or automatic means, pressurizes the container of
dry chemical and forces the agent through the
piping network or hose lines (fig. 8-39).
2. Stored pressure systems that consist of a
container of dry chemicals that is constantly
pressurized, usually with nitrogen.
Operating devices are used to release the
expellant gas from its container for the
pressurization of the dry chemical tank or to
release the dry chemical if it is stored under
In fixed systems, expellant gas is released
from its container by electrically, pneumatically,
or mechanically dropping a weight that opens a
cylinder valve or by mechanically releasing a
spring that punctures the sealing disk of a gas
cartridge. The dry chemical when stored under
pressure is released by pneumatically or
mechanically dropping a weight that opens the
discharge valve. Pressure trips may be used to
release the weights of more than one unit for
simultaneous discharge of expellant gas.
Pressure trips are operated by gas pressure taken
from the low-pressure side of the expellant gas
Hose line systems are actuated at the cylinder by
turning a handwheel or by moving a lever.
The distribution system (piping) should be
constructed of standard weight (schedule 40)
galvanized steel pipe and standard weight
galvanized steel or malleable iron fittings.
It is important for the piping system to be
balanced, so the pressure drop to any one nozzle
is about the same as to any other nozzle.
Although dry chemical suspended in a gas may be
homogeneous during flow, certain effects, such
as inertia and sudden expansion of the gas, may
cause some separation of the two phases. For
example, if several nozzles are installed
consecutively at right angles to a straight run of
pipe, the inertia of the dry chemical carries most
of it past the first nozzles. Therefore, these
nozzles discharge more gas and less dry chemical
than those farther down the piping system. To
eliminate this, you can balance all branch piping
by the use of tees (the dry chemical enters the
side port and leaves through the two end ports).
Nozzles have various designs and discharge
patterns. Nozzles used for distributing the dry
chemical must be approved for a particular
Figure 8-39.Dry chemical and expellant gas storage cylinders with piping connection.