What Is the Difference Between Single and Double Mechanical Seals

Mechanical seals are critical components in pumps and other rotating equipment, protecting against leaks and ensuring safe, efficient operation. However, not all mechanical seals are created equal – the choice between single and double seals can have major implications for reliability, maintenance, and cost.

In this blog post, we’ll explain the distinctions between these two seal types and provide guidance on selecting the right configuration for your specific application and operating conditions.

Single Mechanical Seals

A single mechanical seal is a common sealing solution that consists of a single set of sealing faces. One face is stationary while the other rotates with the shaft. The seal faces are pressed together by a spring force and fluid pressure, creating a barrier that prevents leakage. Single mechanical seals are relatively simple in design and are suitable for many applications with lower pressure and temperature requirements.

Double Mechanical Seals

Double mechanical seals provide an enhanced level of sealing performance and reliability by utilizing two sets of sealing faces. The seals are arranged in series, with a barrier fluid circulating between them. This configuration offers several advantages over single mechanical seals, particularly in applications with stringent sealing requirements or hazardous fluids.

Barrier Fluid System

A key component of double mechanical seals is the barrier fluid system. The barrier fluid, which can be a liquid or gas, circulates between the two seal faces, providing lubrication, cooling, and a pressure barrier. The barrier fluid is maintained at a higher pressure than the process fluid, preventing leakage to the atmosphere. The barrier fluid system typically includes a reservoir, circulation pump, heat exchanger, and pressure control devices.

Types of Double Mechanical Seals

Back-to-Back

In a back-to-back arrangement, the two seals face away from each other, with the barrier fluid chamber located between them. This configuration is well-suited for high-pressure applications and provides excellent sealing performance. The barrier fluid pressure is maintained higher than the process fluid pressure, ensuring no leakage to the atmosphere.

Tandem

Tandem seals have both seals facing the same direction, with the barrier fluid chamber located outboard of the process fluid. This arrangement is commonly used in applications with lower pressures and less hazardous fluids. The tandem configuration allows for easier monitoring of the barrier fluid and simplifies maintenance.

Face-to-Face

Face-to-face seals have the sealing faces oriented towards each other, with the barrier fluid chamber located between them. This configuration is less common but can be advantageous in certain applications. Face-to-face seals offer balanced pressure on the sealing faces, reducing wear and extending seal life. They are also compact in design, making them suitable for space-constrained installations.

Key Differences Between Single and Double Mechanical Seals

Number of Seal Faces

• Single seals have one set of seal faces (stationary and rotating).
• Double seals have two sets of seal faces, providing an additional layer of sealing.

Lubrication

• In single seals, the sealed fluid itself often serves as the lubricant for the seal faces.
• Double seals utilize a barrier fluid, separate from the process fluid, to lubricate the seal faces. The barrier fluid is contained between the two sets of seal faces.

Leakage

• With single seals, leakage of the process fluid to the atmosphere can occur if the seal fails.
• Double seals significantly reduce leakage risk. If the inboard seal fails, the barrier fluid will leak into the sealed fluid. If the outboard seal fails, a small amount of barrier fluid will leak to atmosphere, but the process fluid remains contained.

Seal Life

Double seals generally have a longer operational life than single seals. The barrier fluid in double seals helps dissipate heat and prevents contamination or degradation of the seal faces, contributing to extended seal life.

Cost

Double seal systems have higher upfront costs due to the additional components (seal faces, gland plates, barrier fluid system).

Complexity

• Double seal systems are more complex, with additional parts and auxiliary systems for barrier fluid circulation and pressurization.
• Single seals have a simpler design and fewer components, making them easier to install and maintain.

Applications

• Single seals are suitable for non-critical, non-hazardous applications with lower pressures and temperatures.
• Double seals are used in more demanding services, especially for hazardous, high-pressure, or high-temperature fluids where enhanced safety and reliability are crucial.