Page 288 - Compression Machinery for Oil and Gas

Screw Compressors Chapter 6 273

back into the suction line. Normally the leak oil is polluted by the process gas
and therefore cannot be reused.
The discharge end seal is balanced to the suction line. Thus both seal ends
can be buffered with seal oil at the same pressure level.
Normally lube oil and seal oil are the same and have one common oil res-
ervoir. Therefore, the requirements of the seals must be checked when choosing
an oil type. The oil must not contain zinc additives because these tend to form
layers on the faces of the seals.
A combined seal is any seal that incorporates more than one primary seal
type/medium into a single seal arrangement. The water buffered restrictive ring
seal mentioned above is technically a combined seal because it includes a sep-
aration seal that is purged with gas, which prevents sealing water from reaching
the oil, and lubrication oil from the bearings from reaching the water return con-
nections of the compressor. Another common type of combined seal is a gas-
purged carbon ring seal on the process side with an oil-purged mechanical seal
on the oil side. The oil-purged seal reduces the consumption of seal gas from the
carbon ring seals and provides positive shut-off during standstill, while the gas-
purged carbon ring prevents oil from the mechanical seal from contaminating
the process.
Mechanical seals are a reliable and cost effective seal type which can be
used in a variety of applications. In case of seal oil failure, the compressor
must be shutdown immediately. Failure of seal oil supply may damage the seal
faces, but if the seal faces remain intact the seal will contain the gas during
standstill without gas leakage to the atmosphere. For dry screw compressors
mechanical seals are normally used up to 1.5MPa suction pressure although
higher pressures are possible. It should be noted that the oil system will be
more expensive due to larger oil flow and higher oil pressure than for other
seal types. The power losses of the mechanical seals are similar to those of
the bearings which increases power consumption compared to the other
seal types.
Dry gas seals are typically arranged in double or tandem design. Fig. 6.20
shows a double dry gas seal. The basic arrangement is the same as for the double
mechanical seal but the seal medium in this case is nitrogen. Nitrogen is fed into
the annulus between the two seal faces at a pressure of 2–3bar above the gas
pressure. A very small amount of nitrogen leaks via the inboard seal face into
the process and via the outboard seal face to a safe location. A separation seal
consisting of carbon rings is fed with nitrogen at approximately 20kPa gage and
prevents bearing oil from polluting the dry gas seal. The dry gas seals are sep-
arated from the process by carbon ring bushings. The discharge end seal is bal-
anced to the suction line.
This seal can only be used, if nitrogen is available at a pressure higher than
the highest possible suction pressure [including settle-out pressure (SOP)]. The
nitrogen consumption of double dry gas seals at high pressure is much less than
for carbon ring seals. Dry gas seals are very sensitive against pollution or