A system comprising: a high side heat exchanger configured to remove heat from a refrigerant; a flash tank configured to store the refrigerant; a first low side heat exchanger configured to use the refrigerant to cool a space proximate the first low side heat exchanger; a second low side heat exchanger configured to use the refrigerant to cool a space proximate the second low side heat exchanger; a first compressor configured to compress refrigerant from the first low side heat exchanger; a second compressor configured to compress refrigerant from the second low side heat exchanger and the first compressor, the high side heat exchanger positioned vertically above the second compressor; first piping configured to direct refrigerant from the second compressor to the high side heat exchanger, the first piping comprising: a first p-trap positioned vertically above the second compressor and vertically below the high side heat exchanger; and a second p-trap positioned vertically above the first p-trap and vertically below the high side heat exchanger; second piping configured to direct refrigerant from the second compressor to the high side heat exchanger, a diameter of the second piping is larger than a diameter of the first piping, the second piping comprising: a third p-trap positioned vertically above the second compressor and vertically below the high side heat exchanger; and a fourth p-trap positioned vertically above the first p-trap and vertically below the high side heat exchanger; a first valve configured to control a flow of refrigerant and oil from the second compressor to the first piping; and a second valve configured to control a flow of refrigerant and oil from the second compressor to the second piping. a high side heat exchanger configured to remove heat from a refrigerant; a flash tank configured to store the refrigerant; a first low side heat exchanger configured to use the refrigerant to cool a space proximate the first low side heat exchanger; a second low side heat exchanger configured to use the refrigerant to cool a space proximate the second low side heat exchanger; a first compressor configured to compress refrigerant from the first low side heat exchanger; a second compressor configured to compress refrigerant from the second low side heat exchanger and the first compressor, the high side heat exchanger positioned vertically above the second compressor; first piping configured to direct refrigerant from the second compressor to the high side heat exchanger, the first piping comprising: a first p-trap positioned vertically above the second compressor and vertically below the high side heat exchanger; and a second p-trap positioned vertically above the first p-trap and vertically below the high side heat exchanger; second piping configured to direct refrigerant from the second compressor to the high side heat exchanger, a diameter of the second piping is larger than a diameter of the first piping, the second piping comprising: a third p-trap positioned vertically above the second compressor and vertically below the high side heat exchanger; and a fourth p-trap positioned vertically above the first p-trap and vertically below the high side heat exchanger; a first valve configured to control a flow of refrigerant and oil from the second compressor to the first piping; and a second valve configured to control a flow of refrigerant and oil from the second compressor to the second piping. a first p-trap positioned vertically above the second compressor and vertically below the high side heat exchanger; and a second p-trap positioned vertically above the first p-trap and vertically below the high side heat exchanger; second piping configured to direct refrigerant from the second compressor to the high side heat exchanger, a diameter of the second piping is larger than a diameter of the first piping, the second piping comprising: a third p-trap positioned vertically above the second compressor and vertically below the high side heat exchanger; and a fourth p-trap positioned vertically above the first p-trap and vertically below the high side heat exchanger; a first valve configured to control a flow of refrigerant and oil from the second compressor to the first piping; and a second valve configured to control a flow of refrigerant and oil from the second compressor to the second piping. a third p-trap positioned vertically above the second compressor and vertically below the high side heat exchanger; and a fourth p-trap positioned vertically above the first p-trap and vertically below the high side heat exchanger; a first valve configured to control a flow of refrigerant and oil from the second compressor to the first piping; and a second valve configured to control a flow of refrigerant and oil from the second compressor to the second piping.