Advantages and Disadvantages

The preceding paragraphs have discussed all AFFF generating systems currently installed aboard naval ships. Some of the advantages and disadvantages of AFFF generating systems areas follows: 

. Balanced-pressure systems have the advantage of being cost effective in the use of AFFF concentrate. The disadvantage is the pressure drop, which is a pressure loss between the inlet and discharge openings of the proportioner.

. Injection systems have the advantage of no pressure drop, but a disadvantage in the high use of AFFF concentrate. 

. A disadvantage is that the positive displacement pump rated for 60 gpm will inject only that amount, regardless of the demand for the fire-fighting agent. 

. A disadvantage of the water motor

proportioners is pressure drop and no AFFF/water solution or water is available if the proportioner does not rotate.

TWIN AGENT EQUIPMENT

The twin agent equipment gets its name from the use of two fire-fighting agents: AFFF and potassium bicarbonate (PKP). The twin agent system is built into the ship, while the twin agent unit (TAU) is self-contained and mounted on a movable tractor.

Twin Agent System

The twin agent system (figs. 5-34 and 5-35) consists of an AFFF generating station and a dry-chemical unit. The twin agent concept gives the fire party the ability to combat fuel oil spray fires and liquid pool fires. The AFFF nozzle should be used on liquid pool fires. The drychemical nozzle should be used on fuel oil spray fires. When both a fuel oil spray fire and a liquid pool fire exist simultaneously, both AFFF and dry chemicals should be discharged at the same time. On all ships that have dry-chemical capability, except battleships, the dry-chemical unit actuates the AFFF generating station through the 3-way interlock valves or a pressure switch.

Figure 5-34.\Flow diagram of twin agent system.

Figure 5-35.-Twin agent fire-extinguishing system (electric pump).

The dry-chemical unit (fig. 5-36) consists of a dry-chemical tank, nitrogen cylinder, regulator, twin agent hose reel, and various valves and piping that are necessary to operate the unit.

The dry-chemical tank holds 125 pounds of dry chemical (PKP). A large diameter threaded cap is provided for refilling the dry-chemical tank. The storage tank has the three piping connections listed below:

l One dry chemical discharge (black ball valve) to the twin agent hose reel

l One nitrogen inlet (from the nitrogen regulator)

l One nitrogen vent (yellow ball valve) to relieve the nitrogen pressure from the storage tank, or one nitrogen hose blowdown (blue ball valve) to vent the storage tank and blow down the drychemical hose

When the blue ball valve is installed in conjunction with the yellow ball valve, it will only blow down the dry-chemical hose. In this case,

Figure 5-36.-Cutaway of PKP assembly of the twin agent system.

the yellow ball valve is used to vent the storage tank. When the blue ball valve is installed independently, it functions as a hose blowdown and a tank vent valve. Dry chemical is forced out of the storage tank by nitrogen gas stored in the nitrogen cylinder.

Nitrogen is stored for each dry-chemical unit in a separate nitrogen cylinder. The cylinder has a capacity of 110 cubic feet of nitrogen. The nitrogen is released from the cylinder to charge the dry-chemical storage tank by a quick-opening valve located on the cylinder's throat. This valve can be actuated by either the quick-opening (pull) handle or the handwheel. When the quickopening handle is pulled, the valve opens

immediately and charges the dry-chemical tank with nitrogen. This action pressurizes the 3-way interlock valve or pressure switch, which, in turn, actuates the AFFF generating station. Both the dry chemical and the AFFF/ water solution are then available for fire fighting. The quick-opening valve is equipped with a safety device called a rupture disk.

The rupture disk prevents excessive pressure buildup in the nitrogen cylinder and ruptures at 3,600 to 4,000 psig. Once the rupture disk has ruptured, all of the contents within the cylinder will be drained out. The nitrogen is stored in the cylinder at a maximum pressure of 2,200 psig. The cylinder must be replaced when the pressure drops below 1,500 psig.

Some dry-chemical installations do not actuate the AFFF generating station. Instead the station may be actuated by an SOPV.

The handwheel can also be used to open the valve, but it is much slower and reduces the initial surge of nitrogen into the dry-chemical storage tank.

A pressure regulator on the nitrogen cylinder discharge line reduces the nitrogen pressure from the cylinder pressure down to 210 to 230 psig. The regulator has a preset relief valve that will release excess pressure before nitrogen enters the storage tank. The relief valve will vent at 260 or 325 psig, depending on the type of regulator. Unlike the rupture disk, the relief valve will reset when the pressure drops below the lift pressure of 260 or 325 psig.

The twin agent hose reel (fig. 5-37) consists of 50 feet of 1 1/2-inch noncollapsible hose for the AFFF/water solution, 50 feet of 3/4-inch noncollapsible hose for dry chemical, an AFFF nozzle, and a dry-chemical nozzle. Two types of AFFF nozzles are used with the twin agent hose reel. One nozzle (fig. 5-37, view A) is rated for a constant flow of 90 gpm with a nonadjustable fog pattern. The other (fig. 5-37, view B) is rated for a constant flow of 95 gpm with an adjustable fog pattern and a variable-stream pattern nozzle. Both nozzles have pistol grips and are trigger operated. The dry-chemical nozzle discharges dry chemical at a rate of 2 pounds per second. It throws a tight stream of dry chemical about 10 feet before the stream breaks up and becomes a rolling cloud. The AFFF and dry-chemical nozzles are attached to a tie-bar for ease in handling.

Figure 5-37.-Configurations of the twin agent system.