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. This
feature is valuable when prolonged fire fighting
operations may be encountered.
Skid-mounted balanced pressure proportioning systems
perform proportioning at a central location,
avoiding long runs of concentrate lines. They are
well suited for systems 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.
Line proportioners (venturi type) are fixed-flow
rate devices which are useful on relatively small
systems which have only one set flow rate per
proportioner. They require no electric power but
have limited application, since they require high
water pressure and low discharge back pressure.
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Size system for required flow rates. Provide separate proportioner for
each [tank] [,] [riser] [, or] [group of discharge devices required to
operate simultaneously]. Ensure suction lift of proportioner(s) and system
back pressure do not exceed limits specified by the proportioner
manufacturer.
2.15.1
AFFF Concentrate Storage Tank
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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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NOTE: Consult the Division or District Fire
SECTION 13957N
Page 31
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