Mechanical Seal Working Principle Explained with Seal Diagram

Introduction

Mechanical seals play a critical role in pumps, compressors, mixers, and other rotating equipment by preventing fluid leakage. Understanding how they work is essential for engineers, maintenance teams, and industries that rely on reliable sealing systems. In this updated guide, we explain the mechanical seal working principle, supported with clear explanations, practical insights, and detailed seal diagrams to help you visualize the process effectively.

Whether you are new to mechanical seals or want a deeper technical understanding, this guide will walk you through the components, functions, and working mechanism step‑by‑step.

What Is a Mechanical Seal?

A mechanical seal is a device installed on rotating equipment shafts to prevent fluid from leaking out of the system. It forms a sealing interface between a stationary part and a rotating part. Unlike traditional packing, mechanical seals offer higher reliability, reduced friction, and longer service life.

Mechanical seals are widely used in:

Centrifugal pumps

Compressors

Mixers and agitators

Reactors

Blowers

Mechanical Seal Diagram

A seal diagram helps you clearly understand how each component interacts to prevent leakage. It visually represents the rotating seal ring, stationary seal ring, springs, secondary seals (O-rings / bellows), and gland plate.

What the seal diagram typically shows:

Rotating face attached to the shaft

Stationary face fixed to the gland

Springs/bellows that maintain axial force

Secondary seals (O‑rings) sealing internal leak paths

Interface where the sealing film is formed

This diagram helps simplify the complex interaction of components so that the working principle becomes clearer.

Key Components of a Mechanical Seal

Understanding the components is essential before learning the complete working principle.

1. Rotating Seal Face

Mounted on the shaft

Rotates with the equipment

Usually made of carbon, silicon carbide, or tungsten carbide

2. Stationary Seal Face

Fixed inside the gland plate

Stays static

Material selected based on application temperature and fluid compatibility

3. Secondary Seals

These seals prevent leakage through internal gaps:

O‑rings

Gaskets

Rubber bellows

PTFE wedge rings

4. Springs / Bellows

Maintain proper contact between rotating and stationary faces

Compensate for wear and misalignment

5. Gland Plate / Housing

Holds the stationary face

Provides the structure for the seal assembly

6. Drive Mechanism

Ensures the rotating face moves with the shaft

Can be in the form of drive pins, keys, or clutches

Mechanical Seal Working Principle 

The mechanical seal works based on maintaining a controlled interface between two faces: the rotating face and the stationary face.

Here is the working principle explained clearly:

Step 1: Shaft Rotation Starts

When the pump or equipment starts, the shaft begins to rotate. The rotating seal face, which is tightly fixed to the shaft, rotates along with it.

Step 2: Seal Faces Come in Contact

Axial force from springs or bellows pushes the rotating face against the stationary face. This ensures continuous contact between the two faces, which is essential for sealing.

Step 3: A Thin Lubrication Film Forms

A tiny amount of process fluid enters the seal face interface, forming a lubrication film. This film:

Minimizes friction

Prevents overheating

Ensures smooth sliding between the seal faces

This part is usually illustrated clearly in a seal diagram, showing the microscopic film layer.

Step 4: Leakage Is Controlled to Microscopic Level

The seal faces are lapped to extremely flat surfaces (light‑band accuracy). Because of this precision, only a microscopic amount of vapor escapes — far below visible leakage levels.

Step 5: Springs Compensate for Wear

Over time, seal faces wear due to sliding contact. Springs or bellows automatically adjust the contact force, ensuring continuous sealing performance.

Step 6: System Runs Leak‑Free

With proper lubrication, alignment, and pressure balance, the mechanical seal maintains consistent sealing without leakage.

Types of Mechanical Seal Designs

While the working principle remains similar, seal design variations help suit different industrial applications.

1. Pusher Seals

Use O‑rings that move axially with the seal face.

Common in pumps

Suitable for moderate temperatures

2. Non‑Pusher Seals (Bellows Seals)

Use welded metal bellows or elastomeric bellows instead of O‑rings.

Ideal for high temperatures

No dynamic O‑rings → no fretting

3. Balanced Seals

Designed to reduce hydraulic pressure on seal faces.

Suitable for high‑pressure applications

Lower heat generation

4. Unbalanced Seals

Best for low-pressure applications.

5. Cartridge Seals

Preassembled and pre‑set seal arrangement.

Easy installation

Reduced installation errors

6. Double Mechanical Seals

Contain two sets of seal faces.

Used in toxic, hazardous, or abrasive applications

Offers high reliability and safety

Why Mechanical Seals Are Preferred Over Packing

Mechanical seals offer significant advantages:

Lower leakage

Lower power consumption

Less wear on shaft/sleeves

Longer service life

Better for high-speed and high-pressure systems

Common Failure Reasons

Mechanical seals may fail due to:

Dry running

Incorrect installation

Misalignment

Abrasive or dirty fluids

Thermal shock

Chemical attack on elastomers

Proper installation, monitoring, and lubrication significantly improve seal life.

Real-World Applications

Mechanical seals are used across various industries:

Petrochemical plants

Water treatment

Oil & gas

Dairy and food industries

Pharmaceuticals

Chemical processing

Conclusion

Mechanical seals are highly effective components designed to prevent leakage in rotating equipment. Using seal diagrams helps simplify how the rotating and stationary faces work together to maintain sealing under different conditions.

With a clear understanding of components, working principles, and seal operation, equipment reliability and performance can be dramatically improved.

If you need customized mechanical seal solutions, the QMSeals team can help you choose the right seal for your application.

Frequently Asked Questions

1. What is a mechanical seal diagram?

A mechanical seal diagram is a visual representation showing how seal components interact — including rotating face, stationary face, springs, O‑rings, and lubrication film — to prevent leakage.

2. How do mechanical seals prevent leakage?

Mechanical seals prevent leakage by maintaining a controlled contact and lubrication film between two extremely flat faces. Springs or bellows maintain the pressure needed for sealing.

3. Why is lubrication film important in mechanical seals?

The microscopic lubrication layer reduces friction, lowers heat, and ensures smooth operation. Without it, seals would fail due to overheating and excessive wear.