In a gridded system,all the air (except for outriggers) must leave the
system before water is delivered to sprinklers. A preaction system
reduces the tip time, but does very little to reduce the water delivery
time.
Anybody who has access to FDT can see how it works in a tree system
since the software gives both the trip time and delivery time. A grid is
worse.
Central did extensive testing at FM several years ago in an effort to
find a way to make a workable dry grid by combining fast-tripping valves
and end-of-main exhausters. They found that they could reduce the trip
time to almost zero and exhaust from the end of the cross mains and
still not get an acceptable water delivery time.
Some installers have claimed success with dry grids, but they did so by
plaving the test connection on the bottom of a cross main. With this
arrangement, you can get a steady discharge as soon as there is enough
water in the cross main to fill the test connection, long before air is
exhausted from the system.
Joe
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