What Is Double Mechanical Seal and How Does It Work

Leaking machinery can cause costly downtime and safety hazards in industrial facilities. Even small leaks waste resources and pose risks. Double mechanical seals provide an effective solution to prevent leakage in rotating equipment. These specialized components use a unique dual-seal design to reliably contain fluids and gases.

This introduction will explain what double mechanical seals are, how they work, and the benefits they offer for enhancing machinery performance and reliability.

What is a Double Mechanical Seal

A double mechanical seal is an advanced sealing solution that utilizes two independent seals to provide superior protection against leakage in rotating equipment such as pumps, mixers, and compressors. Unlike single mechanical seals, which rely on a single point of contact between the rotating and stationary components, double mechanical seals feature two sets of sealing surfaces that work together to prevent fluid escape.

The primary seal, which is the seal closest to the process fluid, operates under the same principles as a single mechanical seal. It consists of a rotating face and a stationary face that maintain a thin fluid film between them to prevent direct contact and minimize wear.

The secondary seal, located behind the primary seal, serves as a backup in case the primary seal fails. This redundancy significantly enhances the reliability and safety of the sealing system, making double mechanical seals ideal for applications involving hazardous, toxic, or environmentally sensitive fluids.

How Do Double Mechanical Seals Work

When the pump shaft rotates, the primary seal acts as the first line of defense, containing the process fluid within the pump housing and preventing leakage. The pressurized barrier fluid in the cavity between the seals lubricates the seal faces and carries away any heat generated due to friction. This fluid is maintained at a pressure higher than the process fluid, ensuring that any leakage flows from the buffer fluid to the process rather than the other way around.

When the pump shaft rotates, the primary seal acts as the first line of defense, containing the process fluid within the pump housing and preventing leakage. The pressurized barrier fluid in the cavity between the seals lubricates the seal faces and carries away any heat generated due to friction. This fluid is maintained at a pressure higher than the process fluid, ensuring that any leakage flows from the buffer fluid to the process rather than the other way around.

Purpose of Double Mechanical Seals

Enhanced Safety

The redundant seal arrangement of double mechanical seals significantly reduces the risk of leakage and seal failure. If the primary seal fails, the secondary seal contains the process fluid, preventing potentially hazardous or environmentally damaging leaks.

Zero Emissions Compliance

Strict environmental regulations in many industries mandate near-zero emissions of volatile organic compounds (VOCs) and other pollutants. Double mechanical seals, when used with appropriate barrier fluids, can reliably contain process fluids and prevent fugitive emissions.

Extended Seal Life

In a double seal configuration, the secondary seal operates in a clean, controlled environment isolated from the process fluid. This prevents contamination and extends the life of the sealing surfaces. Additionally, the barrier fluid between the seals lubricates and cools the seal faces, reducing wear and heat generation.

Isolation from Atmospheric Conditions

Certain process fluids can degrade or become hazardous when exposed to atmospheric conditions like moisture or oxygen. The sealed chamber between the primary and secondary seals isolates the process fluid from the environment.

Backup Seal

The secondary seal in a double seal arrangement serves as a backup in case of primary seal failure. The backup seal allows for continued operation until a planned maintenance outage can be scheduled to replace the primary seal.

Barrier and Buffer Fluids in Double Mechanical Seals

Barrier Fluids  

Barrier fluids are used in double mechanical seals to provide a complete physical barrier between the process fluid and the atmosphere. The barrier fluid is typically maintained at a pressure higher than the process fluid, preventing any leakage of the process fluid to the atmosphere in case of seal failure. This pressurized barrier fluid also helps to lubricate and cool the seal faces, extending their service life.

Buffer Fluids  

In contrast to barrier fluids, buffer fluids are not pressurized and serve as an unpressurized liquid barrier between the inner seal and the atmosphere. Buffer fluids are used when the process fluid is non-hazardous, and a pressurized barrier is not required. The buffer fluid captures any minor leakage from the inner seal, preventing it from escaping to the atmosphere.

Double Mechanical Seal Configurations

Back-to-Back

In a back-to-back configuration, also known as an opposed arrangement, the primary and secondary seals are oriented with their rotating seal faces facing away from each other. The seals are mechanically loaded against each other, with barrier fluid pressurized between them. This configuration provides excellent leak tightness and is suitable for high-pressure applications. However, it requires careful management of barrier fluid pressure to maintain proper seal face loading.

Tandem

Tandem seals, also called in-line seals, have both seal faces oriented in the same direction, facing the process fluid. The primary seal handles the full pressure of the process fluid, while the secondary seal serves as a backup. Buffer fluid between the seals is typically at a lower pressure than the process fluid. Tandem seals offer simplified piping and pressure control compared to back-to-back seals. They are commonly used in moderate-pressure applications and where leakage to atmosphere is permissible in small quantities.

Face-to-Face

Face-to-face seals have their rotating faces oriented toward each other, with barrier fluid between them. This configuration is less common than back-to-back or tandem, but offers some unique advantages. Face-to-face seals are inherently balanced, requiring no additional pressure balancing features. They also have a smaller axial footprint, making them suitable for space-constrained installations. However, face-to-face seals require a separate cooling system for the barrier fluid, as heat generated by the seal faces can build up in the closed cavity between them.