Tandem Seal Vs Double Seal

The main difference between tandem seal and double seal is their configuration and purpose. Joints tandem use two seals in series to prevent process fluid leakage and often vent to atmosphere. Double seals also use two seals but pressurize the barrier fluid between them to prevent leakage into the environment.

Tandem Mechanical Seals (Api Arrangement 2 Type)

Detailed Definition and Design Architecture

Un tandem garniture mécanique arrangement consists of two individual mechanical seals mounted in series along the shaft, typically within the same chambre d'étanchéité or a specially designed cartridge. The seal positioned closer to the process fluid is termed the primary or inboard seal, while the seal on the atmospheric side is the secondary or outboard seal. The fundamental concept is that the secondary seal acts as a backup to the primary seal.  

The most common physical configuration for tandem seals is the “face-to-back” orientation, where both seals are installed facing the same direction, with the back of the primary seal towards the back of the secondary seal. This arrangement creates an inter-seal cavity or chamber that is filled with a buffer fluid. According to API 682, for a face-to-back configuration used as an Arrangement 2 seal, the buffer fluid is situated on the inside diameter (ID) of the inner seal and the outside diameter (OD) of the outer seal.  

Operating Principle: The Unpressurized Buffer Fluid System

The defining characteristic of a tandem seal (Arrangement 2) is its use of an unpressurized or low-pressure buffer fluid system. The buffer fluid in the inter-seal cavity is maintained at a pressure lower than the process pressure being sealed, often close to atmospheric pressure or slightly elevated to facilitate circulation and ensure the cavity remains filled. API 682 explicitly states that “a buffer fluid as a fluid between the inboard and outboard seals, which is at a lower pressure than the seal chamber pressure”.  

During normal operation, the primary seal handles the full differential pressure from the process fluid. The secondary seal experiences minimal pressure differential, typically less than 5 PSI. This configuration allows the secondary seal to remain in standby mode, experiencing minimal wear until the primary seal begins to fail.

The buffer fluid circulates through natural convection or with assistance from a anneau de pompage. Heat generated by the primary seal drives the circulation pattern, with cooler fluid entering at the bottom and warmer fluid exiting at the top.

Double Mechanical Seals (Api Arrangement 3 Type)

Detailed Definition and Design Architecture

Garnitures mécaniques doubles, typically conforming to API Arrangement 3, consist of two sets of faces d'étanchéité arranged within a seal chamber or cartridge, with a dedicated barrier fluid circulated in the cavity between them.

The defining characteristic of a double seal is that this barrier fluid is maintained at a pressure that is always higher than the process fluid pressure being sealed. This positive pressure differential is typically in the range of 1 to 2 bar (15 to 30 psig) above the process pressure, or as specified by standards like API 682 or specific manufacturer recommendations (e.g., minimum 2 bar or 10% above pump pressure ).  

Operating Principle: The Pressurized Barrier Fluid System

The pressurized barrier fluid system distinguishes double seals from tandem configurations. Barrier fluid pressure typically maintains 20 PSI above the seal chamber pressure through various methods: nitrogen bladder accumulators, piston accumulators, or direct pressure systems.

Both seals in a double arrangement share the pressure load, reducing stress on individual seal faces. The inboard seal prevents process fluid from entering the barrier fluid, while the outboard seal contains the barrier fluid. The process fluid itself is thus effectively contained within the pump or equipment.

Double Mechanical Seals Types

• Arrangement dos à dos: In this configuration, the rotating seal rings are typically mounted between the stationary mating rings, causing the seals to effectively face in opposite directions, away from each other. This is a common arrangement for general-purpose double seals and is specified under API 682 Arrangement 3. The barrier fluid is typically introduced to the outside diameter (OD) of both seal pairs.
• Arrangement en face à face: Here, the stationary mating rings are mounted between the rotating seal rings, causing the seals to face towards each other. This configuration is also used for API 682 Arrangement 3, with barrier fluid often on the OD of both seals.
• Face-to-Back Arrangement (Tandem Orientation with Pressurized Barrier): This involves one mating ring mounted between the two seal rings and one seal ring mounted between the two mating rings, essentially a tandem physical layout but operated with a pressurized barrier fluid. This configuration can also be used as an API Arrangement 3 seal, with the barrier fluid typically on the inside diameter (ID) of the inner seal and the OD of the outer seal.