That media leaks from the limited space inside is discharged outside or into the limited space inside from the outside of undesirable phenomenon, namely the sealing surface of the medium flowing through the inner and outer space leak interface. Resulting in leakage of the root causes of the gap is due to the presence of the contact surface, and the difference in pressure on either side of the contact surface, the concentration difference is the driving force of the leak. Due to the sealing surface form and precision and other factors, there is a gap sealing surface is inevitable, which will result in the sealing surface is not completely consistent, so that leak. To reduce leakage, it is necessary to maximize the contact surface fitting, i.e. to reduce the leakage path cross-sectional area, increase leakage resistance, and the driving force is greater than the leak.