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CB FP 2010 20

The dry pipe sprinkler system uses compressed air or nitrogen in the sprinkler pipes to keep water out until a sprinkler opens. The dry pipe valve uses the differential principle to keep water from entering prematurely - it has a smaller waterway than the air chamber above the clapper valve, allowing a lower air pressure to hold back a higher water pressure. For example, 10 psi of air could resist 60 psi of water pressure. This differential principle allows as little as 10 psi of air to keep the valve closed against higher water pressures. When a sprinkler opens, it releases the air, allowing the larger surface area above to push the valve open and admit water flow to the sprinkler.

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130 views1 page

CB FP 2010 20

The dry pipe sprinkler system uses compressed air or nitrogen in the sprinkler pipes to keep water out until a sprinkler opens. The dry pipe valve uses the differential principle to keep water from entering prematurely - it has a smaller waterway than the air chamber above the clapper valve, allowing a lower air pressure to hold back a higher water pressure. For example, 10 psi of air could resist 60 psi of water pressure. This differential principle allows as little as 10 psi of air to keep the valve closed against higher water pressures. When a sprinkler opens, it releases the air, allowing the larger surface area above to push the valve open and admit water flow to the sprinkler.

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forevertay2000
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Eligible for Continuing Education Units (CEUs)

at www.nfaonline.dhs.gov
For archived downloads, go to:
www.usfa.dhs.gov/nfa/coffee-break/
Automatic Sprinklers: Dry Pipe Differential Principle
No. FP-2010-20 May 18, 2010
Learning Objective: The student shall be able to explain the differential principle as it pertains to dry pipe sprinkler systems.
Dry pipe sprinkler systems commonly are used where
the ambient temperature of the space they are protecting is
expected to be less than 40 F (4.4 C). The sprinkler pipe
is lled with compressed air or nitrogen that is released
when a sprinkler opens and allows the dry pipe valve to
open, lling the overhead pipes with water.
What keeps water from entering the sprinkler pipes
prematurely? The dry pipe valve is designed so that the
pressure from the compressed air or nitrogen keeps the
valve closed until it is needed.
Look at the interior of the valve assembly in the photograph.
The waterway at the bottom is smaller than the air chamber
above the clapper valve. This design enables it to enjoy the mechanical advantage of the differential
principle. The larger surface area under relatively low air pressure is able to hold back the water pressure
from the smaller orice.
In most dry pipe valves, this differential principle operates on a ratio of about 1:6; one unit of air pressure
will resist six units of water pressure. If, for example, the incoming water pressure were 60 psi (4.1 bar),
the differential principle created by the larger surface area would allow as little as 10 psi (0.7 bar) air
pressure to keep the valve closed. Some low-differential dry pipe valves operate with an air to water
pressure ratio of 1:1.2.
While the minimum air pressure will keep the dry pipe valve closed during normal conditions, most
sprinkler tters will put an additional 20 psi (1.4 bar) air pressure on the system to prevent inadvertent
valve operation in the event of a small air leak. National Fire Protection Association (NFPA) 13, Standard for the
Installation of Automatic Sprinkler Systems, provides guidance on minimum air pressure that must be maintained.
Another important feature of this dry pipe valve is the latching device pictured in the upper left hand
corner. This attachment is designed to hold the heavy dry pipe valve in the open position once it operates
so that it does not interfere with water owing to control a re.
For additional information, refer to NFPA 13, Standard for the Installation of Automatic Sprinkler Systems.
This open dry pipe valve housing illustrates the size differ-
ence between the incoming waterway and the upper valve
surface.

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