The critical pressure is the vapor pressure of a fluid at the critical temperature above which distinct liquid and gas phases do not exist. As the critical temperature is approached, the properties of the gas and liquid phases become the same, resulting in only one phase.
For control-valve work, critical properties should be reviewed together with vapor pressure at operating temperature, the complete sizing process data, and whether the service can reach choked-flow conditions; none of these inputs should be inferred from pressure class alone.
