Figure 8-7. - Air pressure maintenance device.

The clapper in the low-differential dry pipe valve is only slightly larger on the air side than on the water side. The air pressure in the system is maintained approximately 15 to 20 psi greater than the water pressure. Because the sprinkler system piping contains air pressure about equal to the water pressure, the sudden rush of water is slowed and only a slight amount of water is diverted into the branch lines, which do not have operating sprinklers after the valve opens.

With either a differential or low-differential dry pipe valve an automatic air maintenance device (fig. 8-7) must be used to maintain air pressure and prevent accidentally tripping the dry pipe valve. Also, an automatic drain or high-water- level alarm is required for the priming water level so the water does not accumulate. (If there is too much priming water, the valve cannot operate.)

Figure 8-8. - Mechanical dry pipe valve.