Page 284 - Compression Machinery for Oil and Gas

Screw Compressors Chapter 6 269

arrangement has the advantage that the speed of both stages can be chosen to
their optimum and thus optimum efficiency is ensured. Also both stages can
use their own torsion shaft and have their thrust bearing loads reduced.
The tandem arrangement has a simpler gearbox with only one pinion. On the
other hand, the speed of the second stage and the interstage pressure cannot be
chosen freely and may not result in optimum efficiency. As a further disadvan-
tage the first stage has to transmit the torque of the second stage which results in
higher torsional stresses in the driveshaft of the first stage.
Therefore, the parallel arrangement is a more versatile design.

Typical Shaft Seal Arrangements

Shaft Seals for Dry Screw Compressors
The purpose of shaft seals is to prevent leakage of process gas to the bearings
and the oil system and to the atmosphere. In vacuum compressors shaft seals
must prevent air from entering the compression chamber. The optimum seal
choice depends on the actual process conditions (dry or liquid injected, seal
pressures, availability of suitable seal medium, compatibility of seal medium
with the process) and of course economic considerations. The seal type should
be chosen by the compressor manufacturer together with the process engineer.
Typically, dry screw compressor seals are one of these types:

l Carbon ring seals in various arrangements
l Liquid injected restrictive ring seals
l Oil-cooled mechanical seals
l Double or tandem dry gas seals
l Labyrinth at driving shaft
l A combined seal incorporating multiple of the above elements
Fig. 6.15 shows a simple carbon ring seal arrangement where the discharge end
seal is balanced to the suction line. The suction end seal and the outboard end
seal on the discharge end are buffered with nitrogen which prevents process gas
from escaping into the oil reservoir. This arrangement is very simple and cost
effective because it needs only one nitrogen control valve with pressure mon-
itoring and nitrogen pressure and flow are relatively low. If the nitrogen supply
fails gas will escape into the oil reservoir.
Fig. 6.16 shows a more complex carbon ring seal with two different seal gas
supplies. Buffer gas is injected at suction and discharge end with pressure con-
trol above suction and discharge pressure, respectively. One part of the buffer
gas enters the process chamber and the other part is routed to a flare. The buffer
gas may be nitrogen but other gases like natural gas or steam could also be used.
The outboard seal acts as a separation seal and is fed with nitrogen controlled
above flare pressure.
Carbon ring seals are simple, cost effective, and reliable seals. They are typ-
ically used in applications with pressures <1MPa. Nitrogen is required as