Gas leakage

Single-acting air-cooled and water-cooled air compressors are available in sizes up to about 75 kW (100 hp). Such units are available in one, two, three, or four stages for pressure as high as 24 MPa (3500 IbFin"). These machines are seldom used for gas compression because of the difficulty of preventing gas leakage and contamination of the lubricating oil. 

Dry gas seals, when eeonomie eonsiderations allow, are another alternative for redueing gas leakage. Dry faee-seal systems have been used sueeessfully and field experienee has proven their effeetiveness. 

The second option, dry face or gas seals, was developed in the early 1970s. In dry face seals, the hydrodynamic pressure that develops between the rotating and stationary seal faces produces positive sealing with minimal seal gas leakage. 

Pressure (Barg) Static gas leakage (Nm /hr) Dynamic gas leakage... 

Turboexpanders with Dry Gas Seals. As mentioned above, the application of dry gas seals in centrifugal compressors dates back to the early 1980s. They are useful in process compressors because seal gas leakage is considerably reduced. Also, the contact of process gas and other constituents that may dilute lubricating oil is minimized. 

The turboexpander dry gas seal consists of the conventional dry gas seal mating ring and primary ring, an outboard labyrinth, an inboard labyrinth, and tlie cavity to be vented, if desired. Tlie outboard labyrinth reduces warm seal gas leakage to the process side efficiency deterioration is thus minimized. The inboard labyrinth, on one hand, provides an additional seal between the process and lubricating fluids. On the other hand, it allows injection of an inert gas, if desired. In the latter case, inert gas leaks to the bearing side and to the cavity between the... 

In this design, the inboard labyrinth of the DGS is fed with seal gas that is compatible with the process gas. The outboard labyrinth is injected with an inert gas. With this arrangement, the bearing housing is purged with inert gas, the seal gas leakage is minimized, and the mixture of seal gas and inert gas is vented to a flare or disposal system. Figure 6-21 shows a cross-section of a turboexpander with AMB and DGS. 

Process gas leakages from the inboard or high-pressure side of the seal. 

First, one must estimate air or other gas leakage into the vacuum system. Of course every effort is made to keep it as tight as possible. The author is aware of possible leak points being sealed with polystyrene, which produces an excellent seal. When tests cannot be made, one must use rules of thumb. Many such rough estimating techniques exist. 

Fhe mechanical contact seal is a very positive seal. The seal is normally oil-buffered. The mechanical seal, which is the most complex and expensive, is used where gas leakage to the atmosphere cannot be tolerated. This may be due to the cost of the gas, as in closed-loop refrigeration, or where the process gas is toxic or flammable. The mechanical contact seal requires more power than the other seals, which is a deterrent to its use on lower power compressors. 

The liquid film seal uses metallic sealing rings and is liquid buffered to maintain a fluid film in the clearance area and thereby preclude gas leakage. It is not unusual in the screw compressor to find the radial bearing and seal combined. 

In controlling gas leakage, shaft end seals are either restrictive or positive in nature. The labyrinth seal is one form of restrictive seal. The reasoning for the labyrinth end seal is generally the same as discussed in the interstage seal section. A procedure for the calculation of restrictive seal leakage is given in Appendix D. 

Inert gas-filled motors can also be used in refineries and chemical plants, but their applications are limited. They have tightly fitted covers and oil seals around the shaft to minimize gas leakage, are continually pressurized with an inert gas or instrument air, and are equipped with an internal air-to-water heat exchanger. Inert gas-filled motors are suitable for any hazardous location but require auxiliaries such as cooling water, gas pressurizing system, and control accessories. 

Capable of preventing inward test gas leakage of more than 0.5% ... 

It is of prime importance to keep air and gas leakage from ductwork at a minimum, as it represents increased fan running cost and the waste of treated air. Leakage into extract ductwork reduces the efficiency of the collection system. 

Airtightness ductwork class A description of the quality of a ductwork system and its ability to contain air with the minimum of air or gas leakage, classified... 

S. Varadarajah, Report on Scientific Studies on the Factors Related to Bhopal Toxic Gas Leakage, Indian Planning Commission, New Delhi, India, Dec. 1985. 

Figure 12-56 can be used to estimate process gas leakage across a single or tandem gas seal arrangement. 

Applications where positive sealing is desirable and only small amount of buffering gas leakage tolerable. Not suitable for very high speed or very high pressure application. Only gases may be used for buffering. 

The bearings and gears are mounted externally to the gas chamber and are separated by stuffing box seals to prevent gas leakage along the shaft. The packing is usually vented to the suction, so that it is necessary to pack only against suction pressure. 

The maximum bubble size, where the total gas leakage through the bubble boundary equals the total jet flow, can be obtained from either Eq. (32) or Eq. (33) ... 

The total amount of gas leakage from the bubble at a bubble size I), is Eq. (36)... 

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