Dry Gas Seals are able to seal the compressor without the use of seal oil and thereby significantly reduce emissions of process gas to atmosphere.
Main benefits of Dry Gas Seals are:
- Elimination of oil consumption and oil leakage to process
- Significant reduction or even elimination of process gas to atmosphere
- Increase of compressor efficiency
- Higher compressor reliability
- High lifetime expectancy
Dry Gas Seals are designed as a single cartridge for easy installation. Retrofits to Dry Gas Seals require minor or even no machining to shaft and compressor housing.
The most widely used dry gas seal for petrochemical applications is the tandem dry gas seal. It contains a process side seal (primary seal) and a bearing side seal (secondary seal). The process side seal breaks down the process pressure to the pressure of the plant’s flare system. The bearing side seal acts as a back-up seal. Between both seals nitrogen can be fed as an additional buffer. An intermediate labyrinth minimizes the required nitrogen flow. If considered more suitable for your application, Dry Gas Seals can be delivered as a double seal set.
Each seal is equipped with a spring-loaded stationary seal ring and a rotating seal ring. The rotating seal ring has aerodynamically designed grooves on its sealing face. When rotating, pressure will build up in these grooves. This will cause a lift-off between both seal rings. During operation the rings will be separated by a gas film thickness of only a few microns.
This gas film accomplishes the actual sealing. Gas leakage over the seal faces will be very low, resulting in low leakage rates from the compressor. The lift-off also prevents contact between the seal faces. This makes the power loss, life time and reliability of the Dry Gas Seal excellent compared to oil lubricated seals.
At outboard side of the Dry Gas Seal - between the dry gas seal and the bearing - the Dry Gas Seal is equipped with a tertiary seal. This seal is fed with an inert gas e.g. nitrogen. The flow from this tertiary seal to the bearing side effectively prevents the migration of oil and oil mist to the Dry Gas Seal. This is necessary to achieve reliable operation of the Dry Gas Seal.
Positive Seal Barriers
In order to increase the reliability of the Dry Gas Seals, Positive Seal Barriers are applied between the Dry Gas Seal and the compressor process side. Positive Seal Barriers consists of a stationary seal ring and a labyrinth ring mounted on the rotor running against each other.
The labyrinth is fed with dry, clean and filtered process gas. If not available an external can be used. The seal gas partially flows away over the labyrinth to process side. A much smaller part of this gas feeds the Dry Gas Seal.
The Positive Seal Barrier is an effective buffer: it keeps contaminated process gas away from the Dry Gas Seals. The stationary seal ring is lined with abradable material. This allows the application of tight clearances resulting in low buffer gas consumption.
To ensure safe and reliable operation of the Dry Gas Seal, seal gas is supplied through a specially designed control system. The control system supplies filtered gas and controls the gas flows. It is designed for easy monitoring of leakage rates and seal performance.
The Dry Gas Seal control system will monitor and control the following flows in case of a tandem configuration.
- (1) Seal gas (dry and filtered process gas): returned to compressor process where a small quantity leaks away over the primary seal to plant’s flare system.
- (5) Nitrogen as intermediate seal gas to separate inboard and outboard seal: to plant’s flare system and a small leakage over the secondary seal to atmosphere.
- (4) Nitrogen to the tertiary seal: partially leaking to vent of outboard seal, partially leaking to compressor bearing housing.
- (2) Primary vent line: leakage over inboard seal and leakage over intermediate labyrinth are both vented to plant’s flare system.
- (3) Leakage of the outboard seal and part of leakage of tertiary seal: this nitrogen flow, can be vented to an atmospheric safe location.
Dry Gas Seal control systems are engineered and manufactured by Howden Thomassen Compressors. This way we can combine Howden Thomassen Compressors experience with customer’s preferences to deliver a tailor-made and easy-to-use control system. The Dry Gas Seal control system can be equipped with additional provisions like heat tracers, coalescent filters, nitrogen generators etc.
All Howden Thomassen Compressors Dry Gas Seal control systems comply with the API 614 safety requirements.
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