Soft-start valves consist of two halves of throttle flaps supported on a common axis and maintained in the open position by a spring. The axis is held in an ISO-KF centering ring that is formed as a body with valve seat.
When mounting them between two ISO-KF flanges (instead of a centering ring), the open flaps must be orientated against the air flow in the vacuum line. When the roughing system goes into operation, a strong air flow occurs in the roughing line, which immediately closes the valve and reduces the pipe section by about 99%. When reaching a differental pressure of approx. 15 mbar, the valve opens and leaves the pipe sectionnearly unobstructed for further pumping.