A recycle valve is needed for surge control as well as for the conditions listed above for reciprocating compressors. At constant speed the head-capacity relationship will vary in accordance with the performance curve. For a constant compressor speed:
• If the flow rate to the compressor decreases, the compressor approaches the surge point and a recycle valve is needed.
• If the suction pressure decreases, and discharge pressure remains constant, the compressor head must increase, approaching the surge point in the process.
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