The condenser dump valve of the nuclear power plants is always kept closed during normal operation, but modulating operation is required to control the pressure of the main steam system when heat up or cool down the power plant, and the valve is required to quick open to relieve the overpressure rapidly. During the commissioning of the valve, modulating operation was not smooth and it was estimated as interference between the internal components. An analytical method was used to investigate whether interference could occur due to the thermal expansion of the valve internal components influenced by the temperature of the system medium.