Method and system for operating a back-to-back compressor with a side stream

A method for operating a gas compressor, the method comprising:

• providing a first compressor stage and a second compressor stage in a back-to-back arrangement;
• providing a seal arrangement between the first compressor stage and the second compressor stage;
• providing a side stream line between the first compressor stage and the second compressor stage;
• providing an antisurge system for the first compressor stage, comprised of a bypass line and an antisurge valve;
• feeding a first gas having a first molecular weight to a suction side of the first compressor stage and compressing the first gas through the first compressor stage;
• feeding a side stream flow of a second gas through the side stream line, the second gas having a molecular weight lower than the molecular weight of the first gas;
• compressing a gas mixture of the first and second gas through the second compressor stage;
• recirculating gas mixture from the second compressor stage to the suction side of the first compressor stage;
• and
• regulating the side stream flow only if the antisurge system is active to correct a pressure ratio alteration across the compressor caused by a variation of molecular weight of the gas processed by the first compressor stage provoked by recirculated gas mixture.

A compressor system comprising:

• a compressor comprised of:
• a first compressor stage having a suction side and a delivery side, the suction side configured to receive a flow of a first gas having a molecular weight;
• a second compressor stage, having a suction side and a delivery side, the first compressor stage and the second compressor stage arranged in a back-to-back arrangement;
• and
• a seal arrangement between the first compressor stage and the second compressor stage;
• a side stream line in fluid communication with the suction side of the second compressor stage, configured to deliver a flow of a second gas having a molecular weight lower than the first gas, a mixture flow of the first gas and second gas being processed through the second compressor stage;
• a side stream valve on the side stream line configured to adjust the flow of the second gas;
• a side stream controller configured to control the side stream valve;
• an antisurge arrangement comprised of:
• a bypass line configured to recirculate gas from the delivery side to the suction side of the first compressor stage and an antisurge valve on the bypass line;
• and
• a pressure transducer arrangement configured to detect at least one pressure parameter of the first compressor stage,
• wherein the side stream controller is configured to reduce the flow of the second gas when the pressure transducer arrangement detects an alteration of said pressure parameter indicative of a reduction of a pressure ratio across the first compressor stage provoked by a recirculation of gas through the antisurge arrangement, and enable reduction of the side stream flow if the antisurge arrangement is active or if the first compressor stage is operating near a surge limit line.

A method for operating a gas compressor comprising a first compressor stage and a second compressor stage in a back-to-back arrangement, a seal arrangement between the first compressor stage and the second compressor stage, and a side stream line between the first compressor stage and the second compressor stage, the method comprising:

• feeding a first gas having a first molecular weight to a suction side of the first compressor stage and compressing the first gas through the first compressor stage;
• feeding a side stream flow of a second gas through the side stream line to the second compressor stage, the second gas having a molecular weight lower than the first gas;
• compressing a gas mixture of the first and second gas through the second compressor stage;
• detecting a pressure parameter of the first compressor stage, wherein the pressure parameter is a pressure ratio across the first compressor stage or a suction pressure at the suction side of the first compressor stage;
• providing an antisurge system for the first compressor stage, comprised of a bypass line and an antisurge valve;
• and
• regulating the side stream flow only if the antisurge system is active to correct a pressure ratio alteration across the compressor caused by a variation of molecular weight of the gas compressed by the first compressor stage provoked by a recirculation of the gas mixture from the second compressor stage to the first compressor stage.