Page 290 - Compression Machinery for Oil and Gas

Screw Compressors Chapter 6 275

flows inboard via the carbon rings and mixes with the process gas. A very small
amount of buffer gas passes the primary seal in the outboard direction. A part of
the leak gas flows to the primary vent. The other part passes the secondary seal
and flows to the secondary vent. In some cases there is a requirement for an
intermediate labyrinth between primary and secondary seal which is buffered
with nitrogen at the inlet of the secondary seal. Due to the complex casing
geometry and axial space restrictions this is in many cases not possible for screw
compressors. Between the secondary seal and the bearings there is a separation
seal purged with nitrogen at 20kPa gage (same as for the double dry gas seal)
which prevents oil from polluting the dry gas seals.
Tandem seals are longer in axial direction than double dry gas seals and
therefore a balance line to suction may not be possible due to space restrictions.
In this case the buffer gas must be supplied at a pressure higher than discharge
pressure. An independent supply of filtered and dry buffer gas is necessary.
Tandem dry gas seals are the best choice for high seal pressures (>2MPa)
provided that the required buffer gas quality and availability can be maintained
at all times. Typical buffer gases are natural gas or light hydrocarbons. Nitrogen
is of course the safest choice if available.
Both double and tandem dry gas seals are very sensible against pollution.
Ingress of dirt or liquids may lead to immediate damage of the seals. The buffer
gas must be filtered to 3μm. The gas temperature should be at least 20K above
the gas dew point at the actual pressure. Both seal types and the seal support
systems are very expensive compared to other seal systems.
It must be noted that any kind of seal can work properly only if the sealing
medium is supplied at the required pressure, temperature, and quality. Failure of
the seal medium supply or failure of the seal causes a drop in supply pressure
which is monitored with alarm and trip. The compressor must be tripped by low
sealing medium pressure and depressurized to atmospheric pressure to prevent
process leakage. If in case of a seal failure the compressor is not depressurized
this may lead to a process gas leakage through the seals and into the oil reser-
voir. The oil reservoir is vented to a safe location. The sizing of the vent line
must consider the seal failure case and must be sufficient to maintain the pres-
sure inside the oil reservoir at an acceptable level.
Any seal with nitrogen flow to the bearing drain has the positive side effect
that the oil reservoir is automatically inertized and can therefore even in case of
a seal failure not contain an explosive gas mixture. The shaft seal at the driving
shaft of screw compressors separates the bearing compartment from the atmo-
sphere. Inside this compartment there is air with oil mist at atmospheric pres-
sure. Therefore, a simple labyrinth is sufficient. In some cases, a purge with
instrument air or nitrogen at a pressure slightly above atmosphere may be used
to prevent oil migration along the shaft.
The driveshaft seal of an oil-free screw compressor is typically either a lab-
yrinth seal or a single mechanical seal, oil purged. Since the internal seals at the
conveying chamber described above can be designed with the level of safety