Why monitoring of reciprocating compressors ?
Bahia de Bizkaia as a project involves the
construction and commercial operations of an
LNG regasification plant (Bahia de Bizkaia
Gas), which will have an annual regasification capacity
of 800 000 Nm3/h, and an 800 MW combined cycle
power plant (Bahia de Bizkaia Electricidad). The
starting point for Bahia de Bizkaia lies within the
energy strategy devised by the Basque government,
which has been brought about by means of several
energy plans.
The liberalisation of the electricity market paved the
way for another project: the construction of a combined
cycle power plant (Bahia de Bizkaia Electricidad). This
was intended to make the most of the possibilities
offered by the regasification plant, as well as benefit
from the different synergies between both projects, and
help correct the shortage of power production in an
area which, as mentioned above, has a great demand
for this kind of energy, but not enough generation
capacity.
The total investment for the project amounts to € 655 million; € 338 million has been invested on
the power plant (Bahia de Bizkaia Electricidad), and the remaining € 317 million has been invested on
the regasification plant (Bahia de Bizkaia Gas, BBG).
Table 1 reflects the current trend towards a continuous
increase in gas consumption and production.
Boil off gas recovery system
This system consists mainly of two gas recovery compressors together with their associated pipeline.
Burckhardt Compression built these two vertical two
stage laby compressors in 2001. The compressors
are operated at 496 rpm with a maximum flow of
2560 kg/h each, driven by induction motors with a
power of 520 kW to recompress boil off gas from
the tanks. The capacity is controlled by unloading
of suction valves and by switching fixed clearance
pockets. The regulation allows the load steps of
100%, 75% (head end clearance pockets open at both
stages), 50% (crank end unloaded at both stages),
25% (crank end unloaded, head end clearance pockets
open at both stages) and 0%for startup.
During normal operations, a single boil off gas
compressor at approximately 50%of its capacity
is used to deal with vapour originated as a result
of heat in the tanks. Such vapour is then sent to a
recondenser, which is able to handle the whole flow
of vapour arisen in the terminal, thus keeping tank
pressure ur\der the maximum permitted values.
Upon LNG carrier unloading operations, a
noticeable increase in the gas flow is generated in
relation to gas produced during normal operations;
therefore, both compressors are used, sending out
to the vessel excess gas that has not been absorbed
by gas tanks onboard down to the recondenser. These
two compressors have a capacity of 2.560 kg/h each.
Natural gas propelled by compressors is sent to the
recondenser where primary pumps liquefy it.
The reliquefier houses an inner cylinder packed with
stainless steel pall rings, which forms an annulus with
the outer wall of the recondenser. Boil off gas and LNG
enter the inner cylinder from the top of the recondenser
and are contacted in the packing.
The annulus pressure and liquid level
are controlled and this maintains a
constant pressure at the suction to
the sendout pumps.
Pressure in the packed bed and
annulus are individually controlled by
splitting the flow of boil off gas into
each one of them. Regulating the LNG
flow into the recondenser controls the
volume of liquid in the vessel. The bed
liquid level varies with pressure and
vapour/liquid load on the recondenser.
Recondenser pressure is kept by
passing a portion of boil off gas from the
compressors around the packed bed section of the recondenser. In the event
that insufficient boil off gas is available
from the compressors, sendout gas
taken upstream of the metering station
will be used for recondenser pressure
control.
Read the full article from LNG Magazine, January 2009 here. |
