Those familiar with dry pipe system designs know there are four basic factors that affect the design, installation and performance of dry pipe systems. These four principles are:
Air and Water Supply Pressures A. System air pressure: The air pressure within dry system piping that keeps the dry valve closed and prevents water from entering the system. This is dictated by the static water supply pressure and the dry pipe valve design.
B. Static water pressure: The water supply pressure at the base of the riser with no flow into the sprinkler system. This pressure dictates the system air pressure required to keep the dry pipe valve closed.
C. Residual water pressure and flow: The water supply pressure at a given water flow rate. Each water supply follows a unique curve in which the pressure drops as the flow rate increases. This significantly impacts the time required after the dry valve trips for a steady discharge of water from the test connection to be established.
System Capacity and Piping Configuration A. Capacity. Total volume of all piping on the system side of the dry pipe valve. Affects the amount of air that must be discharged from the system before steady water discharge is established at the test connection.
B. Piping configuration (tree, loop, grid):
I. Tree. Consists of dead-end branch lines, cross-mains and feed main. Only the feed main, cross-main and the one branch line with the test connection must have air displaced by water in order for water to reach the test connection. Air in the remainder of the system can be compressed into dead-end piping. This piping configuration produces the fastest water delivery times.
II. Loop. Has dead-end branch lines, but cross-mains are looped. Increases the amount of air that must be displaced by water in order for water to reach the test connection.
III. Grid. Currently not allowed in dry systems because the air in all branch lines and cross-mains must be displaced by water in order for water to reach the test connection, resulting in very long water delivery times.
Size of Test Sprinkler The orifice size of the test sprinkler controls the rate at which air is discharged from the system piping. The rate of discharge and volume of air being discharged control the water delivery time after the dry pipe valve has been actuated.
Dry Pipe Valves Dry pipe valves have traditionally been designed as differential pressure valves. The clapper, which holds water out of the dry system, is designed with the surface area of the airside greater than the surface area of the water side. In this way, a lower air pressure can hold back a higher water pressure, thus reducing the volume of air in the dry system that must be discharged in order for water to reach the test connection.
Other designs of dry pipe valves include “low pressure latch type” that depend on actuators and a quick opening device to work together for the valve to function properly.