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APPENDIX A
TM 07661B-14/1
CHAPTER 2
OPERATING INSTRUCTIONS
Section I. THEORY OF OPERATION
A 2-1. GENERAL. The relationship of the operator to the model 450 PKP/100 AFFF 03638MC fire extinguisher is
basically that of a fire-fighter using the extinguisher as an emergency tool, rather than that of an operator of a machine in
continual use. For this reason, the extinguisher and its remote hose reel are to be kept in the ready status at all times
when not in use. The extinguisher can then be put into use quickly with a minimum amount of time and motion on the
operator's part. After the fire has been extinguished by the dry chemical and sealed against reignition by the AFFF, the
operator can then take the time necessary to recharge the unit and return it to an operational ready status.
A 2-2. MECHANICAL THEORY . The twin-agent fire extinguisher consists of two completely independent systems, one
for the dry chemical fire-fighting agent, the other for the AFFF. The arrangement of the systems is illustrated in figure 2-
1. Nitrogen gas (N2) is used to expel both chemical agents. The high pressure nitrogen supply is reduced to 220-240 psi
system working pressure by pressure regulators installed in the nitrogen supply lines. Each system has its own nitrogen
supply sufficient to completely discharge the chemical agent and purge all lines of residual agents.
a.
Dry Chemical System. When the quick-opening gas valve on the dry chemical system UN cylinder is
opened, nitrogen flows through the regulator, is reduced to system operating pressure 230 psi, then is sent through a
check valve into the piping for the dry chemical tank. The check valve prevents back-flow of agent to the regulator when
the nitrogen charge on the cylinder is low. From the check valve, the nitrogen flows through the normally open charge
valve and is induced into the dry chemical agent through two gas tubes inside the dry chemical tank. Rubber sleeves on
the gas tubes prevent agent back-up into the tubes. Inside the tank, the nitrogen fluidizes or aerates the dry chemical
agent and pressurizes the system up to the product valve. A relief valve installed in the piping at the top of the dry
chemical tank protects the system from over-pressurization in event of regulator malfunction. The check valve next to
the relief valve provides for blowdown and purging of the discharge lines, but also serves to prevent the nitrogen charge
from entering the dry chemical tank except through the gas tubes. To bring the fluidized dry chemical up to the nozzle
the operator opens the dry chemical shutoff valve by turning the control rod one-quarter turn counterclockwise.
Dispensing of the dry chemical is accomplished by squeezing the trigger on the dry chemical nozzle.
b.
AFFF System. The foam (AFFF) system is very similar to the dry chemical system except aeration is not
required because the AFFF solution is already in a liquid state. This system will pressurize more rapidly than the dry
chemical system because the void in the top of the AFFF tank is relatively small and nitrogen is induced directly into the
top
A 2-1
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