Normally the flare gases bubble through water seal in the
seal drum upstream of the flare stack. The liquid level in
the seal drum imposes a positive backpressure in the flare
header and thus ensures that air is not drawn into the flare
system. For providing a better suction pressure while avoiding
air ingress to suction of flare gas recovery compressor, modifications
are required to be carried out to increase the water level
in the water seal drum.
FGRS is located downstream of knockout drums as all flare
gases from various units in the refinery is available at this
single point. It is located just upstream of the seal drums
as pressure control at the suction to compressor will be maintained
precisely, by keeping increased height of water column in
The flare gas enters the compressors at 1.1 Kg/cm2 Abs and
380C. A continuous re-circulating flow of process water enters
the compressors for compression, sealing and cooling of gas.
After leaving the compressors the gas, water and hydrocarbon
mixture enters the Gas/Liquid separator where the compressed
gas is separated from the water and condensed hydrocarbons
by gravity due to lower gas speed. The separated flare gas
passes through demister before leaving the G/L separator in
order to have a minimum water and condensed hydrocarbons content
in the outlet gas stream and leaves from the top at 8.0 Kg/cm2
Abs and about 500C. The process water leaves the G/L separator
from the bottom where it is pushed back to the compressors
due to the pressure differential between G/L separator and
compressor inlet (liquid ring).
The normal operating liquid ring flow rate is about 32 m3/hr.
at 380C. A common shell and tube cooler on the liquid ring
line assures the continuous cooling of the ring and therefore
isothermal gas compression. The cooler is designed for the
duty of two compressors.
The flare gas is dry but during compression process it becomes
saturated with water. This causes a continuous water decrease
in the system. Due to this fact and to clean up the liquid
ring process water from hydrocarbons, a continuous process
water make up line in the compressor suction line is provided.
The condensed hydrocarbons overflow into OWS from where they
are discharged by means of level control valve. The process
water overflows through a weir into a collecting compartment.
The level into the compartments is guaranteed by the control
valve that is controlled by the level transmitter. The excess
water is sent to OWS.
The unit has a by-pass between the inlet and outlet of the
unit to control the inlet line pressure. If the inlet pressure
decreases below a certain value the valve starts opening till
full recycle is established (no gas available).
The gas stream from the G/L separator is routed to the Flare
gas Amine Absorber where the gas is amine treated to remove
H2S present in the gas. The treated gas from the Amine Absorber
goes to the Fuel Gas header.
If the volume of gas relieved into the flare system exceeds
the capacity of FGRS, the excess gas volume will flow to flare
stack. If the volume of gas relieved into the flare system
is less than the full capacity of the recovery unit, a spillback
valve will divert the discharged gas back to the suction header,
to maintain the capacity of FGRS.
Choice of Compressor
Liquid-ring compressor is preferred and most suitable over
the other types (i.e reciprocating, screw, centrifugal etc.)
available in the market for this service, since it ensures
near isothermal compression and an intrinsically explosion-proof
operation. It can handle a wide variation in flow rate, dirty
gas, liquid slugs and solids. Liquid-ring compressors use
a liquid (water) to form a seal in the shape of a ring between
the outer ends of the impeller and compressor housing. The
centrifugal force of the rotating impeller forces liquid to
the outside wall forming a seal.
As the operating fluid absorbs most of the heat of compression,
there is minimal rise in recovered gas temperature during
compression. The compressor doesn’t require any after
cooler. After separation of the compressor operating liquid
from the gases, the operating liquid will get discharged from
the separator and is cooled through a heat exchanger. Once
the operating liquid is cooled, it returns to the compressor
where it is re-used to create the compressor seal. No separate
booster pump is required to move the operating fluid from
the separator to the compressor. Due to presence of sour gases
the quality of operating liquid needs to be strictly maintained
to prevent acid build up and contamination of the operating
fluid. Liquid bleed and fresh water make-up capabilities have
been provided which will operate as necessary.
Flare Header Pressure Control
For safe operation of FGRS it is mandatory to eliminate the
possibility of air being sucked into the flare gas system,
since the composition is normally of hydrocarbons and hydrogen
sulphide but also of hydrogen, what when mixed with air can
be highly explosive. To avoid ingress of air into flare header,
it is required to maintain always a positive suction pressure
at around 150-200 mm water gauge (water seal level in drum
needed adjustment) . In case of reduction of suction pressure,
compressor spillback valve will open to recycle compressed
gas back to compressor suction.
Seal Drum Modifications
Modifications to the existing seal drums were necessary to
have a bandwidth for pressure control of the flare gas recovery
system. The seal height has to be increased from existing
127 mm to 1000 mm. The increase in submerged portion of dip-legs
was achieved by increasing the liquid level in the vessel
by 873 mm.
The above modification called for an investment of Rs. 10.08
Cr and the savings achieved are Rs. 14.0 Cr./annum with a
payback period of 9 months.