The leakage of valve is very common in life and industrial production, and the impact is very big, it will not only cause waste, but also bring danger to life,such as the leakage of tap water valve,will lead to serious consequences.Valve packing is one of the most prone parts of the valve to leakage,but the reasons can be divided into the following aspects.

1. The structure of the sealing pair

Under the change of temperature or sealing force, the structure of the sealing pair will change.And this change will affect and change the force between the sealing pairs,so that the performance of the valve seal is reduced.Therefore, when choosing seals, we must choose elastic seals.At the same time, we should pay attention to the width of the sealing surface.The reason is that the contact surface of the sealing pair is not completely consistent. When the width of the sealing surface increases, it is necessary to increase the force required for sealing.
2.The specific pressure of the sealing surface
The specific pressure of the sealing surface affects the sealing performance and the service life of the valve.Therefore, the specific pressure of the sealing surface is also a very important factor.Under the same conditions, too much specific pressure will cause the damage of valve, but too little specific pressure will cause the leakage of valve.Therefore, we need to fully consider the appropriate amount of specific pressure in the design.
3.The physical properties of the medium
The physical properties of the medium also affect the sealing performance of the valve.These physical properties include temperature, viscosity, hydrophilicity of the surface and so on.The change of temperature not only affects the relaxation of the sealing pair and the change of part dimension, but also has an inseparable relationship with the viscosity of the gas.The viscosity of gas increases or decreases with the increase or decrease of temperature.Therefore, in order to reduce the impact of temperature on valve seal performance,when we design the sealing pair, we should design it into a valve with heat compensation.Viscosity is related to the permeability of fluid.Under the same conditions，the greater the viscosity, the less permeable the fluid will be.The hydrophilicity of the surface means that when there is a thin film on the metal surface, the film should be removed.Because this thin film of oil can destroy the hydrophilicity of the surface, causing the blockage of the fluid channel.
4.The quality of sealing pair
The quality of the sealing pair mainly refers to we should check the selection,matching and manufacturing accuracy of materials.For example, a good fit between the disc and the seat sealing face can improve sealing.