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While operation and maintenance of centrifugal compressors in series, parallel or a combination of arrangements has been extensively studied, centrifugal and reciprocating compressors are now being operated together in a mixed configuration. The centrifugal compressor is typically a large, gas turbine-driven one supplying base-load compression while the smaller recips supply cyclical or peaking demand. In such cases, the centrifugal compressor may experience some pulsations from the reciprocating compressor on both the common suction and discharge headers.
A good reciprocating compressor bottle and manifold design can minimize the impact on the operational stability of the centrifugal compressor. But in some designs, reciprocating compressors are placed in series with centrifugal compressors for either high-pressure ratio applications or to take advantage of the operational flexibility of a centrifugal compressor (i.e., arrangements with the reciprocating compressor upstream and downstream of the centrifugal compressor are proposed).
For example, a centrifugal compressor may be placed upstream of a reciprocating compressor in gas-reinjection applications to handle the higher flow volumes at low pressure while the reciprocating compressors are better equipped for high pressures and pressure ratios. Centrifugal compressors are often installed downstream of one or multiple reciprocating compressors in gas gathering applications where multiple, very low-pressure streams from the reciprocating compressor(s) are combined into one larger volume medium pressure stream to be compressed to pipeline pressure by the centrifugal compressor. These plant arrangements can lead to significantly higher relative pulsations on the centrifugal compressor than parallel operation, and the operational stability of the compressor may be affected, according to turbomachinery experts Rainer Kurz and Klaus Brun.
In a paper, “The effects of pulsations on the surge margin of centrifugal compressors in stations with both reciprocating and centrifugal compressor”, presented at the 2009 Turbomachinery Symposium in Houston, Texas, they analyzed various aspects of these operations.
Their paper notes that typically, a centrifugal compressor operating with a suction pressure of 40 bar (600 psi) and discharge pressure of 70 bar (1000 psi) may experience upstream pulsations from a reciprocating compressor exceeding 10 bar (150 psi) peak-to-peak, if no pulsation attenuation devices (bottles, choketubes, or orifice plates) are utilized or an acoustic pipe resonance exists. These suction pressure variations of up to 25 percent will clearly present a challenge for any centrifugal compressor aerodynamic stability, especially if the compressor operates near its surge line. Thus, the piping arrangement between reciprocating and centrifugal compressors must be properly analyzed to avoid acoustic resonance conditions and/or attenuate the pressure pulsations from the reciprocating compressor.
The paper shows that by properly analyzing and designing the interconnecting piping between the compressors,utilizing pulsation attenuation devices, and matching the compressor’s mass-flow rates, a satisfactory functional compression system design can be achieved for even the worst cases of mixed centrifugal and reciprocating compressor operation. The paper warns that even small analysis errors, design deviations, or machine mismatches, can lead to a severely restricted (or even inoperable) compression system. Also, pulsation attenuation designs often lead to significant piping pressure losses.