A system for controlling pressure comprising: a CO2 refrigeration system comprising a receiving tank, a compressor and a gas cooler/condenser; a pressure sensor configured to measure a pressure within the receiving tank; a gas bypass valve fluidly connected to an outlet of the receiving tank and arranged in series with the compressor; a parallel compressor fluidly connected to the outlet of the receiving tank and arranged in parallel with the gas bypass valve; and a controller configured to: receive a pressure measurement from the pressure sensor; determine a pressure (P rec) within the receiving tank based on the measurement from the pressure sensor; compare the pressure (P rec) within the receiving tank to a first threshold pressure (P thresh_valve) and a second threshold pressure (P thresh_comp) higher than the first threshold pressure (P thresh_valve); and characterized in that the controller is configured to control (506, 508) the pressure (P rec) within the receiving tank using: only the gas bypass valve in response to a determination that the pressure (P rec) within the receiving tank is between the first threshold pressure (P thiesh_valve) and the second threshold pressure (P thresh_comp), and both the gas bypass valve and the parallel compressor in response to a determination that the pressure (P rec) within the receiving tank exceeds the second threshold pressure (P thresh_comp). a CO2 refrigeration system comprising a receiving tank, a compressor and a gas cooler/condenser; a pressure sensor configured to measure a pressure within the receiving tank; a gas bypass valve fluidly connected to an outlet of the receiving tank and arranged in series with the compressor; a parallel compressor fluidly connected to the outlet of the receiving tank and arranged in parallel with the gas bypass valve; and a controller configured to: receive a pressure measurement from the pressure sensor; determine a pressure (P rec) within the receiving tank based on the measurement from the pressure sensor; compare the pressure (P rec) within the receiving tank to a first threshold pressure (P thresh_valve) and a second threshold pressure (P thresh_comp) higher than the first threshold pressure (P thresh_valve); and characterized in that the controller is configured to control (506, 508) the pressure (P rec) within the receiving tank using: only the gas bypass valve in response to a determination that the pressure (P rec) within the receiving tank is between the first threshold pressure (P thiesh_valve) and the second threshold pressure (P thresh_comp), and both the gas bypass valve and the parallel compressor in response to a determination that the pressure (P rec) within the receiving tank exceeds the second threshold pressure (P thresh_comp). receive a pressure measurement from the pressure sensor; determine a pressure (P rec) within the receiving tank based on the measurement from the pressure sensor; compare the pressure (P rec) within the receiving tank to a first threshold pressure (P thresh_valve) and a second threshold pressure (P thresh_comp) higher than the first threshold pressure (P thresh_valve); and characterized in that the controller is configured to control (506, 508) the pressure (P rec) within the receiving tank using: only the gas bypass valve in response to a determination that the pressure (P rec) within the receiving tank is between the first threshold pressure (P thiesh_valve) and the second threshold pressure (P thresh_comp), and both the gas bypass valve and the parallel compressor in response to a determination that the pressure (P rec) within the receiving tank exceeds the second threshold pressure (P thresh_comp). only the gas bypass valve in response to a determination that the pressure (P rec) within the receiving tank is between the first threshold pressure (P thiesh_valve) and the second threshold pressure (P thresh_comp), and both the gas bypass valve and the parallel compressor in response to a determination that the pressure (P rec) within the receiving tank exceeds the second threshold pressure (P thresh_comp).