The capacity of hot water relief valves has been determined with an empirical correlation describing a critical flow. To clarify the correlation, critical flow experiments were conducted by using an industrial relief valve. The experimental data approximately agreed with the correlation. On the other hand, a nonequilibrium homogeneous model based on the isentropic equation of state was proposed and the nonequilibrium factor in the valve was determined. The nonequilibrium effects appeared on the critical flow rate only at relatively low inlet subcooling conditions. At high subcooling conditions, the critical flow rate could be described by the Bernoulli equation where the saturated pressure corresponding to the inlet water temperature was used at the valve throat.