Page 453 - Compression Machinery for Oil and Gas

Compressor System Design and Analysis Chapter 11 433

above 2.4 the running speed. Especially on high-speed machines the 2.4
limit can be difficult to achieve and it is often necessary to consider secondary
bottles as a part of the pulsation control such as the inlet separator or a secondary
pulsation bottle.

Cylinder Orifice Plates
As discussed above for Fig. 11.2, the gas enters (and leaves) the cylinder in dis-
crete slugs, 2 per revolution for a double-acting cylinder. Depending on the
velocity of the gas flowing through passageway the velocity head results in a
pressure pulsation. Applying a fast Fourier transform (FFT) analysis to this
gas flow gives the harmonic content of the flow. Considering the gas passage-
way from the compressor valve to the pulsation bottle volume this acts as a
closed-open pipe and the column of gas has a natural frequency of a quarter
wave. For a 1200-rpm natural gas compressor with a sonic velocity of
519m/s, and a distance from the valve to the bottle volume of 1.2m, the quarter
wave resonant frequency would be 519/(1.2 4)¼108Hz. The operating speed
of 1200rpm is 20Hz so the quarter wave resonant frequency is 108/20¼5.4
the running speed. Looking at Fig. 11.2 a double-acting cylinder will have sig-
nificant pulsation energy at 4 and 6 the running speed. Considering that the
sonic velocity for a given gas composition will vary primarily with gas temper-
ature (higher temperatures have higher sonic velocity), the pulsation analyst
will often consider an orifice plate in the cylinder nozzle to dampen the quarter
wave pulsation resonance. The orifice plate creates a pressure drop that is quite
effective at damping quarter wave resonances at higher running orders. It is
important to note however that orifice plates will normally increase 2 exci-
tation which is in phase with the compressor valve resistance and will cause
an increase in HP and rod load. If the orifice bore is well sized, this increase
should not be significant, but if poorly (under) sized or if the cylinder nozzle
response is near resonance, the resulting 2 response can sharply increase
resulting in increased HP and, at times, compressor valve damage.
In addition, the pressure drop will cause an increase in overtoe and undertoe
losses in the Paucis Verbis (PV) card causing an increase in compression HP
and rod load. API-618 gives an allowable pulsation bottle pressure drop of
ΔP¼1.67[(R 1)/R]%. This gives a value of 1% when the pressure ratio is
2.5. Many packagers use a value of 1% pressure drop for all applications even
though this is higher than that allowed for compression ratios below 2.5. In addi-
tion, the limit is calculated on the basis of steady flow. As shown in Fig. 11.2,
the steady flow is only a small fraction of the peak flow rate for the nozzle ori-
fice plate, usually around 2 on the inlet and 3 on the discharge. As pressure
drop through an orifice is a function of flow velocity squared the effective
instantaneous pressure drop through the orifice plate, it is typically 4–10
the value calculated based on steady flow through the orifice plate. This can
have a significant impact on compression horsepower and rod load possibly