Double Mechanical Seal vs Single Mechanical Seal: What Is the Difference

What Is a Single Mechanical Seal

A single mechanical seal is a type of seal that consists of a stationary and a rotating component, with one seal face. It is the most common and economical type of mechanical seal used in pumps, mixers, and other rotating equipment. The stationary component is typically made of a hard material such as ceramic, while the rotating component is usually made of a softer material like carbon. These two surfaces form a flat, annular sealing surface that prevents leakage of the process fluid along the rotating shaft.

Single mechanical seals are used in a wide range of applications, from water pumps to chemical processing equipment. They rely on a thin film of fluid between the seal faces to lubricate and cool the sealing surfaces. This fluid can be the process fluid itself or an external barrier or buffer fluid.

What Is a Double Mechanical Seal

A double mechanical seal, also known as a dual seal, consists of two sets of sealing faces arranged in series. This configuration provides an additional level of safety and reliability compared to single mechanical seals. In a double seal arrangement, the primary (inboard) seal operates as a conventional single seal, while the secondary (outboard) seal acts as a backup in case of primary seal failure.

The space between the two seals, called the buffer or barrier chamber, is filled with a compatible fluid maintained at a pressure higher than the process fluid. This arrangement ensures that any leakage from the primary seal is contained within the buffer chamber and does not escape to the atmosphere.

Types of Double Mechanical Seal

There are three main types of double mechanical seal arrangements:

Back to Back

In a back-to-back arrangement, the two seals are oriented with their rotating faces facing away from each other. The buffer fluid is maintained at a pressure higher than the process fluid, ensuring that any leakage from the primary seal is directed into the buffer chamber. This configuration is suitable for high-pressure applications and prevents process fluid from reaching the atmosphere.

Face to Face

Face-to-face double seals have their rotating faces oriented towards each other, with the buffer fluid between them. This arrangement is less common than the back-to-back configuration and is typically used in applications with lower pressures or when the process fluid is compatible with the buffer fluid.

Tandem

In a tandem seal arrangement, the two seals are installed with their rotating faces in the same direction, facing the process fluid. The buffer fluid is maintained at a lower pressure than the process fluid, and any leakage from the primary seal is collected in the buffer chamber. Tandem seals are used in applications where the process fluid is not compatible with the buffer fluid or when the buffer fluid needs to be kept clean.

Barrier Vs Buffer Fluids

The fluid used in the space between the two seals in a double mechanical seal arrangement can be either a barrier fluid or a buffer fluid.

Barrier Fluid

A barrier fluid is a clean, compatible liquid maintained at a pressure higher than the process fluid. Its purpose is to prevent the process fluid from reaching the atmosphere by providing a physical barrier between the two seals. The barrier fluid is typically circulated through a separate system with a heat exchanger and pressure control devices to maintain its temperature and pressure within the desired range.

Buffer Fluid

A buffer fluid is maintained at a lower pressure than the process fluid. Its primary function is to collect and contain any leakage from the primary seal, preventing it from reaching the atmosphere. Buffer fluids are often used when the process fluid is not compatible with the barrier fluid or when the barrier fluid needs to be kept clean for reuse.

When to Use Double Mechanical Seal

Double mechanical seals are recommended in the following situations:

1. When handling hazardous, toxic, or expensive fluids where leakage is unacceptable
2. In applications with strict environmental regulations or where emissions must be minimized
3. For processes requiring a higher level of safety and reliability
4. In high-pressure or high-temperature applications where a single seal may not provide adequate sealing performance
5. When the consequences of seal failure are severe, such as in the chemical, petrochemical, or pharmaceutical industries

When to Use Single Mechanical Seal

Single mechanical seals are suitable for a wide range of applications and are generally preferred in the following situations:

1. When handling non-hazardous, non-toxic fluids where minor leakage is acceptable
2. In applications with lower pressure and temperature requirements
3. For processes where the cost of a double seal arrangement is not justified
4. In equipment with limited space for seal installation, as single seals have a smaller footprint compared to double seals
5. When the process fluid is compatible with the seal materials and does not require additional protection