Mechanical seals: how they work, cartridge and dual seals, flush and barrier plans, how seals fail, and fitting them so they last
Mechanical seals from the maintenance side: the faces, springs and O-rings and how a seal runs on a liquid film, component and cartridge seals, single and dual arrangements, the common API 682 flush, buffer and barrier plans, how seals fail and what the faces show, and the pump condition and fitting checks that decide seal life. With a half section drawing of a cartridge seal.
Key takeaways
- A mechanical seal is two flat faces, one turning and one still, held together by springs and running on a liquid film about a micron thick.
- Cartridge seals take the setting out of the fitter's hands. Dual seals with a pressurised barrier keep the pumped liquid off the faces altogether.
- The flush or barrier plan matters as much as the seal. Most are named by their API 682 plan number.
- Most seals fail from the duty or the pump, not the seal: dry running, solids at the faces, vibration and misalignment.
- A seal that fails repeatedly is a symptom. Look at the faces and O-rings, record what was found, and check the pump and its operating point.
A seal on a process pump fails every few months. Each time, the leaking seal is replaced with the same model. The old one goes in the bin. Nobody looks at its faces, which would show heat checking from dry running, and nobody connects the failures to the tank that is drawn down low on night shift until the pump loses suction.
This guide covers mechanical seals from the maintenance side: the parts, component and cartridge, single and dual arrangements, the common flush and barrier plans, how seals fail and what the faces show, and fitting them so they last. It sits beside the guides to centrifugal pumps, slurry pumps and shaft alignment. The drawing is schematic.
How a mechanical seal works
The seal turns a leak along a moving shaft into a leak across two flat faces, which can be made very small. The faces are lapped flat to within about a micron. Liquid pressure and spring force push them together, and the liquid film between them keeps them from touching. If the film is lost, from dry running or vapour at the faces, the faces heat and crack within minutes.
| Part | What it does | Notes |
|---|---|---|
| Rotating face | The ring that turns with the shaft and runs against the stationary face | Often carbon graphite, or silicon or tungsten carbide for abrasive duties |
| Stationary face (seat) | The ring held in the gland plate that the rotating face runs on | Usually the harder of the two faces |
| Springs or bellows | Hold the faces closed when the pump is stopped and take up face wear | Clog in solids or crystallising liquids, which lets the faces open |
| Secondary seals | O-rings, wedges or bellows that seal each face to the shaft or gland | Chosen for the liquid and temperature, and the first thing to harden or swell |
| Sleeve and drive collar | Carry the rotating parts on the shaft and drive them | Set screws or a clamp collar hold the setting |
| Gland plate | Bolts to the pump and carries the stationary face and the flush connections | Must sit square to the shaft |
Types of mechanical seal
Seals are described by how they are built and how many sets of faces they have. A cartridge seal is now the default for most pumps. Single seals suit most clean duties. Dual seals are used where leakage would be dangerous, where the liquid would damage the faces, or where a backup is wanted.
| Type | What it is | Where it suits |
|---|---|---|
| Component seal | Loose parts fitted onto the shaft and set by measurement | Cheaper, but the setting depends on the fitter |
| Cartridge seal | Assembled and set on its own sleeve, slid on and clamped | Faster and more consistent to fit, the setting clips come off before start |
| Single seal | One pair of faces, sealed by the pumped liquid | Most water and process duties |
| Dual unpressurised | Two pairs of faces with a buffer liquid at lower pressure between them | Contains leakage of hazardous liquids, the outer seal as a backup |
| Dual pressurised | Two pairs of faces with a barrier liquid at higher pressure than the seal chamber | The pumped liquid never crosses the faces, so it suits abrasive, toxic or sticky duties |
| Slurry seal | Heavy section hard faces, springs outside the liquid, open seal chamber | Built to survive solids, some designs run without flush water |
Flush, buffer and barrier plans
A seal lives or dies by its environment: the temperature, the pressure and the cleanliness of the liquid at the faces. Flush plans control that. API 682 numbers the arrangements, and the numbers are used well outside the oil and gas plants it was written for. The plan should be on the pump datasheet and in the maintenance log, so a changed plan is visible.
| Plan | What it does | Used for |
|---|---|---|
| Plan 11 | Liquid from the pump discharge through an orifice into the seal chamber | The default for clean liquids, cools and flushes the faces |
| Plan 13 | From the seal chamber back to the pump suction | Vents vertical pumps and those with a high chamber pressure |
| Plan 23 | A pumping ring circulates seal chamber liquid through a cooler | Hot liquids, keeps the faces cool without cooling the whole flow |
| Plan 32 | Clean liquid injected from an outside source | Dirty or abrasive liquids, at the cost of dilution |
| Plan 52 | A vented reservoir supplies buffer liquid between dual seals | Dual unpressurised seals, a level or pressure rise shows the inner seal leaking |
| Plan 53A | A reservoir pressurised by gas supplies barrier liquid | Dual pressurised seals, a falling level shows a seal leaking |
| Plan 54 | An outside system circulates pressurised barrier liquid | Several seals, or where a reservoir is not enough |
| Plan 62 | A quench of water or steam on the atmospheric side | Stops crystals or coke forming at the outer edge of the faces |
A seal and its flush plan are one system. A good seal with no flush or a blocked orifice lasts no longer than a bad one.
How mechanical seals fail
A failed seal is evidence. The faces show heat, wear, chips or deposits, and the O-rings show chemical attack or extrusion. Inspecting them, photographing them and recording what was found against the pump is the start of any root cause analysis. Often the answer is in the pump or how it is run, such as a pump running far from its best efficiency point, bearings failing, or a coupling out of alignment.
| Failure | What the seal shows | Likely cause |
|---|---|---|
| Dry running | Heat checking, cracks or a glazed, blistered carbon face | Suction lost, a pump run against a closed valve, or no flush |
| Abrasive wear | Grooves in the faces and worn sleeve | Solids between the faces, flush missing or too dirty |
| Chipped face edges | Chips round the face edges | Vibration, cavitation, misalignment or a bent shaft |
| Secondary seal attack | O-rings hard, cracked, swollen or extruded | The wrong elastomer for the liquid, temperature or pressure |
| Hang up | Faces held open, springs packed with solids or crystals | Solids or crystals forming on the atmospheric side, no quench |
| Leak from the first start | Leaks as soon as the pump runs | Setting clips left in, faces damaged or dirty at fitting, wrong setting |
Fitting and maintaining seals
Seal life is set at fitting. The pump has to be in good condition, the faces clean and undamaged, the setting right, and the pump and motor aligned after the pipework is connected. A work instruction with the checks and values for each pump keeps fitting consistent between crews.
| Task | What it involves | When |
|---|---|---|
| Pump condition | Shaft runout, end float and the seal chamber face square to the shaft | Before a seal goes in, since a worn pump destroys new seals |
| Fitting | Clean faces, lubricate O-rings as the maker says, cartridge setting clips removed | At every seal change, by the job plan |
| Alignment | Pump and motor aligned once the pipework is connected | After every seal change that disturbs the coupling |
| Flush and barrier | Flow, pressure and reservoir level checked against the setting | On rounds |
| Leakage | Drips or vapour at the gland compared with the allowance | On rounds, recorded so a trend shows |
| Failure record | Each seal failure recorded with the face and O-ring condition | Every change, so repeat failures point to the cause |
Common mistakes
- Replacing failed seals without looking at the faces and O-rings.
- Leaving cartridge setting clips in place at start up.
- Running pumps dry, or against a closed valve, even briefly.
- Fitting a new seal to a pump with a bent shaft, worn bearings or excessive end float.
- Blocked or isolated flush lines, or barrier reservoirs left empty.
- The wrong O-ring material for the liquid or temperature.
- No seal failure history, so bad actors are never found.
Common questions
How does a mechanical seal work?
A mechanical seal stops liquid leaking where the shaft leaves the pump. A rotating face fixed to the shaft runs against a stationary face held in the gland plate, and springs or bellows hold the faces together. A very thin film of liquid between the faces lubricates them, so a small amount of vapour or liquid crosses the faces, usually too little to see. O-rings or bellows seal each face to the shaft and the gland.
What is the difference between a gland packing and a mechanical seal?
Gland packing is a set of soft rings compressed around the shaft sleeve, which must leak a steady flow to stay cool and lubricated, and is adjusted by tightening the gland. A mechanical seal uses two lapped faces and leaks very little, needs no adjustment, and causes less sleeve wear, but costs more and is less forgiving of solids, dry running and a worn pump.
What is a cartridge seal?
A cartridge seal is a mechanical seal supplied assembled on its own sleeve with its gland plate, preset at the factory. It slides onto the shaft, bolts to the pump and is clamped in place, then the setting clips are removed. It removes most of the measurement and setting that a component seal needs, which makes fitting faster and more consistent.
What are seal flush plans?
Seal flush plans are standard arrangements of piping and equipment that keep a mechanical seal's environment right, such as flushing the faces with process liquid, injecting clean liquid, cooling the flush, or supplying buffer or barrier liquid between dual seals. They are numbered in API 682, so Plan 11 is a flush from pump discharge, Plan 32 is a clean flush from an outside source, and Plan 53A is a pressurised barrier reservoir.
Why do mechanical seals fail?
Common causes are running dry, solids getting between the faces, heat, the wrong elastomer for the liquid, vibration and misalignment, and errors at fitting such as leaving the setting clips in or damaging the faces. Pumps running far from their best efficiency point also shorten seal life. The faces and O-rings of a failed seal show which of these happened, so they should be inspected and recorded before they are thrown away.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
- API Standard 682, Pumps, shaft sealing systems for centrifugal and rotary pumps (American Petroleum Institute)
- Hydraulic Institute standards for rotodynamic pumps (Hydraulic Institute)
- ISO 14224:2016 Collection and exchange of reliability and maintenance data for equipment (ISO)
Related case studies and tools
- Predictive maintenance and condition monitoring (case study)
Related reading
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