Page 108 - Compression Machinery for Oil and Gas

96 SECTION II Types of Equipment

the seal. To monitor proper supply, the gas temperature, flow, or the differential
pressure is measured and compared with setpoint values. An alarm is issued by
the compressor control system if mismatch occurs. Frequently, orifices are
installed in each supply line downstream of a common control valve to limit
the amount of seal gas in the event of a seal failure and to avoid harm to other
DGSs connected to the same supply.
A support system for DGSs with intermediate labyrinth implements two
vents, primary and secondary vents. Both vents have the function to safely
remove the gas mixture from the leakage of the upstream DGS and the leakage
of the intermediate labyrinth or the partial leakage of the bearing seal. Special
attention is required for the primary vent as the seal health monitoring function-
ality for both the primary and the secondary seal is usually hosted there. To pro-
tect the necessary instruments, a rupture disk or a pressure relieve valve is
installed in parallel with the vent line. This limits the pressure increase in the
vent in the event of primary seal damage.
Because primary seal leakage is typically flammable or toxic, the primary
vent line often discharges into a flare system charged slightly above atmo-
spheric pressure. This requires the installation of a back-pressure valve at the
discharge of the vent line, to control the pressure upstream about 0.5–1bar
above the flare pressure.
A failure of the primary or secondary seal usually leads to a sharp increase in
leakage; this principle is used for monitoring. Failure of the primary seal leads
to an increase in flow, while failure of the secondary seal leads to a drop of gas
flow in the primary vent. Flow is measured and compared with setpoint values.
An alarm (low or high) followed by a shutdown (low low or high high) is trig-
gered by the compressor control system.
Alternatively, pressure measurement for monitoring is common. For this
purpose, installation of a flow orifice between the pressure measurement and
the back-pressure valve is necessary to allow flow-dependent pressure changes.
In low-pressure applications, monitoring with pressure measurement reaches its
limits. To avoid reverse pressure on the primary seal, the primary vent pressure
must be correspondingly low. Pressure fluctuations due to a DGS failure can
practically no longer be measured reliably.

Wet
Wet seals are generally not selected for new equipment due to their higher leak-
age rates and cost of ownership as compared to DGSs. However, a very large
population of wet seals has been installed in the past, and these applications con-
tinue to operate reliably. As the name implies, wet seals require a liquid; and for
oil and gas applications, that liquid is typically oil such that these seals are often
called oil seals. Two types of wet seals are discussed:
l mechanical (contact) seal and
l liquid ring seal