What Is Double Mechanical Seal and How Does It Work

Have you ever wondered how machinery prevents leaks and maintains efficiency? Double mechanical seals are the answer. These critical components play a vital role in various industries, ensuring safe and smooth operation.

In this blog post, we’ll explore the inner workings of double mechanical seals, breaking down their components and functionality. By understanding how these seals operate, you’ll gain valuable insights into their importance and the benefits they provide.


What Is Double Mechanical Seal

A double mechanical seal is a type of seal used in pumps and other rotating equipment. It consists of two sets of sealing faces. One set is stationary, while the other rotates with the shaft. The two sets of sealing faces are arranged in pairs. Each pair has a primary seal and a secondary seal.

The inner set of sealing faces, known as the primary seal, is in direct contact with the process fluid. The outer set of sealing faces, known as the secondary seal, is in contact with the barrier fluid. The secondary seal, located behind the primary seal, provides an additional barrier.

A buffer fluid, such as oil or glycol, is circulated between the two seals. This fluid lubricates the seal faces and removes heat generated by friction. If the primary seal leaks, the buffer fluid is the first to escape, serving as an early warning sign.

Barrier Fluid or Buffer Fluid

Barrier fluid is a clean, compatible liquid that is pressurized higher than the process fluid. It is introduced between the two seal faces of a double mechanical seal. The barrier fluid lubricates the seal faces and prevents process fluid from entering the sealing interface.

Buffer fluid is typically the same as the process fluid. It is used in the outer seal chamber of a double mechanical seal. The buffer fluid is maintained at a lower pressure than the barrier fluid. This creates a pressure differential that prevents the process fluid from entering the inner seal chamber.

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Types of Double Mechanical Seal


In a back-to-back double mechanical seal, the two seals face away from each other. The primary seal faces the process fluid, while the secondary seal faces a buffer fluid. The buffer fluid is maintained at a higher pressure than the process fluid. This arrangement prevents leakage of the process fluid to the atmosphere.

The back-to-back design is commonly used in applications with high-pressure process fluids.


A tandem double mechanical seal consists of two seals facing the same direction. The primary seal faces the process fluid, and the secondary seal is mounted behind it. A barrier fluid is circulated between the two seals at a pressure lower than the process fluid.

The tandem arrangement is used in applications where the process fluid is hazardous or toxic.


In a face-to-face double mechanical seal, the two seals face each other with a common cavity between them. The cavity is filled with a barrier fluid maintained at a pressure higher than the process fluid. This arrangement creates a double barrier between the process fluid and the atmosphere.

The face-to-face seals are suitable for applications with high temperatures or pressures.

Advantages of Double Mechanical Seal

Increased Reliability

Double mechanical seals offer increased reliability compared to single seals. The secondary seal provides a backup in case the primary seal fails.

Longer Seal Life

The dual seal design allows for a lubricating fluid between the seals. This fluid cools and lubricates the seal faces. It reduces friction and wear on the seals. The result is a significantly longer seal life compared to single seals.

Improved Leakage Control

Double seals provide superior leakage control. The secondary seal contains any leakage from the primary seal. The barrier fluid is maintained at a higher pressure than the process fluid. This prevents process fluid from escaping to the atmosphere.

Suitable for Harsh Applications

Double seals are ideal for harsh or dangerous applications. They are commonly used with corrosive, toxic, or flammable fluids. The dual seal and barrier fluid provide an extra layer of protection. They help contain dangerous liquids and gases.

Lower Maintenance Requirements

While double seals are more complex, they often require less frequent maintenance. The longer seal life and leakage control mean less downtime for repairs.


Applications of Double Mechanical Seal

  • Extra safety and reliability are required. This is especially true for processes involving hazardous, toxic, or expensive fluids.
  • The process fluid is extremely hot or cold.
  • The process fluid is abrasive or contains solids.
  • Applications where process downtime is extremely costly.

Factors to Consider When Selecting a Double Mechanical Seal

Fluid Properties

Consider the properties of the fluid being sealed. This includes the fluid’s chemical composition, temperature, and pressure. Ensure the seal materials are compatible with the fluid to prevent corrosion or degradation.

Operating Conditions

Evaluate the operating conditions of the equipment. Take into account the speed, load, and vibration levels. The seal must be capable of withstanding these conditions without failing or leaking.

Seal Arrangement

Determine the appropriate seal arrangement for the application. This includes the orientation of the seal faces and the type of spring used. The arrangement should be optimized for the specific operating conditions and fluid properties.

Seal Face Materials

Select the appropriate seal face materials based on the fluid properties and operating conditions. Common materials include carbon, silicon carbide, and tungsten carbide. The materials should provide adequate wear resistance and chemical compatibility.

Environmental Factors

The environment where the seal will be used should be assessed. This includes exposure to dust, dirt, or other contaminants. Seals in dirty or abrasive environments may need additional protection. Barrier fluids or cyclone separators can help keep contaminants away from the seal faces.

Cooling and Flushing

Consider the cooling and flushing requirements of the seal. Determine if the seal needs external cooling or flushing to maintain proper operating temperatures and prevent the build-up of contaminants.

Application of Double Mechanical Seal

  • Pumps handling hazardous, toxic, or expensive fluids in the chemical, petrochemical, and pharmaceutical industries.
  • Rotating equipment in the oil and gas sector, such as compressors, mixers, and agitators.
  • Food and beverage processing machinery, where maintaining product purity and preventing contamination are paramount.
  • Water treatment and wastewater management systems, where double mechanical seals are employed in pumps and other rotating equipment.
  • High-pressure applications, such as boiler feed pumps in power plants.
  • Reactor vessels and mixing tanks in various process industries.


What types of barrier fluids are used in double mechanical seals

Common barrier fluids used in double mechanical seals include oil, glycol, water, and specially formulated synthetic fluids.

How is the barrier fluid pressure maintained in a double mechanical seal

The barrier fluid pressure is typically maintained using an external system, such as a pressurized reservoir or a thermosyphon system.

What is the difference between an unpressurized and pressurized double mechanical seal

In an unpressurized double mechanical seal, the barrier fluid is not actively pressurized and relies on the centrifugal force generated by the rotating seal components to maintain the fluid film. In a pressurized double mechanical seal, the barrier fluid is actively pressurized using an external system.


Double mechanical seals provide superior leak protection through two sets of sealing surfaces. Their reliable design is ideal for demanding applications.

To learn more about selecting the right mechanical seal for your needs, contact our knowledgeable team today.

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