Seal gas will be provided to each turbine driven compressor as a barrier
between the process gas lift and the dry gas seals. The primary source of seal
gas is from the discharge of the 2nd
stage gas lift compressor. The gas is regulated by inlet control valves (2x100%
configuration) to maintain the pressure at 95 barg.
The seal gas scrubber will remove any entrained liquid from the supply gas.
The filter coalescer is provided to removed 99.9% any liquid / solid particle
greater than equal to 2 micron. Liquid
drop is removed in the seal gas scrubber located immediately downstream of the
pressure control valve.
The gas will be heated such that it will remian superheated by at least 10
degC to prevent condensation. The heater are equipped with electric heating
elements, controlled by seal gas outlet temperature controller set at 85.6
degC.
The seal gas is provided with high-high and low-low pressure trips,
high-high and low-low temperature trips, high-high and low-low level trips.
Control loop destription
Pressure Controller
The controller PIC-A maintains a
constant pressure in the seal gas scrubber at 95 barg by actuating control
valve A. If the pressure continues to fall, PIC-B will actuate control valve B.
PIC-B shall be less than the setpoint of
the PIC-A.i.e, set at 92 barg such that the control valve B will normally
remain closed. The control valve is fail closed type. The controllers output
signal shall be reverse acting and control valve OPDIR is direct. As the
pressure falls below the setpoint. The controller shall open the control valve
to increase the pressure.
Level Controller
Level control in the scrubber is performed via level controller by opening
the control valve between ON and OFF levels.
The control valve is fail closed type. As the level reached the ON setpoint,
the controller will open the valve to decrease the liquid level and close the
control valve when the level reaches the OFF set point
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