The seal then leaks with both faces still in good order, which is why the elastomer gets blamed last.
Grit. Abrasive particles get between the faces and lap them into each other. This is what wears seals out early on a sewage works where the grit removal is not doing its job, and it is the case for a flush.
Heat. The seal, not the pump body, usually fixes the temperature limit. Grundfos publishes the CR's liquid temperature window by seal code rather than by pump, and Ebara's standard ceramic, carbon and NBR seal on the 3S runs -10°C to +90°C while the harder face builds take it to +110°C. Run outside the published range and seal life shortens, which is the failure everyone then blames on the pump.
Cavitation and vibration. A centrifugal pump run off the end of its performance curve puts vibration through the shaft, and the faces are what cannot cope with it. The fault is in the suction conditions and it presents as a failed seal.
Installation. Working length set wrong, a face contaminated by a fingerprint, a shaft that is not running true. Cartridge seals exist mostly to remove this category.

If a seal has failed twice on the same pump, the seal is not the problem. Look at the suction side, the medium and the duty point before you fit a third one.

The question that decides the arrangement is what a failure would do

Sealing comes late in the questions we ask on the phone, and it is one question rather than a specification exercise: single, flush, double or cartridge, and what a seal failure would do to the product if it happened. Answer that honestly and the arrangement follows.

On clean water in a plant room, a failed single seal makes a mess and someone mops it up. On a food line, the same failure puts barrier fluid or product on the wrong side of a hygienic boundary and you lose a batch. On a chemical duty it is different in kind: a failed seal on water makes a mess, a failed seal on a chemical makes an incident. That is why corrosive duties get the seal designed out wherever the process allows it, and why chemical plant specifies containment at selection rather than bolting it on afterwards.

Where a seal choice is genuinely difficult, the seal manufacturer's advice is the one to take. Saying so is not an admission, it is the practice. Seal manufacturers hold face and elastomer data on media that no pump datasheet covers, and on a hard duty we go to them rather than guessing on your behalf.

What to send us when the seal is the problem

Send the medium, its temperature and what solids it carries, alongside the flow and the head. Those four decide the faces, the elastomers and whether the duty wants a flush or a double seal, and they matter more here than the performance curve does.

Say what a leak would reach. A drain, a food contact surface, an operator, a bunded area: the answer changes the arrangement, and it is the thing enquiries leave out most often. How to select a pump walks the full sequence the seal sits inside. If your existing pump is correctly specified and the seal keeps going because the tank runs empty, that is the answer you will get from us, and it is cheaper than a new pump.

What a seal specification actually names

The face materials: One face is usually hard and the other softer, so it can bed in. Carbon against silicon carbide runs quietly and forgives a moment of poor lubrication. Silicon carbide against silicon carbide takes abrasives and lasts longer, and is less forgiving when it runs dry. Tungsten carbide is the other hard face you will see quoted.
The elastomers: The O-rings behind the faces are specified against the medium, not against the casing. EPDM, FKM, sold as Viton and PTFE cover most duties between them, and their chemical windows barely overlap. This is the part of a seal code people skim, and it is a common reason a new seal fails early.
Balanced or unbalanced: An unbalanced seal takes the full seal chamber pressure as closing force on the faces. A balanced seal steps the shaft or the sleeve so part of that force is carried elsewhere, which lets the same faces run at higher pressure and generate less heat. Above a modest chamber pressure, balanced is what gets specified.
The principal dimensions: Seals for European pumps follow BS EN 12756, which sets the principal dimensions, the designation and the material codes for a seal fitting a cavity to ISO 3069, as found on centrifugal pumps to EN 733 and EN 22858. It replaced DIN 24960, which is withdrawn, and it is why a seal is quoted as a string of letters.

Frequently asked

What is a mechanical seal on a pump?

It is the seal that stops the pumped liquid escaping where the shaft leaves the pump. Two flat faces run against each other, one turning with the shaft and one fixed in the housing, separated by a thin film of the liquid being pumped. That film lubricates the faces and seals the gap.

How does a mechanical seal work?

Springs, a bellows or seal chamber pressure hold two lapped faces together. In normal running the faces do not touch. A very thin film of the pumped liquid sits between them, carries the load and takes away the heat the rubbing generates, so the film is doing the sealing and the lubricating at once.

What causes a mechanical seal to fail?

Running dry is the fastest cause, and it usually traces back to suction conditions or a closed valve. After that come the wrong elastomer for the medium, grit lapping the faces, running above the seal's temperature range, vibration from cavitation, and installation errors such as the working length being set wrong.

What is the difference between a single and a double mechanical seal?

A single seal has one pair of faces lubricated by the process liquid. A double seal has two pairs with a fluid between them. Held below seal chamber pressure it is a buffer and contains what the inner seal passes; held above it is a barrier and nothing reaches atmosphere.

What is a cartridge mechanical seal?

A seal supplied as one pre-assembled unit, with the faces, springs, elastomers, sleeve and gland set at the factory. Nothing is measured or adjusted during fitting, so the commonest installation errors are designed out. Cartridge describes how the seal is supplied, not which arrangement it is.

Can a mechanical seal run dry?

No, not for long. The faces rely on a film of the pumped liquid to lubricate and cool them. Without it, friction heats the faces and the seal fails quickly. Some dual seals tolerate it because the barrier fluid keeps the outer faces lubricated, but that is a design decision, not a default.

How long does a mechanical seal last?

There is no catalogue figure, because seal life is set by the duty rather than by the seal. A correctly specified seal on a clean, well-flooded duty can outlast the bearings. The same seal on grit, or on a pump that runs its tank empty, can go in weeks. Fix the duty, not the seal.

What do you need to know to supply a replacement mechanical seal?

The pump model and the serial number from the nameplate, plus the medium and its temperature. Model and serial are what the manufacturer needs to look up the build and quote the right kit. Photograph the nameplate and send the picture with the enquiry.

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