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COMPRESSOR ENGINEERING

Shaft Seal

Elliott Ebara Turbomachinary Corporation

TOKYO JAPAN

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AA SHAFT SEAL

Shaft seals are provided to restrict or prevent process-gas leakage to the atmosphere.

When handled gases are flammable and/or toxic, mechanical contact seals and liquid

film seals were used before self-acting gas seals are becoming familiar. Recently,

self-acting gas seals are commonly used on process gas compressors.

1. MECHANICAL CONTACT SEAL

Pressurized seal oil (normally supplied from the combined seal and lube oil system)

is injected into the seal as shown. Between the seal oil area and the gas zone is a

metal-reinforced carbon ring. The ring is sandwiched between a stationary metal

sleeve and a rotating seal ring. The sealing face of each of these elements is

ground and lapped to a flatness of unusual accuracy, measured in light bands.

Figure A-1 Mechanical contact seal

(Elliott type Iso-Carbon Seal)

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Because the carbon ring is symmetrical, no net axial thrust is imposed on the ring

due to variations in oil or gas pressure. The system of springs acting through the

stationary sleeve thus can apply an accurately controlled, fixed pressure between

the three elements.

A slight flow of seal oil takes place through the seal faces toward the gas zone

preventing any outward flow of gas. This flow, which is recoverable and does not

enter the main gas stream, it from 5 gallons (19 liters) per day, depending on size of

machine. It serves continually to flush the seal faces, discouraging the

encroachment of damaging abrasives into the seal faces. A series of orifices around

the circumference of the seal direct cooling jets of oil against the carbon ring.

Actual installations have proved that even hydrogen does not pass the carbon seal

and enter the oil.

In the outboard direction, a floating ring (or a series of rings, if seal oil pressure

demands it) breaks down seal oil pressure to atmospheric pressure and thus forms a

pressurized isolating oil chamber.

During idle periods, automatic devices will permit the seal oil system to be shut

down even though the compressor is still pressurized. The seal is automatic in its

shutdown situation and is nearly 100% effective.

Since mechanical contact seals allow high differential pressure between seal oil and

process gas, seal oil pressure can be controlled by a simple regulating valve and no

elevated tank is required.

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2. LIQUID FILM SEAL

Pressurized seal oil, fed through the seal oil inlet, is broken down to atmospheric

pressure by a series of floating Babbitt-faced steel rings. This oil is returned to the

clean oil reservoir. A portion of the seal oil is forced thorough the stationary seal

sleeve where it opposes the outward leakage of process gas and keep the gas in the

compressor.

This oil is discharged via the contaminated oil drain, to be reclaimed or

discarded.

Fig A-2 Liquid film seal

(Elliott type Iso-Sleeve Seal)

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3. ACTIVE GAS SEAL (Dry Gas Seal)

Active gas seals control process gas leakage to the atmosphere. Dry gas seals are

commonly used in tandem arrangement for the process gas compressor. A buffer

gas system with a fine filter protects the seal from foreign materials.

Figure A-3 Dry gas seal

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Seal surface has grooves and acts as an compressor and pressure rise the gas at seal

face and keep small clearance at sealing surface.

Figure A-4 Spiral grooves on seal surface

BENEFIT of DRY GAS SEAL

1) Free from oil contamination

2) Low emission to environment

3) Save maintenance time

4) Save oil

5) Save gas

6) Save energy