Chapter 6: Dry Gas Seal Retrofits

Overview

Thousands of centrifugal compressors were manufactured before the advent of dry gas sealing technology. Most of these compressors are equipped with oil film seals (fig. 6-1).

Figure 6-1: Typical oil film seal

Oil film seals require a supply of sealing oil to be injected into the compressor seals at a pressure slightly higher than the required sealing pressure. The differential pressure is set by an overhead tank mounted at a level 15 feet above the centerline of the compressor. The gas reference pressure is provided at the top of the tank. This sets a differential pressure of approximately 5 psi over the gas reference pressure. The seal oil flows between seal rings on the compressor shaft, forming a barrier against the process gas. Most of the seal oil flows outboard, through the outer seal ring to a sweet drain back to the reservoir. Some of the oil flows inboard, through the inner seal ring, making contact with the process gas. A small amount of process gas mixes with the oil flowing through the inner seal ring. The oilgas mixture is drained to a trap system. In the traps, the seal oil absorbs a considerable amount of process gas. This sour oil is routed to a sour oil drain and either reclaimed or disposed of. If the sour oil is reclaimed, it is generally circulated and purged of entrained process gas oil using heaters and/or degassing tanks (fig. 6-2). The recovered process gas is typically routed to the flare system.