Centrifugal compressors use shaft end-seals to:
1. Restrict or prevent leakage of air or oil vapors into the process gas stream.
2. Restrict or prevent leakage of process gas from inside the compressor.
Various types of seals are used, depending on the gas being compressed, the pressures involved, safety, operating experience, power savings, and process requirements.
Shaft end-seals are separated into two broad categories:
• the restrictive seal which restricts but does not completely prevent leakage; and
• the positive seal designed to prevent leakage.
Restrictive seals are usually labyrinths. They are generally limited to applications involving non-toxic, non-corrosive, abrasive-free gases at low pressures. In some cases, ports for injection or withdrawal of the gas are used to extend the range of effectiveness. Some possible arrangements are shown in Figure 200-42.
Another form of the restrictive seal is the dry carbon ring seal, often used on overhung single-stage compressors where maximum sealing and minimum axial shaft spacing are important. Since this seal can be held to close clearances, leakage is less than with the labyrinth seal. Also, less axial shaft space is required (see Figure 200-43).
Positive seals, while varying somewhat in design between manufacturers, are either liquid-film or mechanical contact type.
The liquid-film type is shown in Figure 200-44. A schematic of a seal system is shown in Figure 200-45. Sealing oil is fed to the seal from an overhead tank located at an elevation above the compressor set to maintain a fixed five psi (typically) differential above “seal reference” pressure. (Seal reference pressure is very close to suction pressure.)
The oil enters between the seal rings and flows in both directions to prevent inward leakage to the process gas or outward leakage of the gas to the atmosphere. “Buffer ports” are often available for injection of an inert gas to further ensure separation of the process from the sealing medium. The oil-film seal is suitable for sealing pressures in excess of 3000 psi. (See Figure 200-46 for an illustration of a buffer-gas injection.)
The tilting-pad oil seal (shown in Figure 200-47) is a design that recognizes that in some cases the seal operates as a bearing. It can be used in high-pressure, high-pressure-rise applications to improve rotor stability.
The mechanical contact seal (Figure 200-48) is used at pressures up to 1000 psi, and has the added feature of providing more positive sealing during shutdown. Sealing is provided by means of a floating carbon ring seal riding between a stationary and a rotating face. The seal medium (oil) functions primarily as a coolant. Seal oil differential is controlled by a regulator rather than an overhead tank.