Page 231 - Compression Machinery for Oil and Gas

220 SECTION II Types of Equipment

bank using one common inlet and one common discharge bottle, similarly for
the second stage on the opposing bank.
For the case of a single double acting cylinder, the H.E. and C.E. pulses are
not identical, because they have different swept volume (C.E. is less due to area
of piston rod. Internal clearances are often different and the instantaneous piston
velocity is different on the outboard and inbound stroke due to the slider crank
mechanism. Note that because there are two pulses of gas the second harmonic
(2 running speed) dominates, but there is a small excitation at almost all the
harmonics. For a four throw, the two cylinders sharing a bottle will have pulses
every 90 degrees of crank rotation so the max pulse harmonic with be 4 . For a
six throw, three cylinders will share a bottle and with be phased every 120
degrees (pulses every 60 degrees for double-acting cylinders) and the sixth har-
monic will dominate.
There are two basic types of pulsation damper that are commonly used.
Surge volume, here the cylinder feeds into a simple volume bottle. This uses
the principle of a volume and some pressure drop, either an orifice plate or
choke at the bottle line connection. Typically, volumes of 10–30 cylinder
swept volume and pressure drop at the line connection of 1% are adequate to
meet the API-618 pulsations requirements. This bottle may have an internal baf-
fle to control the shaking forces and an orifice at the cylinder nozzle if necessary
to control the quarter wave nozzle resonance. Simple surge volume bottles are
typically used on low-molecular-weight gases and slow-speed compressors.
Volume-choke-volume acoustic filters. These are highly effective pulsation
control devices where the two volumes and connecting choke are designed so
that the Helmholtz resonant frequency is below either the 1 or in case of
double-acting cylinders with very low 1 pulsation, below the 2 frequency.
Then the pulsation bottle will effectively attenuate almost all frequencies
greater than 1.4 the Helmholtz frequency. Acoustic filters are used on
high-speed compressors and at higher molecular weights (to the right of
the curve).
Surge volumes are typically used for applications on the left of the curve and
acoustic filters to the right.
For guidance in sizing acoustic filters refer to Ref. [20].

Bottle Design
The sizing and selection of pulsation bottles are done as a part of a pulsation
study which is an acoustic simulation of the cylinders and piping and is done
in accordance with API-618 (Reciprocating compressors and API-688 (pulsa-
tion and vibration control for positive displacement compressors—due to be
published in 2019). API 618/688 describes the theory of pulsation control
and provides limits for pulsation levels at the cylinder and also in the piping