Page 291 - Compression Machinery for Oil and Gas

276 SECTION II Types of Equipment

and reliability desired, the driveshaft seal is rarely regarded as a primary pro-
tection to keep process gas from reaching the atmosphere. It would require
multiple concurrent failures for the process gas to reach the driveshaft seal,
so normally this seal’s function is only to keep atmospheric air out.

Driveshaft Seals for Oil-Flooded Screw Compressors
As described above, there are no internal seals at the conveying chamber of an
oil-flooded screw compressor. The only seal is at the driveshaft. Depending on
the design of the machine, the internal side of the driveshaft seal may be at suc-
tion pressure, discharge pressure, or some intermediate pressure. Regardless,
the seal must be designed for the maximum allowable working pressure
(MAWP) of the machine, and should provide a positive shut-off even without
the presence of seal oil.
One option is to use a single mechanical seal, purged with oil from the com-
mon oil system. Since the seal typically requires oil at the same pressure and
temperature as the rest of the oil system, this is a simple arrangement. The
oil outlet from the seal is routed internally to the discharge flange of the
machine, so there is not a separate oil outlet connection on the machine. At
the outlet of the machine, the seal oil mixes with the rest of the oil being injected
into the bearings and rotor chamber, along with the discharge gas, and is routed
to the discharge bulk oil separator. From there, the oil is pumped to a higher
pressure, cooled, filtered, and reinjected into the machine, and the cycle con-
tinues. Since the oil always carries a small amount of process gas in it, the seal
materials must be compatible with the process gas. The seal must also be
designed to minimize internal pressure drop—if the pressure drop is too high,
the gas will flash out of the oil, which can damage the seal. As with any mechan-
ical seal, there is always some amount of leakage to the outboard side. In this
arrangement, the leakage oil carries a small amount of process gas with it. Nor-
mally the amount of gas coming from the leakage oil is so small that it cannot
even be detected by a gas monitor, and in many cases this amount of leakage is
acceptable to the plant operator. However, in the event of a seal failure, the
amount of leakage oil—and thus the amount of gas carried by the leakage
oil—increases, and when the oil system is shutdown, the process gas inside
the machine may also leak out at a higher rate than what is acceptable. When
this risk is not acceptable to the user, then a double mechanical seal is used.
When a double mechanical seal is required, it is most common to also
require a separate oil system for the seal. By using a separate oil system, it
can be ensured that the seal oil is not in contact with the process gas. There
is still outboard seal leakage, but that leakage oil does not carry any process
gas with it. There is a small amount of inboard leakage as well, at a quantity
that is not enough to significantly alter the properties of the primary oil system.
However, the seal oil (which may be a different viscosity than the primary lube
oil) must still be compatible with the primary lube oil, with the process gas, and