The shut-off seals of valves
can be divided into three categories: forced seal, semi-forced seal and passive seal.
A. Forced seal
: The sealing pressure of forced seal can be increased by the further rotation of hand wheels.
B. Semi-forced seal
: The sealing pressure of semi-forced seal is generated by other structures, but is often set in advance, which can not be increased by the rotation of hand wheels.
C. Passive seal
: The sealing pressure of passive seal(also known as self-sealing) is obtained by the pressure of media acting on valve clacks and then pressing valve seats.
Valves which have the characteristic of forced seal are mainly wedge gate valves, parallel gate valves
, globe valves
and orbit ball valves. Most of these valves use wedge-shaped principle, that is, the rotating hand wheels make the valve stems go down and continually increase the sealing pressure between the valve clacks and the valve seats. Diaphragm valves and eccentric butterfly valves also belong to valves with forced seals, namely, increasing the sealing pressure of sealed auxiliaries by the further rotation of hand wheels. But increases of diaphragm valves' sealing pressure are limited by the strength of non-metallic sealing parts.
The main conditions affecting the sealing performance are namely: materials of sealing surfaces, working accuracy and finish degree of sealing surfaces(for metal sealed auxiliaries) and the sealing pressure. Increases of the sealing pressure can improve the sealing performance of sealed auxiliaries when other conditions are the same.
In addition, with the increasing opening and closing of the valves, the valve clacks and the valve seats will wear due to relative motion of the valve clacks and the valve seats. The forced sealing structure helps to compensate for wear in order to maintain the good sealing performance of the valve in the middle and later periods of the valves' service life.
Comparatively speaking, valves that are of the forced seal structure have the most reliable sealing performance. They have the best sealing performance and the least amount of leakage under the same conditions.
To be continue...