Diaphragm pressure proportioning systems operate by
water pressure, require no electrical power, and
minimal control circuitry for automatic operation.
Maintenance requirements are minimal, however
refilling the tank is a difficult operation
requiring the services of a qualified technician to
avoid rupturing the diaphragm.
Balanced pressure proportioning systems require
reliable electrical power and more complex control
circuitry for automatic operation. In some cases an
emergency generator will be required. The primary
advantage of the non-diaphragm systems is the ease
in refilling the tanks. Tanks may be refilled even
while the system is in operation, if necessary.
Skid-mounted balanced pressure proportioning systems
perform proportioning at a central location,
avoiding long runs of concentrate lines. They are
well suited for systems such as deluge sprinklers
and monitor nozzles which have a relative narrow
range of flow rates.
In-line balanced pressure proportioning is useful
when there are multiple hazards with widely varying
discharge rates which are to be supplied from the
same proportioning system, and any time it is
desired to proportion foam remotely at risers or
discharge devices instead of at the pump room. Their
disadvantage is the need for much more concentrate
piping in the field.
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Foam solution shall be produced by introducing AFFF concentrate into the
water stream by the balanced pressure proportioning method using a
diaphragm pressure tank and ratio controller. [Provide proportioning
system and storage tanks for hose lines independent of main proportioning
system and tanks.]
2.15.1
Diaphragm Pressure Proportioning Tanks
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NOTE: When large quantities of AFFF concentrate are
required, consider two or more tanks in parallel vs
one large tank. (This is in addition to reserve
tanks.) Approved diaphragm tanks larger than 9475 -
11370 liters 2,500 - 3,000 gallons are not readily
available.
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NOTE: Designer must calculate foam tank capacity
based on maximum flow for maximum duration to
determine size of tank and space required. Do not
label foam tank capacity on drawing. Exact tank
size (which may be larger) will be determined by
Contractor's hydraulic calculations.
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SECTION 13958N
Page 30
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