Page 452 - Compression Machinery for Oil and Gas

432 SECTION III Applications

the cylinder nozzle if necessary to control the quarter wave nozzle resonance.
Simple surge volume bottles are typically used for low molecular weight gases
when operating with low-speed compressors.
Volume-choke-volume acoustic filters are highly effective pulsation control
devices where the two volumes and connecting choke are designed such that the
Helmholtz resonant frequency is either below 1 or, in the case of double act-
ing cylinders with very low 1 pulsation, below 2 frequency. This type of
pulsation bottle will effectively attenuate almost all frequencies greater than
1.4 the Helmholtz frequency. Acoustic filters are used on high-speed com-
pressors and at higher molecular weights. Fig. 11.3 can be used as a rough
guideline in selecting the type of filter for a particular application.
For specific guidance in sizing acoustic filters refer to the GMRC guideline
noted in [1]. Sizing and selection of pulsation bottles are done as a part of a pul-
sation study which is an acoustic simulation of the cylinders and piping
performed in accordance with API-618 (reciprocating compressors) and API-
688 (pulsation and vibration control for positive displacement compressors—
due to be published in 2019). API 618/688 describe the theory of pulsation
control and provide limits for pulsation levels at the cylinder as well as in
the off-skid piping. It provides acceptable limits for pressure drop in the pulsa-
tion bottle as a function of the compression ratio.
There is a tradeoff in sizing pulsation bottles. For a given pulsation limit the
designer can use smaller bottles with higher pressure drop (higher friction loss
at the orifice plate and choke) versus large bottles with lower pressure drop. But
the user is cautioned about the use of excessively large bottles. They have three
technical problems: potentially high shaking forces due the gas pulsation acting
on the baffles, lower and more easily excited mechanical natural frequencies
(MNFs), and, because they are large and heavy, difficulty in mechanically sup-
porting bottles on the inlet of the compressor safely above the cylinders. Piping
and bottles are usually designed to have a mechanically resonant frequency

FIG. 11.3 Surge volumes are typically used for applications on the left of the curve and acoustic
filters to the right.