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TM 9-2320-279-10-3
Principles of Operation (Cont)
An around-the-pump foam system is used. System includes an eductor in the
line from the pump discharge to pump intake. Eductor uses a venturi effect
to create a vacuum, which draws foam agent into the water stream. A
metering valve is placed in the foam line to control injection percentage.
Foam is injected into the foam line that will pump foam agent at a percentage
set by a metering valve to all discharges.
Foam agent system is broken into two (2) halves: Class A foam and Class B
foam. One is an "automatic" valve (multi-metering valve). The other is a
manual metering valve. Multi-metering valve is used for the two (2) turret
discharges and four (4) ground sweeps, all of which are controlled in the
personnel cab. When each respective discharge is opened, a signal will be
sent to the multi-metering valve to open the corresponding port, which will
have an orifice sized to inject foam at a preset rate, for a pre-determined flow.
It is important to note that the roof turret and bumper turret must be run at
the same flow every time (250 GPM [946 L/min]) for the bumper turret,
(500 GPM [1,893 L/min]) for the roof turret) in order for foam system to
function correctly. A manual metering valve is used for side and pre-connect
discharges, and is located on pump operators panel. This valve is operated by
pump operator, depending on percentage and flow requirements. If flow
changes at any point, metering valve must also be adjusted.
Intake pressures higher than 5 psi (34 kPa) are not allowed. To facilitate
operating from either a hydrant or from relay pumping, an automatic tank
refill will be used. When water level in tank falls below a certain
predetermined level, refill valve will open. When water tank is full, refill
valve closes. This system allows vehicle to pump continuously from water
tank with accurate water/foam concentration levels.
In order to enable simultaneous use of pre-connects and turrets, pressure
reducing valves are used on pre-connect lines. Pump will need to maintain a
pressure of 210 psi (1,448 kPa) for optimum turret flow and reach. Pressure
reducing valves are provided to reduce drivers side A and B pre-connects
operating pressure to a more manageable pressure of 150 psi (1,034 kPa). 2
in. (6.35 cm) discharges are not equipped with pressure reducing valves.
A two-tank selector is located inside personnel cab and at pump operators
Foam system engagement controls are located inside personnel cab and at
pump operators panel.


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