Patent ID: 8678342

Claim:
An air-operated valve which is provided with a valve chamber body which has a valve chamber which forms an inflow part of a controlled fluid, an outflow part of said controlled fluid, and a seat between said inflow part and said outflow part, a valve mechanism which is provided with a valve part which has a seal which advances to and retracts from said seat to seal the same and with a diaphragm which is formed at an opposite side from said seal side and which is attached inside said valve chamber, and a housing body which houses a drive mechanism which uses working air which flows in and out from an air port so as to make said valve mechanism advance and retract to thereby drive and control an opening and closing operation of said seat, said drive mechanism provided with a first piston which is connected to said valve mechanism and which has a first pressure receiving part which receives said working air which flows in from said air port, a first spring which biases said first piston to said seat side, a second piston which is slidably housed in said housing body, which slidably houses said first piston inside of it, and which has a second pressure receiving part which receives said working air which flows in from said air port, and a second spring which biases said second piston to said first piston side, said second spring has a spring force larger than a spring force of said first spring, when the working air from said air port falls in feed pressure, the load of said first spring counters a rising force of working air which occurs in said first pressure receiving part whereby said first piston descends before said second piston, and, after that, said second piston descends delayed from said first piston whereby said second piston biases said first piston in the seat direction to thereby increase a seal strength between said valve part and said seat in said seated state.