Why should the cut-off valve be hard sealed as far as possible?

The lower the leakage requirement of the cut-off valve, the better, the leakage of the soft seal valve is the lowest, and the cutting effect is of course good, but it is not wear-resistant and the reliability is poor. From the double standard of small leakage and reliable sealing, soft seal cutting is not as good as hard seal cutting. Such as the use of sealed stack with wear-resistant alloy protection of the cut-off valve, its reliable performance, leakage rate of 10 ~ 7, has been able to meet the requirements of the cut-off valve.