Page 454 - Compression Machinery for Oil and Gas

434 SECTION III Applications

causing overloading. As previously noted resonant pulsations do need to be con-
trolled and API-618 gives allowable pulsation values at the cylinder nozzle, but
the point of interest is actually the compressor valve which can be damaged by
excessive high-frequency gas pulsations. Too often orifice plates are used “just
to be safe” and sized based on average flow. It is recommended that orifice
plates be sized based on instantaneous flow so that the effective pressure drop
is calculated. A low (nonconservative) estimate of effective flow rate can be
estimated based on the total flow divided by the percentage of time the valve
is actually open (70% on inlet and 50% on discharge is typical). The actual pres-
sure drop will be even higher than this calculation, but this is a useful and simple
to calculate method for estimating the cylinder nozzle orifice plate pressure
drop for dynamic flow. For further discussion on this topic refer to the paper
by Hinchliff et al. [2].

Centrifugal and Axial Machines
Centrifugal and axial compressors produce a continuous flow of compressed gas
thusproducingtypicallyinsignificantamountsoflow-frequencyexcitationinthe
fluid flow. However, some machines produce high-frequency excitations,
between 1000 and 4500Hz, depending on the size of the compressor, often
excited by the interaction between rotating and stationary blades. These high-
frequency pulsation can radiate into the suction or discharge piping system.
When aresonance occurs, it is commonly expressedas loud tonal noise or a buzz-
ing type of vibration that can result in vibrations of small bore instrumentation
connections or pipe nipples. Acoustic liners in the form of Helmholtz resonator
arrays can remove the noise and exciting frequencies. The Helmholtz arrays are
installed in recesses milled into the diffuser wall thus forming large cells that act
likeadeadvolumetothemeanflow,butcandissipateacousticenergy.Excitation
or frequencies external to the compressor casing can be attenuated in a similar
manner or through modifications to the resonating piping to decouple the corre-
sponding resonant acoustic or mechanical natural frequency.

Mixed Compression
The mixed operation of centrifugal and reciprocating compressors in a single
compression plant has become common design practice over the last 20 years
as this arrangement is beneficial for operational flexibility or when needing
additional capacity at an existing older station. Pressure and flow fluctuations
can create an oscillating operating point for a centrifugal compressor. As the
operating point varies in location on the compressor map, it may pass through
regions of instability resulting in the compressor experiencing multiple tran-
sient surge events. This creates operational and safety concerns in mixed com-
pression stations as pressure pulsations into the suction or discharge of a
centrifugal compressor can move its operating point to operational instability
regions such as surge, rotating stall, or choke [3].