Page 443 - Compression Machinery for Oil and Gas

422 SECTION III Applications

discharge pressure, as the gas flow requirement varies. Reciprocating compres-
sors often include a head end suction valve unloader system to provide “step
unloading” of each cylinder, and is very effective with four and six throw recip-
rocating compressors. Various performance points may be looked at using the
integral slide valve on a rotary screw compressor, at different suction pressures
and flow requirements. Centrifugal compressors would utilize inlet guide vanes
and/or diffuser vane control systems to improve the performance at critical
power cost evaluation points. The VFDs are often considered for all of the com-
pressor designs.
For these reasons, it is important to consider the compressor performance
across the entire suction pressure range, because FGC’s rarely operate at the
design (lowest suction pressure and highest flow/discharge pressure) condi-
tions, for the compressor ROI evaluation.
Life cycle and maintenance costs for each design are another important ROI
evaluation points. For a given set of conditions, a centrifugal design will have
the lowest costs, followed by a rotary screw and then the reciprocating design.
However, by selecting a reciprocating compressor with a slower speed, shorter
stroke, and/or lower discharge temperatures, the maintenance costs can be
greatly reduced.
Most FGC’s use electric motor drivers, rather than natural gas engines, due
to lower initial cost, lower maintenance costs, and lower emissions. However,
for power plants operated by a natural gas producer or entity with very low nat-
ural gas costs, the ROI may make sense to use an engine driver. Another advan-
tage is for black-start capabilities, when there is no sufficient electrical power
available to start the electric motor drive on an FGC.
For projects with multiple compressors and multiple turbines, a master inte-
grated FGC control system can help reduce energy costs. It may sound funny,
but the least expensive way to operate an FGC (or any compressor) is off. As
stated earlier, the FGC is generally selected for the worst-case conditions, but
rarely operates at this design point. Therefore, in these applications it makes
sense to have a “lead-lag” sequencing logic which will automatically start
and stop the “lag” FGC, as needed, to maintain the overall current gas flow
and pressure requirements. This is most commonly achieved by programming
logic from the PLC panel on each FGC in the system, and they are linked
together for communicating with each other. In some cases, a DCS or separate
master control system may also be used.
Another important consideration in a fuel gas system is the buffer volume in
the piping between the FGC and the turbine. If there is a turbine trip (drops
100% of the fuel gas required) or the turbine sheds load to an idle mode (typ-
ically drops 60%–70% of the fuel gas required), then the compressor recycle
valve needs to react quickly to this large reduction in gas flow. Depending
on the response time of the recycle valve and the volume between the FGC
and the turbine, there will be a short and sometimes large pressure spike in
the system. It is possible to have a recycle valve for the FGC that has been