The Rapidrop 6” Dry Pipe Valve Model “B” is essentially a differential check valve. The bronze clapper carries two rubber gaskets. The larger (air) gasket is rubber and seats against pure tin. The smaller (water) gasket is a specially designed rubber disc, which also seats on pure tin. These seats are so proportioned that one pound (.454 kg) of air pressure will hold back approximately six pounds (2.722 kg) of water pressure.
When the air pressure on the surface of the priming water is relieved by the opening of a sprinkler, the upward pressure of the water underneath the water gasket causes the clapper to lift, the intermediate chamber instantly fills, sounds the alarm, and the water pressure acting on the entire surface of the clapper, pushes it over to the wide open position and thus leaves a passage for the water to the sprinkler system.