In the design of valves, the valve filler of lift valves are asbestos packing, grafoil and V type PTFE. But these filler are limited by the times of opening and closing of valves. With the increasing of opening and closing times, the filler will be weared and the gap will increase. The valve will leak at the filler after a certain times(it won't exceed 2000 times based on statistics).
If we frequently open and close valves, for example 100 to 300 thousand times in one year, which will add the cost of using and maintaining valves without question.
Is there a kind of vale which can keep its sealing performance well after being opened and closed for one to two million times? The answer is yes.
One way is to put a gasket under the filler, and add a spring under the gasket(you need to know the pretightening force of the spring). When the filler is weared, the spring will repress the filler and keep it sealed. This method has been proved to keep
valves from leaking after being opened and closed for 500 thousand to one million times.
Another way is to adopt sealing technologies of hydraulic and pneumatic, that is, to change original design of valves, to use cylinder sealing slip ring at the filler, to add O-ring(we need to calculate the friction and sealing pressure to know the amount of slip ring and O-ring we need).
This method has been proved to stop valve from leaking after being opened and closed for 2 million times. But O-ring will be aged, its service life only last for 5 years and relatively high cost.
