COMPRESSOR

DRY GAS SEAL TRAINING

Dry Gas Seals

1. Dry gas seals are becoming the most popular shaft seals for centrifugal gas compressors as they do not require a supply of oil either as a lubricant or sealing medium. In fact if oil from the shaft bearings is allowed to enter the seal it is likely to cause seal failure.

2. A buffer gas is used instead of oil as the sealing and lubricating medium.

3. In most cases the buffer gas is process gas tapped from the compressor discharge and then filtered before being fed to the seal chambers.

4. A dry gas seal is in the form of a self-contained cartridge which must be installed as a complete unit. A special plate is supplied fitted to the cartridge to keep the seal components together while the cartridge is not installed. This plate must be used when removing or fitting a cartridge and each cartridge has its own plate which must be stored safely while the seal is in use in the compressor.

Types of dry gas seals

Seal design depends primarily on:

The compressor speed

Contained gas composition

Operating pressure

Process requirements

Safety requirements.

1.unknown

The single seal is used mainly on inert or non-hazardous, low pressure gases, where a small leakage to atmosphere is acceptable.

The tandem seal can be considered a single seal with containment or 'safety' seal. The first seal, or inboard seal, handles the full process pressure and the second, or outboard seal, which is vented to flare, will contain any leakage from the first seal.

The double seal is used where the leakage to the atmosphere cannot be tolerated, the gas is dirty, unstable or there is a danger of negative pressure. An inert gas is injected between the opposing seal faces, at a pressure higher tan the process pressure, thereby creating a small amount of leakage to flow across the seal faces into the process and the bearing cavity.

Triple seals are primarily used for high pressure applications. A triple seal usually has two 'active' seals and one safety seal. The breakdown of pressure occurs across two seal stages, while the third seal acts as a safety seal.

EXAMPLE :

The John Crane Type 28 Dry Gas Seal

This type of dry gas is a face type seal which uses dry, clean filtered gas to seal and lubricate the faces. For simplification the single type is shown in the half section in Figure 28.

Figure 28. Type 28 Single Dry Gas Seal.

2.unknown

1. The seal comprises an 'O'-ring sealed spring-loaded carbon primary ring in a stationary stainless steel container. This primary ring is spring-loaded against a tungsten carbide mating ring which is fixed on and sealed to the rotating compressor shaft.

The ability of a dry gas seal to contain gas without the use of seal oil relies entirely on the design of the tungsten carbide mating ring and its interaction with the carbon primary ring when gas is present between the two sealing faces. Both these sealing surfaces are lapped to a high degree of flatness.

3.The rotating tungsten carbide ring has a series of spiral grooves etched into its running face as shown in Figure 29. (See next page).

Figure 29. The Running Face of the Tungsten Carbide Ring

3.unknown

1. The grooves are only micro-inches deep and are invisible to the naked eye. The pattern of the lapped contact area can, however, be seen as a grey matt finished surface in contrast to the spiral grooves as shown above.

2. The principle of operation of the spiral groove gas seal is shown in Figure 30 below.

Figure 30. The Gas Sealing Action Created by the Rotating Mating Seal Ring.

4.unknown

HOW IT WORK :

1. When the compressor is not rotating, the soft flat carbon ring is pressed against face of the mating ring by the springs, to form a leak-proof seal.

2. Separation of the faces occurs at about 100 rpm. when the gas pressure has increased sufficiently to overcome the pressure of the springs.

The rotating action of the mating ring causes the spiral grooves to pick up gas which is forced inwards in the direction of the centre of the shaft. This 'pumping' action forces gas between the two flat sealing surfaces.

4. In this way a film of gas is formed between the surfaces which acts as a lubricant and a sealing medium, just like the oil in a liquid seal.

5.The gap between the sealing faces is established when the closing force of the springs is equal to the opening force generated by the gas. This gap is in the order of 0.0025 to 0.005 mm. (0.0001 to 0.0002 in.).