Abstract: In this article, the critical issue of icing in high pressure pneumatic relief valves is analyzed with mathematical simulation and flow field calculation. The fact that ice is found in the central cavity of the pilot valve rather than in the exhaust cavity of the main valve with much lower temperature indicats that temperature is not the only factor which causes icing. First, a mathematical model of the pressure relief valve is built, and then curves of the throttle area ratio of the pilot valve vs the pressure of the pilot valve cavity as well as its temperature are obtained by simulation, and the critical area ratio of the 2nd and 3rd grade air source is found to be 1.70 and 1.22 respectively. Simulation results show that the probability of icing can be reduced by decreasing the throttle area ratio of the pilot valve. A grid model of the pilot valve is built, and then the flow field distribution inside the pilot valve is calculated with Fluent, and the pressure distribution corresponds well with the result of theoretical calculation. Velocity contour and streamline indicate that whether icing will occur is concerned with the structure of the valve. Icing can be avoided by designing the flow passage properly.

Key words: high pressure pneumatic, pressure relief valve, pilot valve, icing, computational fluid dynamics