Page 286 - Compression Machinery for Oil and Gas

Screw Compressors Chapter 6 271

FIG. 6.17 Water cooled restrictive ring seal.

Demineralized water is fed at the suction and discharge end seals at pressures
above suction and discharge pressure, respectively. A part of the water enters
the compression chamber and mixes with the process gas. The other part flows
to the drain and may be routed back to the water tank. The separation seal is
purged with nitrogen at approximately 20kPa gage and prevents water from
entering the oil system.
This is a simple, reliable, and cost effective seal but of course it can only be
used in cases where ingress of water into the process is allowable. If the seal
water fails, the compressor must be depressurized immediately in order to pre-
vent process gas leakage. The restrictive rings are not gastight and therefore a
certain leakage to the flare is unavoidable during depressurization. If the nitro-
gen fails, the compressor must be shutdown also in order to prevent water leak-
ing into the oil reservoir.
For cases where no seal water or sealing gas is available an oil-cooled
mechanical seal is a good choice. Fig. 6.18 shows a single oil-cooled mechan-
ical seal and Fig. 6.19 shows a double oil-cooled mechanical seal. In both cases
the seal is a combination of carbon ring bushings with mechanical seals. The
mechanical seal consists of a stationary ring and a rotating ring which form a
narrow seal gap extending in radial direction. Both rings are pressed together
by springs and by hydraulic forces and slide against each other while a thin
oil film separates the faces. Due to the sliding motion of the seal faces heat
is generated. Oil with a pressure higher than gas pressure is injected into the
annulus at the bottom and cools the seal faces. Also the oil seals the gap between
the seal faces and prevents gas from escaping. The differential pressure between
oil and process gas is typically around 0.2–0.3MPa and is controlled by a dif-
ferential pressure control valve at the inlet to the annulus. The outflow of oil is