TYPES OF SYSTEMS

There are basically two types of dry chemical systems.

1. Gas cartridge systems that use a container of expellant gas that, when released by manual

Figure 8-38.-Stored pressure dry chemical cylinder withhose line.

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.

SYSTEM COMPONENTS

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 pressure.

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 regulator.

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 application.

Figure 8-39.-Dry chemical and expellant gas storage cylinders with piping connection. 8-35

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