Internal and Atmospheric Leakage Assessment and Intergate Overpressurization of Isolation Valves

Some problem areas which would lead to internal and atmospheric leakage in valves, specifically for pressurized water reactors, are identified. Frequent thermal transients can induce thermal stresses and promote the propagation of surface faults into the valve body and cause eventual cracking of the seat. Trapping of hard debris around the seat causes seat and disc damage as well as providing leak paths. Cavitation can also produce seat and disc damage, particularly in high-speed flows. Atmospheric leakage is caused mainly by the deficiencies in gland packings and by overpressurization problems in valves. In the case of bellows and diaphragm sealed valves, metal fatigue, stress corrosion cracking and punctures are the usual cause. The overpressurization problem in valves can increase the rate of gland deterioration. The problem is discussed in some detail in this paper. Analysis of theoretical gland packing results suggest that stem friction reduction and discarding lantern rings in stuffing boxes can improve the sealing performance of the packings. Ideally the sealing material should have a low coefficient of friction and a high Poisson ratio.