DIFFERENTIAL DRY PIPE VALVE.-The differential dry pipe valve (fig. 8-3) has a large clapper on the air side that bears directly on a smaller water side clapper. The differential between the areas of the two clappers is approximately 6 to 1. Therefore, relatively low air pressure can hold back a much larger water pressure. For example, 30 pounds per square inch (psi) air pressure can hold back 180 psi water pressure.

To eliminate an accidental trip of the valve and false alarms, air pressure should be maintained at least 20 psi greater than the calculated trip pressure of the dry pipe valve. This is based on the highest normal water pressure of the supply system.

In operation, when there is a fire the heat actuates the sprinklers and allows the air pressure to be relieved from the piping network. The differential is destroyed. The water pressure below the valve opens the clapper, allowing water to flow through the piping to the open sprinklers. This operation has an inherent time delay between the actuation of the sprinklers and the application of water to the fire. This delay can be shortened by adding an accelerator or an exhauster to the dry pipe system.

The accelerator (fig. 8-4) allows air from the system's piping to enter the intermediate chamber in the dry pipe valve, destroy the differential, and open the clapper.

The exhauster (fig. 8-5) opens and exhausts air from the piping system faster than through the sprinklers, destroying the differential sooner.

LOW-DIFFERENTIAL DRY PIPE VALVE.- Occasionally the water supply to dry pipe valves contains debris. With a differential dry pipe valve, the high velocity of water entering the system when the valve trips can carry the debris into the system, plugging system piping and sprinklers. If debris in the water is a problem, the low-differential dry pipe valve (fig. 8-6) may be useful.

Figure 8-3.-Differential dry pipe valve.

Figure 8-4.-Dry pipe system accelerator.

Figure 8-5.-Dry pipe system exhauster.

Figure 8-6.-Low differential dry pipe valve.

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-waterlevel 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. 8-6

MECHANICAL OR LATCHED-CLAPPER DRY PIPE VALVE.- The mechanical or latched-clapper dry pipe valve operates under the same theory as other dry pipe valves. It has system air pressure against a small disk, diaphragm, or clapper. An Arrangement of levers, links, and latches on the valve clapper provides the leverage for the closing force placed on the water clapper (fig. 8-8).

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