Page 233 - Compression Machinery for Oil and Gas

222 SECTION II Types of Equipment

user is cautioned about the use of excessively large bottles. These have two
technical problems, potentially high shaking forces due to the gas pulsation act-
ing on the baffles and because they are large and heavy the difficulty in support-
ing safely above the compressor cylinders. Piping and bottles are usually
designed to have a mechanically resonant frequency 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 separator or a secondary pulsation bottle.

Cylinder Orifice Plates
Gas enters (and leaves) a cylinder in two discrete slugs 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 an FFT
analysis to this gas flow gives the harmonic content of the flow. Considering the
gas passageway 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. So for 1200rpm 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
running speed. Looking at Fig. 5.40 a double-acting cylinder will have signif-
icant pulsation energy at 4 and 6 running speed. Considering that the sonic
velocity for a given gas composition will vary primarily with gas temperature
(higher temperatures have higher sonic velocity), then the pulsation analyst will
often consider that an orifice plate in the cylinder nozzle is necessary to dampen
the quarter-wave pulsation resonance. The orifice plate creates a pressure drop
that is quite effective at damping the quarter-wave resonance (usually at 4 or
6 machine speed), but this does nothing to reduce the 2 pulsation, in fact it is
made worse. In addition, the pressure drop will cause an increase in overtone
and undertone losses in the PV card causing an increase in compression HP
and rod load. API-618 gives an allowable pulsation bottle pressure drop of
ΔP ¼1.67((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 the allowance for compression ratios below 2.5. In
addition, the limit is calculated on the basis of Steady flow. As can be implied
from Fig. 5.40 the steady flow is only a small fraction of the peak flow rate for
the nozzle orifice plate, usually around 2 on the inlet and 3 on the discharge
is typical. As pressure drop through an orifice is a function of flow velocity
squared the effective instantaneous pressure drop through the orifice plate is
typically 4–10 the value calculated based on steady flow through the orifice
plate. This can have a significant effect on compression horsepower and rod
load possibly causing overloading. As previously noted resonant pulsations
do need to be controlled and API-618 gives allowable pulsation values at the