Double Mechanical Seal vs Single Mechanical Seal

When selecting a mechanical seal, the choice between double and single seals is crucial.

In this blog post, we’ll explore the key differences between these two types of seals.

By the end, you’ll have a clear understanding of their advantages and be equipped to make the best decision for your application.


What is Single Mechanical Seals

A single mechanical seal is a type of seal that consists of just one set of sealing faces – a stationary face and a rotating face. The faces are pressed together by a spring and slide against each other, with a thin fluid film between them that is generated by the pumped product. This fluid film lubricates and cools the seal faces to prevent excessive wear.

Single mechanical seals are the most common and economical type, used in around 90% of applications. They are simple, with the least number of parts. The pumped product itself is often used to lubricate the seal faces.

What is Double Mechanical Seals

Double mechanical seals have two sets of sealing faces – a primary seal that contains the pumped fluid, and a secondary seal that prevents leakage of the barrier fluid to atmosphere. The barrier fluid is pressurized to a level above the pump’s discharge pressure. This ensures that if the primary seal leaks, the barrier fluid will flow into the pump rather than the pumped fluid leaking out.

Types of Double Mechanical Seals

There are three main configurations for double mechanical seals:

  1. Back-to-back arrangement: The two seals are mounted facing away from each other. This is the most common configuration, suitable for general applications. It handles pressure in one direction only.
  2. Face-to-face arrangement: Here the two seals face each other. This allows handling of pressure from both sides, but is less popular these days.
  3. Tandem arrangement: Also called face-to-back. The two seals face in the same direction, providing a backup in case the primary seal fails. Suitable when absolute sealing is critical.

In addition to the seal arrangement, double seals can be categorized as:

  • Pressurized seals: The barrier fluid is maintained at a pressure higher than the pump’s discharge. Suitable for most applications.
  • Unpressurized seals: The barrier fluid is at a lower pressure. Used when there are concerns about barrier fluid leaking into the process.
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The Difference between Single and Double Mechanical Seals

Design Differences

single mechanical seal consists of one set of sealing faces – a stationary face and a rotating face that seal against each other to prevent leakage.

double mechanical seal has two sets of sealing faces. The primary seal acts like a single seal, while the secondary seal provides a backup in case the primary seal fails. There is a barrier fluid between the two seals.

Sealing Performance

Double seals provide superior sealing performance compared to single seals. The secondary seal and barrier fluid act as an additional layer of protection against leakage. If the primary seal fails, the secondary seal maintains sealing until repairs can be made.

Single seals have no backup, so any failure results in immediate leakage.


Double mechanical seals are more expensive than single seals due to their more complex design with additional components. The barrier fluid system also adds cost.

Single seals are a more economical choice where the added expense of a double seal is not justified.

Maintenance and Replacement

Both seal types require routine maintenance like monitoring and replacing worn parts. However, single seals are easier and less costly to maintain and replace when needed.

Replacing a double seal is more involved since there are two sets of faces and additional hardware. The barrier fluid must also be drained and refilled.

Application Suitability

Single seals are suitable for many common applications with non-dangerous fluids at lower pressures and temperatures.

Double seals are a better choice for demanding services with hazardous, valuable, or environmentally sensitive fluids. Single seals are not recommended where leakage could create major issues.


When to Use Single Mechanical Seals

  • The fluid is non-hazardous and not environmentally sensitive
  • The fluid is clean and at moderate temperature and pressure
  • Cost is a primary concern
  • Space for the sealing system is limited

When to Use Double Mechanical Seals

  • The fluid is hazardous, toxic, or environmentally sensitive
  • Dealing with high-temperature or high-pressure fluids
  • Extremely low leakage rates are required
  • Enhanced system reliability is crucial


What Is the Role of the Barrier Fluid in a Double Mechanical Seal

The barrier fluid lubricates and cools the sealing faces, prevents the process fluid from reaching the atmosphere, and can also be monitored for leakage detection.

Can a Single Mechanical Seal Be Upgraded to a Double Mechanical Seal

In some cases, it may be possible to upgrade a single mechanical seal to a double mechanical seal, but it often requires significant modifications to the equipment and may not be cost-effective.

In conclusion

Double mechanical seals offer superior reliability, safety, and environmental compliance compared to single mechanical seals.

While they may have a higher initial cost, the long-term benefits and reduced maintenance requirements make them a wise investment.

Upgrade your sealing technology today and experience the peace of mind that comes with double mechanical seals.

Contact our experts to discuss your specific needs.

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