TM-9-2320-328-13-P-1
HEMTT-BASED WATER TENDER (HEWATT)
TECHNICAL MANUAL; OPERATOR’S AND FIELD LEVEL MAINTENANCE MANUAL (INCLUDING REPAIR PARTS AND SPECIAL TOOLS LIST); PART 1 OF 3
MARCH 2009
TM-9-2320-328-13-P-1 - Page 59 of 1318
TM 9-2320-328-13&P-1
0003-1
DRAFT
0003
OPERATOR MAINTENANCE
THEORY OF OPERATION
SYSTEM INTRODUCTION
The HEWATT contains the same functional base systems as the M985 vehicles, with some modifications.
The
HEWATT firefighting systems are described in the following paragraphs.
Refer to TM 9-2320-422-10 for base
M985 vehicle systems not contained in this manual.
WATER PUMPING SYSTEM
The water pumping system for the HEWATT is based on a single stage, hydraulic driven centrifugal water pump,
which is controlled by a pressure governor.
The hydraulic power for the water pump is provided by an engine PTO
driven hydrostatic pump.
The bumper turret and under truck nozzles are operated from the cab instrument control panel, and the side
discharges and hose reel are operated from the pump operator’s control panel.
The controls are interlocked to
ensure single control operation.
In pump and roll/drive mode, water is supplied to the pump from the onboard water tank.
In stationary mode, water
is supplied to the pump from either the onboard water tank or alternate source(s).
Alternate sources include fire
hydrants, remote pumping units, or drafting from open reservoirs.
If water is supplied from the onboard water tank
during stationary firefighting, the water tank can be continuously refilled using the automatic tank fill valve.
This
system maintains the water level in the water tank automatically by opening the direct fill valve when the water level
in tank falls below a certain predetermined level, and closing the direct fill valve when the water tank is full.
PRESSURE GOVERNOR
The pressure governor is designed to control and maintain the water pump output pressure at a selector (manual
operations) or predetermined (automatic operations) value.
FOAM SYSTEM
The HEWATT’s foam system uses an around-the-pump foam system to deliver class A foam agent to the
discharge valves during firefighting missions.
This system uses an eductor in line with the pump discharge and
intake manifolds, to draw foam agent from an onboard tank into the water stream during foam system operations.
Metering valves are placed in line with the eductor to control foam agent injection percentage for bumper turret and
under truck nozzles operations.
Each metering valve has an orifice sized to inject foam at a preset rate for a
pre-determined flow, which is 0.5% for bumper turret operations, and 1.0% for under truck nozzles operations.
When a discharge valve is opened, a signal is sent to the respective metering valve to open the corresponding
port.
The foam agent is then mixed with the water and is pumped to the corresponding discharge valve by the
water pump.
It is important to note that the bumper turret and under truck nozzles must be operated at the same
flow every time in order for foam system to function correctly.
A manual metering valve is also provided so class A foam can be delivered during side discharge and hose reel
operations.
This valve is operated by pump operator when adjustments to percentage and flow rate are required.
The manual metering valve must be adjusted whenever the flow is changed during operations.
Back to Top
