Mechanical Seals: A Complete Guide

What Is a Mechanical Seal and Why Is It Needed?

A mechanical seal is a device used to seal the shaft of a pump, mixer, reactor, or other rotating equipment. It replaced the outdated gland packing and provides significantly more reliable sealing.

How does it work? The design consists of two rings – a rotating (primary) ring and a stationary (mating) ring. One ring is mounted on the shaft and rotates with it, while the other is fixed in the housing. A microscopic gap (less than a micron) forms between them, where a thin film of the pumped fluid acts as a lubricant. A spring presses the rings together to maintain a tight seal.

Purpose of a mechanical seal:

• In surface-mounted pumps – prevents fluid leakage to the outside.

• In submersible pumps – protects the electric motor from water ingress, preventing short circuits.

Advantages Over Gland Packing

Mechanical seals are steadily replacing gland packing due to several key benefits:

Main Types of Mechanical Seals

By Design

• Component seals – the classic type, consisting of separate parts.

• Cartridge seals – ready-to-install modular units. They simplify installation and reduce the risk of errors.

• Bellows seals – use a metal bellows instead of a spring. They perform better at high temperatures and with aggressive media.

By Arrangement

• Internally mounted seals – the process fluid contacts the outer surface of the seal.

• Externally mounted seals – the fluid is between the seal and the shaft.

By Friction Pair Type

Single and dual seals exist, with different dual seal arrangements: back-to-back, face-to-face, and tandem.

Materials of Construction

The choice of material directly depends on the process fluid, temperature, and pressure. Common friction pair materials include:

• Silicon carbide (SiC) – one of the most wear-resistant materials, used in aggressive media.

• Aluminum oxide (CER – ceramic) – a cost-effective choice for water and neutral fluids.

• Carbon (CAR) – impregnated with metals or synthetic resins, works well as a sealing material.

• Tungsten carbide – for extreme conditions, including abrasive media.

Secondary Seal (O-Ring) Materials

• NBR (nitrile) – for oils and petroleum products.

• EPDM (ethylene-propylene) – for water, steam, mild acids, and alkalis.

• FKM (Viton) – for aggressive chemicals and high temperatures.

How to Select a Seal for Your Pump

Most users search for a seal by pump brand. Internationally, the most common pumps are WILO, Grundfos, KSB, Alfa Laval, DAB, EBARA, and Speroni – a wide range of equivalents is available for these.

To select the right seal, you need to know:

1. Pump brand and model (e.g., Calpeda NM 25/160 AE).

2. Shaft diameter – the primary compatibility dimension.

3. Fluid characteristics – temperature, aggressiveness, presence of sand or abrasives.

4. The friction pair material combination suitable for your conditions.

If the exact model is unknown, you can select by mounting dimensions. The 60B(F) series, compliant with European standard EN12756, is widely interchangeable.

Common Failure Causes and How to Avoid Them

The Main Cause – Dry Running

The most common failure cause is overheating due to insufficient lubrication in the gap between the rings. This happens when the pump runs dry without fluid. Overheating leads to thermal deformation, micro-cracks, and rapid wear.

Other Typical Problems:

1. Contaminated pumped fluid – abrasive particles and salt deposits enter the gap, jam the rotating ring, and destroy the friction pair.

2. Incorrect installation – misalignment, insufficient or excessive tightening.

3. Shaft misalignment – causes uneven wear of the friction pair.

4. Wrong material selection – the seal fails due to aggressive media or high temperatures.

How to Extend Service Life

• Never run the pump without fluid.

• For dirty water, use seal flushing systems.

• Always install a correctly selected seal.

• During installation, strictly follow alignment and torque specifications.

Leakage: Normal or a Problem?

Some leakage is normal. A micro-gap always exists between the rings, through which a thin fluid film passes. For a functional seal, leakage can range from 0.01 to 30 ml/hour.

Leakage rate depends on:

• fluid pressure and temperature;

• fluid viscosity;

• shaft rotational speed;

• surface roughness of the friction faces;

• axial and radial shaft runout.

A significant increase in leakage indicates problems – ring wear, contamination, or improper installation.

Where to Buy

Mechanical seals are widely available through industrial suppliers and pump distributors worldwide. Prices range from modest amounts for domestic pumps to several hundred dollars for heavy-duty industrial equipment.

Major suppliers offer seal selection assistance for pumps of any brand, as well as cross-reference services to find equivalents for original parts.

Quick Answers to Frequent Questions

What should I do if the seal starts leaking? Check for contamination in the process fluid and ensure the pump isn't running dry. If the issue persists – the seal most likely needs replacement.

Can I install a seal myself? Yes, cartridge seals simplify installation. For component seals, careful handling and following the instruction manual are essential. Improper installation is a common cause of premature failure.

How do I know which seal fits my pump? Provide the pump brand, model, shaft diameter, and fluid characteristics to your supplier. You can also search by the seal code, for example, MG12/28-Z.

What's the difference between single and dual seals? A single seal is cheaper; a dual seal provides extra reliability and is used for hazardous or expensive fluids.