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.