Page 441 - Compression Machinery for Oil and Gas

420 SECTION III Applications

get the fuel gas requirements across the entire range of conditions from the tur-
bine manufacturer, rather than just consider the highest flow and gas pressure
required.
Larger combustion turbine power plants are typically designed to be oper-
ated continuously. They may be a simple cycle design, with only natural gas
combustion turbines generating electricity, or, a combined cycle design with
one or more additional steam turbines to produce more electricity. The combus-
tion turbine waste exhaust heat is used to generate the steam in conjunction with
natural gas duct burners. Additional fuel gas requirements for the duct burners
are typically at a lower flow and pressure than the combustion turbine, but still
need to be considered in sizing the FGC. If the duct burner service is intermit-
tent, the flow may be added to the turbine requirements in sizing the FGC, and
then regulated down to the required duct burner pressure. This solution is sim-
ple, but not very energy efficient. A separate FGC sized for just the duct burner
and other low flow/pressure gas requirements would be more energy efficient,
but would have higher initial capital costs. If the main FGC has multiple stages,
it can be sized to take the duct burner gas flow off one of the initial stage(s) of
compression for more efficient operation, and the capital cost is generally less
than a separate lower flow/pressure compressor system. A separate lower pres-
sure piping system cost also needs to be considered in the overall evaluation,
with either of the lower pressure compressor solutions.
Continuous-duty power plants typically require redundancy in the FGC sys-
tem. A 2 100% redundant system will generally be less expensive in capital
and installation costs, compared to a 3 50% or 4 33.34% system.
However, there is a cost effective and efficient 3 33.34% alternate solu-
tion, when there is a wide range of suction pressures for a specific project. This
is acceptable for many clients, when the minimum suction pressure only occurs
at a very low percentage of the time, and the 3 33.34% would be sized for the
total flow with this minimum suction pressure. A compressor is capable deliv-
ering a much higher capacity with a higher suction pressure. This is effective if
the much higher historical or average gas pressure is available most of the time,
and is sufficient to allow a single compressor in this 3 33.34% system to pro-
duce at least 50% of the total flow required. So, in essence, this really allows for
a3 50% redundancy most of the time. If a single compressor is not operational
during the rare occurrence of the lowest design suction pressure, the turbine
would throttle back to a partial load operation. This solution is also typically
more energy efficient than operating a compressor sized for 100% of the flow
with the minimum suction pressure, but at the much higher historical or average
gas pressure.
Some power plants are designed for intermittent or peak loading, which may
be only a few hundred hours per year. For these “peaker plants,” the capital costs
are typically a bigger factor in the FGC evaluation than the operating, mainte-
nance, and/or redundancy costs.