Page 444 - Compression Machinery for Oil and Gas

Downstream Chapter 10 423

designed or adjusted for an extremely short response time to mitigate the pres-
sure spike caused during this turbine upset condition. But, this rapid valve
design criteria can impact the FGC discharge pressure stability during normal
operation, because the recycle valve tends to “over react” around the discharge
pressure set point. The volume in the fuel gas system is the other controlling
variable in minimizing the pressure spike and providing a stable pressure during
normal operation. Calculations can be made to determine the proper buffer vol-
ume, but in general “bigger is better.” One simple solution to increase the vol-
ume is go to a larger pipe diameter between the FGC and the turbine. This
solution will also lower the piping pressure drop, so the compressor can operate
at a lower discharge pressure. Large gas receivers or pressure vessels may also
be installed in the system to increase volume. One contingency recommenda-
tion is to include one or more blind flange connections in the piping between the
FGC and the turbine, so that a gas receiver can easily be added later, if needed.
A larger buffer volume also helps during the transition of starting a backup or
additional FGC.
Many new power plants, especially for the smaller CHP/DG/MG sites, are
being constructed near existing homes or businesses. Noise level of the pro-
posed plant was an important criterion for the approval of the construction.
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Most FGC’s do not operate below 85 dBA at 3 and typically can be in the
100+ dBA at 3 range. Multiple FGC systems will be even louder. One solution
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is to provide a low sound enclosure around the FGC. If small enough, the enclo-
sure can be mounted on the FGC skid, but this will increase the freight cost to
the site, especially if it is located in older high-density population areas, such as
New England or California. In that case a field-constructed low sound enclosure
is required. The enclosure would need proper ventilation, lighting, a gas/fire
detection system, and typically a fire suppression system. If the FGC is going
into an enclosure, the electric motors need to be designed for a higher maximum
ambient temperature because the interior temperature of the enclosure will gen-
erally be 10–20°F above the maximum ambient temperature.
An alternate noise attenuation solution, which works especially well for sites
in temperate climates, is to use sound barrier walls. The walls are field con-
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structed around the perimeter of the FGC skids, with about 3–5 maintenance
accessibility around all sides. If there is an air-cooled gas heat exchanger, this
would be installed outside of the sound wall perimeter. The sound barrier walls
are less expensive and generally do not require ventilation, lighting, a gas/fire
detection system, or a fire suppression system. It also makes it easier to pull a
major component (compressor, cylinder, motor) off the FGC skid because there
are no roof panels to remove. Sound barrier walls may not be able to meet very
low near field or far field noise level requirements, but for sound levels at or
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above about 80dBA at 3 , they are a viable alternative.
Power cost comparisons have become increasingly important in FGC selec-
tions and evaluations, even with smaller projects. Attached is an example of a
case study of a CHP project awarded to Norfolk, VA that compares the