Page 188 - Compression Machinery for Oil and Gas

Reciprocating Compressors Chapter 5 177

to as a separable compressor. Separables were built starting around 1970. Sep-
arables are available to match gas engines (less commonly to diesel engines)
and are available in the power range of 200kW up to around 7000kW. Modern
separable gas engine are almost exclusively four-cycle designs.
The primary market for engines used to drive a reciprocating compressor is
for electric generators and they are therefore designed to run at electric synchro-
nous speeds such as 720, 900, 1200, and 1800rpm for 60Hz frequency and 750,
1000, and 1500rpm for 50Hz. Natural gas fueled engines are available from 40
to 6000kW. As engines get larger they run slower.

Integral Engine-Compressors
An engine/compressor configuration that is not prevalent today is the integral
engine-compressor. This is a design where the compressor cylinders are
mounted on the engine frame and driven by the engine crankshaft. Integral
engine-compressors power levels were typically 4500kW or less, running at
360rpm or less, and very few have been built since 2000. Many thousands
of integral engine-compressors are currently operating. Integral engine-
compressors were built both as two and four cycle.

Turbines and Gearboxes
It is possible and maybe advantageous in some cases to drive a reciprocating
compressor with a gas or steam turbine. A gas turbine may allow the burning
of natural gas with some quantity of hydrogen sulfide which cannot be accom-
plished in the current regulatory climate with a reciprocating engine. If a facility
has excess steam (or heat from which steam may be produced), it can potentially
be used to drive a steam turbine. The “fuel” in this case is essentially free or very
low cost. With either kind of turbine drive a speed reducing gearbox will need to
be utilized because the turbine will normally run at rotating speeds much higher
than the driven reciprocating compressor. The addition of a gearbox to the drive
train raises significant torsional vibration concerns and in many cases will
require the use of a torsionally soft coupling between the gearbox and
compressor.
In common practice, gas and steam turbines are rarely used as drivers for
reciprocating compressors. Steam turbines were fairly common in refinery
applications prior to 1980 when the steam supply from within the refinery
was available and highly reliable as a source of power. However since 1980
the steam turbine has been rarely used. Steam turbines operate at 3000–
4000rpm and gas turbines at greater than 4000rpm. So the turbine is connected
to the compressor by a double reduction gearbox. The gearbox will usually have
a double helical gear for smooth power transmission and the absence of thrust.
A large flywheel and torsional soft coupling are generally used to couple the
compressor and gearbox in these cases. The first torsional natural frequency
is tuned to approximately 2/3 of the minimum full-load run speed so that the