Liquid Buffered Seals

The positive seals are positive in the sense that the process gas is completely controlled, and in most applications, can be designed to avoid the loss of any gas, if the process gas and the sealing fluid are compatible to permit safe separation. In any event, the gas taken from the process is orders of magnitude lower than is the case for the restrictive seal. The positive seals take on the form of a liquid film seal or a contact seal, also known as the mechanical seal. The buffer fluid aids in the sealing process in the liquid film type and acts as coolant in both types. Each manufacturer generally has a proprietary form for one or both types of seal. Figures 5-50, 5-51, and 5-52 show the various seals available. The liquid film type operates with a close clearance and is used for high pressure applications. One modification of the liquid film seal uses a pumping bushing to control gas side leakage and, therefore, operates at bearing clearances (see Figure 5-52).

The contact seal can be used under 1,000 psig. It is more complex, but has the advantage of not leaking while shut down. The contact seal is used extensively in refrigeration service where the compressor is part of a closed loop, and the shutdown feature is desirable. As mentioned, the seals must have a source of cooling and buffer fluid. In many cases, this fluid is lubricating oil. If contamination is not a problem, a combined lube and seal system can be used.

Positive seals have been used in flammable and some toxic services. In toxic applications, an isolating seal must be included in the seal configu-

Liquid Film Seal
Figure 5-50. Liquid film shaft seal [12].
Figure 5-51. Mechanical shaft seal. (Courtesy of Elliott Company

ration. By careful application, the isolating seal can also act as a backup to the primary seal.

In all situations, seals must function over the entire operating range, including startup and shutdown. If a compressor shuts down and is to be restarted hot after being down only a short time, the possibility exists of differential growth of the various components, closing the clearances to the point of seizure of the parts. The seal should be selected well inside its operating pressure range. With the liquid buffered seals, a value for the allowable leakage toward the gas side must be determined. This liquid is removed from the compressor by traps, referred to as sour oil pots, even when the fluid can be recycled. On small to intermediate compressors, the leakage flow should not be more than three to five gallons per day (gpd). Large compressors can have larger leakages, but should not average more than ten gpd per seal.

Gas leakages range from less than 1 Scfm to 1 Scfm. The maximum rubbing speed is considered to be 590 fps. Operating pressures may range up to 3,000 psi. The temperature range using elastomers range from -40" F to 450°F. By using non-elastomers in the seal design, the temperature range is widened to -250°F to 650°F. From these values, if can be seen that the dry gas seal has a wide application range potential.

The dry gas seal has numerous advantages, but, as with most things in life, it also comes with some disadvantages. It is fair to state that for most of the applications, the good outweighs the bad and as such these seals are used extensively in the industry. However, each application should be evaluated on its own merits.

Probably the biggest single advantage of dry gas seals is getting rid of the seal oil. The seal oil system, even when part of a combined lube and seal system, is a complex assembly. With the dry gas seal, the lubrication oil system is all that is needed to service the compressor train bearings and, on turbine driven units, to also supply turbine control oil. As an aside, it makes feasible the dream long held by the compressor vendors of having a standardized lube system line.

Eliminating the oil gets rid of the disposal problem of the contaminal ed oil, which must be properly disposed of or cleaned up and recycled. It also eliminates the fouling problems in components downstream of the compressor. Despite all efforts to the contrary, oil from liquid buffered seals finds its way into the gas stream.

In most applications, the net loss of gas is less. The oil buffered seal loses gas both with the contaminated oil due to gas in solution and through the gas leakoff required to keep the various differential pressures in the proper orientation.

In those application where the cross-coupling effects from the oil seal were detrimental to the rotor dynamics, the use of the gas seal is a distinct advantage. However, the down side is that should the oil seal have provided a good measure of damping, the impact on the rotor dynamics is reversed. None of this is irreversible, but certainly must be kept in mind at the time of design.

As stated, the dry gas seal does come with its own set of disadvantages, The biggest of these is that the buffer gas must be reliable. Loss of buffer gas in some cases will reverse the differential pressure across the seal faces, which will damage the seal in short order. The seals will operate at a zero differential pressure level, but when possible, even a small differential in the proper direction is recommended by the manufacturers. Another disadvantage is the requirement for clean and dry gas at the seal faces. The issue of providing a dry gas supply to the seal is covered in Chapter 8. For dirty gas applications, a sidestream from the compressor discharge will have to be filtered and injected on the process side of the seal Of course, all buffer gas must be filtered. A 2-micron nominal level is considered sufficient. While the requirement for cleanliness of the gas is a disadvantage, it is not unique to the gas seal as the liquid buffered seal, particularly the mechanical type, also has a relatively stringent cleanliness requirement.

One final negative comment: some of the dry gas seals are unidirec tional. This is a problem for compressors that are subject to reverse rotation. It is a problem for using a common spare seal for a compressor, because the rotation makes a seal rotor end specific. For compressors prone to reverse rotation and for the spare parts concern, seals that are bidirectional are available. There may be a small leakage penalty. Other considerations are that the compressor bearings may not tolerate reverse rotation, making the seal limitation not the only factor. Also, though dcfi ■ nitely not recommended, unidirectional seals have rotated in the reverse direction for short periods of time without any major problem. The best solution is to address the reverse direction problem itself. The negatives were pointed out only as a caution to the user. The dry gas seal advantages definitely outweigh the negatives and are a significant addition tn compressor shaft sealing.

Seal configurations are single, tandem, and double opposed (shown in Figures 5-53 A, B, and C, respectively). The single configuration, as the name implies, is a single set of sealing faces with the leakage either flared or vented. The tandem seal, which is probably the most common, consists of two single seals oriented in the same direction. The first seal is considered a primary seal and handles full pressure, while the second seal, which is referred to as secondary, operates at near zero differential and acts as a backup to the primary. Figure 5-54 shows a tandem seal. The leakage is removed from between the seals, and either flared, vented, or recovered if the recovery system can maintain a relatively low pressure. In applications where it is undesirable to permit the primary gas to leak through the secondary seal, such as with hazardous gas, a baffle can be installed between the primary and secondary seal. An additional port is added to permit the injection of a secondary gas with inert properties. This secondary gas then flows through the secondary seal. A variation of the tandem seal is referred to as the triple seal, which uses a two-seal arrangement to break down the pressure. By design, the two seals divide the pressure drop approximately in half and use the third seal as a backup.

Figure 5-54. Cutaway of a tandem arrangement non-contacting dry gas seal. (Courtesy of John Crane International)

The double opposed seal is used in applications where a zero process leakage is mandated. The seal consists of two seal faces, with the process side seal reversed. An inert gas is injected between the two seals at a positive differential over the process gas pressure. A small amount of the inert gas leaks into the process. The process must be able to accept the contamination of the buffer gas for this seal to be used.

Dry gas seals use a separation seal on the bearing side of the seal as a barrier. The purpose of the barrier seal is to prevent lubricating oil from migrating along the shaft and into the dry gas seal. This seal also serves the purpose of preventing any gas leakage from the dry gas seal from leaking into the bearing cavity.

The barrier seals come in two basic forms. One is a labyrinth design, which is probably the most common. It has the features of a conventional labyrinth discussed earlier. The alternative is the carbon ring seal. The carbon ring is used either as a single ring or, in some cases, it is of a mui-

tiple ring configuration. In the latter, it is normally a double ring. The carbon ring may be split and use a garter spring around the outside segments or it may be one piece. The carbon ring has the advantage of being a lower leakage seal and uses less barrier gas. It also provides a more effective seal against oil migration. The carbon seal features in general were covered in the earlier discussions on carbon ring seals.

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Responses

  • marta
    How liquid film seal working in centrifugal compressor?
    4 years ago

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