Mechanical seal: what it is, how it works and why it fails
The mechanical seal is the device that seals the rotating shaft's passage through the pump casing using two flat faces — one spinning with the shaft, one stationary — sliding against each other separated by an ultra-thin liquid film. It is the component that fails most in centrifugal pumps, and most of those failures are not the seal's fault. This guide explains how it works, the types, and how to end recurring replacement.
Updated on July 8, 2026 · Reviewed by Hydro Pumps engineering
What it is and what it is for
Every centrifugal pump has a design dilemma: the shaft must cross the casing to spin the impeller, and the pressurized liquid inside wants to escape through exactly that hole. The mechanical seal solves the dilemma with an elegant idea: instead of squeezing a material against the shaft (as packing does), it places two extremely flat surfaces — lapped to millionths of a meter — sliding against each other.
One face spins with the shaft; the other stays fixed to the casing. Between them, a film of the pumped liquid itself, thinner than a human hair divided by fifty, lubricates and cools the contact. The resulting leakage is so small it evaporates before dripping — in practice, total sealing.
That delicate equilibrium is the seal's strength and weakness: while the film exists, it lasts for years; if the film breaks for a few seconds, the faces run dry against each other and the seal dies in minutes.
The components of a seal
A mechanical seal is a set of five component families working together:
Rotating face. Spins with the shaft, pressed against the stationary face. One of the two sliding surfaces.
Stationary face (seat). Mounted in the gland or seal chamber, receives the rotating face's contact.
Force element (springs or bellows). Keeps the faces together despite wear, vibration and axial shaft movement.
Secondary seals. O-rings, wedges or elastomer bellows sealing the static paths — between face and shaft, between seat and casing.
Hardware. Sleeves, drive collars and glands that transmit torque and position the assembly.
Types: single, double, cartridge and balanced
The choice of type is dictated by the fluid and the operating conditions:
Single seal. One pair of faces, lubricated by the pumped fluid itself. Standard for water and clean, non-hazardous fluids — most industrial applications.
Double seal. Two pairs of faces with a clean barrier fluid between them. Mandatory when the product is toxic, flammable, abrasive or crystallizes in contact with air: the process never touches the environment.
Cartridge seal. Any configuration, factory pre-assembled and pre-set on a single sleeve. Eliminates setting measurements at installation — the assembly error that kills most conventional seals. Costs more to buy and less across the life cycle.
Balanced seal. Geometry that reduces the hydraulic force pushing the faces together. Necessary at high pressures: less load means less heat generated in the film and longer life.
Materials: faces and elastomers
Faces work in pairs of different materials, chosen for hardness, thermal conductivity and chemical compatibility. The most common combinations: carbon-graphite against silicon carbide (the general-duty pair, self-lubricating and economical), silicon carbide against silicon carbide (abrasive fluids — maximum hardness) and tungsten carbide where resistance to mechanical shock matters.
Secondary seals follow the fluid's chemistry: NBR for oils and water at moderate temperatures, EPDM for hot water and steam, fluoroelastomers for aggressive chemicals and higher temperatures. A seal with perfect faces and an o-ring incompatible with the fluid fails just the same — the elastomer swells, hardens or dissolves.
That is why specifying a seal by 'model equivalence' without checking materials is gambling: two seals of identical dimensions can have completely different internal chemistry.
Why seals fail early
The design life of a properly applied seal is measured in years. When a seal lasts months, there is a systemic cause — and it is rarely in the seal:
Dry running. Poor priming, empty reservoir or an air pocket at startup: seconds without a liquid film are enough to crack the faces by thermal shock.
Misalignment and vibration. A misaligned shaft or unbalanced impeller makes the faces 'slam' thousands of times per minute, breaking the film and hammering the secondary seals.
Cavitation. Cavitation's pressure pulses destroy the lubrication film. A seal failing repeatedly in a noisy pump is a symptom, not a cause.
Incorrect installation. Wrong setting measurement, faces contaminated with grease, an o-ring cut at assembly — the seal is born condemned.
Fluid out of specification. The process changed, temperature rose, solids appeared — and the seal specified for the old condition no longer fits the new one.
Mechanical seal or packing?
Packing — rings of braided material compressed around the shaft — is the century-old solution the mechanical seal came to replace in most applications. It still has its place: tolerant of abrasive solids, simple to maintain and needing no special seal chamber. But it charges two permanent prices: it must leak continuously to avoid burning (the drip is a requirement, not a defect) and it rubs against the shaft, consuming energy — a tight packing set can consume several times the power a balanced seal consumes in the same position.
The packing-to-seal conversion pays off quickly when the fluid is expensive or treated (every leaked drop is money), when the product is hazardous (leakage is risk, not cost) or when recurring corrective maintenance of packing and shaft sleeve weighs on the budget.
Our independence helps here: we supply both systems, so the recommendation comes from the application analysis — not from the catalog we happen to sell.
How to specify the correct seal
To specify a seal without error, engineering needs a small, objective data set: the pumped fluid (with temperature and concentration), the pressure in the seal chamber, the pump speed, the shaft or sleeve diameter at the sealing area and — ideally — the pump's brand and model, which define the available space and the dimensional standard.
With the pump nameplate and a photo of the current seal, we resolve identification in most cases, including technical equivalents for discontinued seals. A seal is too critical a component to be bought by guesswork.
Need a correctly specified mechanical seal?
We supply single, double, cartridge and balanced seals for pumps of any brand — with engineering-led selection and field replacement, all across Brazil.
Mechanical seal FAQ
A correctly specified seal, installed in an aligned, balanced pump operating within design, typically lasts years of continuous operation. Seals failing in weeks or months indicate a systemic cause: dry running, misalignment, vibration, cavitation or installation error — worth diagnosing before replacing yet again.
In most pumps, yes. The conversion involves assessing the stuffing box space, specifying the seal for the fluid and conditions, and adapting the assembly — frequently with a cartridge seal, which simplifies installation. The return comes from eliminating continuous leakage and reducing friction consumption.
Immediate leakage after replacement almost always points to installation: incorrect setting measurement, a face cracked in handling, an o-ring damaged at assembly or dirt between the faces. If installation was correct, suspect shaft misalignment or a pump condition (vibration, cavitation) breaking the lubrication film.
In critical pumps, almost always. The cartridge eliminates the setting error at installation, reduces maintenance time and standardizes replacement. The extra purchase cost pays for itself at the first avoided failure — especially where assembly labor is not seal-specialized.
It is a geometry that reduces the area over which fluid pressure pushes the faces together. Less closing force means less heat generated in the lubrication film — essential at high pressures or with fluids of poor lubricity. At low pressures, the simpler, cheaper unbalanced seal serves well.
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