A dry system is a fire protection system which is filled with air.
The air is under pressure. The components of a dry system are the
same as a wet system except a dry valve is required. A dry valve
is a specially designed valve which uses a mechanical apparatus
or straight differential to hold a higher water pressure from overcoming
a lesser air pressure in the system. Most dry systems should not
have air pressure greater than 40 psi. It can be seen that both
system size and static air pressure are important in obtaining a
satisfactory trip test. Where a dry system is large and it exceeds
500gpm in capacity a miniature dry valve, when tripped, city water
is piped directly into the intermediate chamber forcing the clapper
open. This event speeds the delivery of water to the open head or
test valve. Also see Standpipe System
for additional information.
Main Points to Establish
* Is the main control valve open and all valves down stream of the
connection to the water supply open?
* Is this and all other valves controlling water to the system locked
in the open position or supervised and functioning?
* There should be at least two on a dry system one indicating the
city water pressure and one indicating the air pressure. In most
cases they should read different amounts, and the air normally less
than the water pressure. Where there is a quick opening device it
also has a gauge and should read the same pressure as the air pressure
in the system.
* On a dry system a water motor gong piped from the dry valve or
a pressure switch attached to a local bell is standard.
* This valve allows the system to be drained and is used to test
to see if the city water valve is open. This is done by opening
the valve wide open while reviewing the pressure drop on the pressure
gauge. The loss should not be significant. If it is the city water
supply should be investigated.
Fire Department Connection
* This is used to allow the fire department to pump into, using
their pumper truck to supplement the city water pressure. Threads
should be checked to see if they meet standard threads used by local
* The siamese connection has to be piped in between the control
valve and the dry pipe valve.
Alarm Testing Apparatus
* This is usually a 1" located somewhere in the building and
piped to the outside or a drain capable of sustaining a full 1"
flow equivalent of one sprinkler orifice. When this valve is opened
the alarm must ring.
* Each dry system must have an approved air source which is capable
of replenishing the air within 45 minutes and must be U.L. approved
of this application.
Air Maintenance Device
* This is an approved device which allows air to flow into the system
at a rate approved by the NFPA standards. It is not installed on
all systems as it is a contractor option.
* All sprinklers in the pendent position must be approved dry type
sprinklers. If the area where pendent sprinklers are installed is
heated, the sprinklers can be ran using a return bend, thus eliminating
the dry pendent sprinklers. All changes in piping directions which
caused a trapped portion of piping must be provided with the appropriate
drain or drum drip assembly.
* There should be a supply of extra sprinklers in the facility as
per NFPA #13 and a sprinkler wrench.
* As you walk through a facility there should be no gaps in coverage
in those areas requiring sprinklers.
* The question should be asked if any area has been changed from
one occupancy to another after original system had been installed.
* What is the age of the sprinklers?
Certification of Inspection
* The owner, under NFPA #13A, is responsible for certain tests and
the collection of certain data to demonstrate the performance capability
of the system. The owner should have documentation of periodic testing