What Is a Mechanical Seal? Working Principle & Replacement Checks

Contents

• What is a mechanical seal?
• Why pumps need mechanical seals
• Main parts of a mechanical seal
• How a mechanical seal works
• Common leakage points
• Mechanical seal vs gland packing
• Common mechanical seal types
• Material selection
• Common failure causes
• Information needed for selection
• Replacement checks
• FAQ

What is a mechanical seal?

A mechanical seal is an assembly that seals the gap between a rotating shaft and a stationary housing. It usually contains one rotating seal face and one stationary seal face. These two faces are pressed together by spring force and hydraulic force.

During operation, a very thin fluid film forms between the two seal faces. This film lubricates the faces, removes heat, and helps the seal run without excessive wear. The seal faces are close enough to control leakage, but they should not run completely dry in normal wet-seal service.

• Centrifugal pumps
• Chemical pumps
• Water pumps
• Sewage pumps
• Boiler feed pumps
• Mixer and agitator equipment
• Compressors and blowers
• Reactors and process vessels

Why pumps need mechanical seals

Inside a centrifugal pump, the impeller rotates and moves liquid from the suction side to the discharge side. The pump shaft must rotate freely, but the casing must also hold pressure. Without a sealing device, liquid would leak from the shaft area.

A mechanical seal solves this problem by creating a controlled sealing interface around the shaft. The seal allows rotation while limiting leakage from the pump chamber to the atmosphere.

In many industrial applications, leakage is not only a housekeeping problem. It may also cause:

• Product loss
• Environmental risk
• Safety risk for hot, toxic, flammable, or corrosive fluids
• Bearing contamination
• Motor or baseplate damage
• Frequent shutdowns
• Higher maintenance cost

Main parts of a mechanical seal

Rotary seal face

The rotary face rotates with the shaft or shaft sleeve. It is usually fixed to the sleeve by a drive pin, set screw, spring holder, or other drive structure.

Stationary seal face

The stationary face is installed in the gland, housing, or seat. It does not rotate with the shaft. The rotary face and stationary face work together as the primary sealing pair.

Secondary seals

Secondary seals prevent leakage around the seal faces, shaft sleeve, gland, and other fitted parts. Common secondary seal materials include NBR, EPDM, FKM, PTFE, and flexible graphite.

Springs or bellows

Springs, elastomer bellows, or metal bellows provide axial force. This force keeps the seal faces in contact during start-up, shutdown, and pressure changes.

Gland and sleeve

The gland connects the seal to the pump casing. The sleeve protects the pump shaft and provides a mounting surface for the rotating seal components.

How a mechanical seal works

The working principle of a mechanical seal is based on controlled face contact and a stable fluid film.

When the pump starts, the rotary face turns with the shaft. The stationary face remains fixed. Springs and process pressure keep the two faces together. A small amount of liquid enters the microscopic gap between the faces and creates a lubricating film.

This fluid film performs three important functions:

• It lubricates the seal faces.
• It carries heat away from the contact area.
• It helps prevent direct dry friction.

If the film is stable, the mechanical seal can run with very low leakage. If the film is lost because of dry running, vaporization, poor cooling, wrong material, or excessive solids, the seal faces may overheat and fail quickly.

Common leakage points

Mechanical seal leakage can come from several locations. Understanding the leakage point helps maintenance teams find the real cause instead of replacing the same seal again.

• Between the rotary face and stationary face
• Between the rotary face and shaft sleeve
• Between the stationary face and gland
• Between the gland and pump casing
• Between the shaft and sleeve
• Through damaged elastomers or gaskets

Face leakage is often caused by poor lubrication, wrong material, vibration, solids, heat, or pressure changes. Leakage around secondary seals may be caused by wrong elastomer material, chemical attack, installation damage, or incorrect compression.

Mechanical seal vs gland packing

Gland packing is still useful in some applications, but mechanical seals are preferred when leakage control, efficiency, and reliability are more important.

Common mechanical seal types

Rubber bellows mechanical seal

Rubber bellows seals are common in clean water pumps and general service pumps. They are compact, economical, and easy to install.

Spring pusher mechanical seal

Pusher seals use springs and dynamic secondary seals to maintain face contact. They are widely used in chemical pumps and industrial pumps.

Metal bellows mechanical seal

Metal bellows seals are often used for high-temperature service, low-temperature service, and applications where ordinary elastomers are not suitable.

Cartridge mechanical seal

A cartridge seal is pre-assembled with sleeve, gland, seal faces, springs, and secondary seals. It reduces installation errors because the working length and compression are set before installation.

Single and dual mechanical seals

A single mechanical seal uses one set of seal faces. A dual mechanical seal uses two sets of seal faces with buffer or barrier fluid between them. Dual seals are often used for toxic, flammable, crystallizing, or difficult fluids.

Material selection

Mechanical seal material selection depends on liquid, temperature, pressure, speed, solids content, and chemical compatibility.

Common seal face materials include:

• Carbon graphite
• Silicon carbide
• Tungsten carbide
• Ceramic
• Stainless steel

Common secondary seal materials include:

• NBR for many oil and water services
• EPDM for hot water and some chemical services
• FKM or Viton for higher temperature and oil resistance
• PTFE for broad chemical resistance
• Flexible graphite for high-temperature service

Final material selection should always be confirmed according to the actual operating condition.

Common failure causes

Many mechanical seal failures are caused by the operating environment rather than the seal alone.

• Dry running during start-up
• Insufficient seal chamber liquid
• Wrong seal face material
• Wrong O-ring or secondary seal material
• Excessive vibration
• Shaft runout or misalignment
• Solids entering the seal faces
• Crystallization or polymerization near the faces
• Poor cooling or blocked flush line
• Pressure or temperature outside the design range
• Incorrect installation length
• Damaged faces during installation

If the same pump repeatedly leaks, check the pump condition, piping plan, operating procedure, and installation method before changing to another seal model.

Mechanical seal piping plans

Mechanical seals often need a support system to create a better working environment around the seal faces. These systems are called piping plans.

For single seals, common plans include Plan 11, Plan 21, Plan 23, and Plan 32. For dual seals, common plans include Plan 52, Plan 53A, Plan 53B, and Plan 54.

The purpose of a piping plan may include:

• Flushing the seal faces
• Removing heat
• Increasing vapor pressure margin
• Supplying clean external fluid
• Cooling the seal chamber
• Monitoring leakage
• Supporting a dual seal with buffer or barrier fluid

For a full explanation, see the related guide about mechanical seal piping plans.

Information needed for mechanical seal selection

Before selecting or quoting a mechanical seal, prepare the following information:

• Pump model and manufacturer
• Shaft size or sleeve size
• Seal chamber dimensions
• Liquid name and concentration
• Operating temperature
• Suction pressure and discharge pressure
• Solids content
• Viscosity
• Rotation speed
• Existing seal type or photos
• Current leakage or failure problem
• Required material or standard

Photos of the old seal, pump nameplate, and installation position can also help confirm the correct replacement.

Replacement checks: turn observations into useful information

A replacement enquiry is easier to assess when it separates what you observed from what you assume. A wet gland does not, by itself, identify a failed face pair: liquid can travel along a surface before it drips.

Record when and where leakage appears

Describe whether the leak first appears at standstill, during start-up, after warming up or throughout steady operation. Note the first visible wet location, recent maintenance and any process change. These observations help organise an investigation; none alone proves the cause.

Distinguish free length from installed working length

The length of a loose seal on a workbench is not automatically its specified installed length. Include the assembly drawing and label the surfaces used for each measurement. Matching the shaft diameter alone does not confirm that the seat, gland, drive arrangement and available axial space are compatible.

Label pressure and material information precisely

Provide seal-chamber pressure when available, and identify suction and discharge readings separately. Do not relabel a discharge reading as seal-chamber pressure. If a face or O-ring material is unknown, write “unconfirmed” rather than assigning a material from its colour or appearance.

A practical example: two seals with the same shaft size

Suppose two removed seals fit the same nominal shaft diameter. One enquiry includes only that diameter; the other includes the stationary-seat dimensions, mounting details, working-length drawing and service conditions. The second provides a basis for checking compatibility. The first still needs those details before a replacement can be confirmed. This is an illustrative comparison, not a customer case study.

When requesting help, send the equipment identification, labelled photographs, drawing and operating details through our mechanical seal enquiry page. Keep unknown values explicit so they can be resolved before selection.

FAQ

What does a mechanical seal do?

A mechanical seal prevents liquid or gas from leaking along the rotating shaft of a pump, mixer, agitator, compressor, or similar rotating machine.

How does a mechanical seal prevent leakage?

It uses one rotating face and one stationary face. These faces are pressed together, and a thin fluid film between them lubricates the faces while controlling leakage.

Is a mechanical seal completely leak-free?

No dynamic seal is absolutely leak-free. A good mechanical seal controls leakage to a very low level. In many pump services, leakage is not visible during normal operation.

What is the most important part of a mechanical seal?

The seal face pair is the primary sealing area. However, the secondary seals, springs, gland, sleeve, and operating environment are also important for reliable performance.

Why do mechanical seals fail?

Common reasons include dry running, wrong material selection, vibration, solids, overheating, blocked flush piping, incorrect installation, and operation outside the design range.

How do I choose the right mechanical seal?

Confirm the pump model, shaft size, liquid, temperature, pressure, solids content, speed, seal chamber dimensions, and current failure history. These details help select the correct structure and materials.

What is the difference between a single seal and a dual seal?

A single seal has one set of seal faces. A dual seal has two sets of seal faces and usually uses buffer or barrier fluid. Dual seals are used when leakage control or fluid safety is more demanding.