A TYPICAL WATER INJECTION SYSTEM
The general purpose of a water injection system has already been stated. In a static or ready condition, injection system piping places a charged supply of freshwater under each missile. Should a missile accidentally ignite, blast pressure will activate the system. Instantly, a pressurized stream of freshwater injects into the core of the rocket motor. When the limited supply of freshwater is depleted, salt water (from the fire main supply) is used to continue the operation.
Once the injection system is activated, it must be secured manually. Additionally, provisions must be made to remove and discharge the large quantity of water remaining in the magazine. That is normally accomplished by a magazine drainage system and an overboard eductor system.
Why use freshwater? Freshwater is used because it does not promote corrosion as quickly as salt water, Salt water can ruin the piping and valves of the system. Normally, an additive to the freshwater slows any corrosive action and helps seal minor leakage.
Many of the water injection system components are located outside the magazine area. They function to maintain the system in a ready state and supply the freshwater and salt water. The components inside the magazine distribute and activate the injection system. Refer to figure 8-24 for a schematic illustration of a typical water injection system.
Figure 8-24.-A typical water injection system; these components are located outside the magazine.
EXTERNAL MAGAZINE COMPONENTS.- The compression tank of the injection system is usually located in a machinery room below or near the magazine. The tank is an enclosed structure, normally filled with freshwater to one half of its 125-gallon (approximate) capacity. The tank is then charged to 200 psi from a ship's HP air supply. The 200-psi freshwater pressure is transmitted throughout the injection piping network and remains in a static state under each missile.
A flow switch is installed in the outlet line of the tank. When the system does activate, freshwater flows through the switch. The switch actuates and energizes a variety of equipments. Examples include ship fire
pumps, alarm circuits, and eductor systems.
A check valve is also installed in the outlet line of the tank. When open, it permits freshwater to flow to the water injector piping. When closed, it prevents salt water from flowing back into the compression tank.
Another hydraulically operated check valve, designated the main check valve, is shown in figure 8-24. It isolates the fire main supply from the injection system until the system activates. In a static state, the 200-psi freshwater supply keeps the valve closed against the ship's fire main supply. (I&us assume fire main supply pressure is rated at 70 psi. Actual fire main pressure varies among ships.) However, when the system does activate, the 200-psi freshwater pressure will decrease. As fire main pressure equals or exceeds freshwater pressure, the main check valve opens. Salt water flows to the piping network of the system. A manually operated shutoff valve above the main check valve must be closed to secure the system.
INTERNAL MAGAZINE COMPONENTS.- The injection system piping network distributes injection water around the magazine base area. At every position a missile can come to rest, a standpipe is threaded into the supply lines. Attached to the top of the standpipe is a water injection (detector) nozzle. The unit is just a few inches below the tail cone of the rocket motor.
The main component parts of a detector nozzle (fig. 8-25) include a closure piston and an actuation piston. Three lock balls and a gold-wire spring pin connect the two pistons. The lock balls (1/4-inch ball bearings) are forced outward by the actuation piston. They serve to hold the closure piston in place against the 200-psi freshwater supply.
Figure 8-25.-Water injector (detector nozzle).
The actuation piston is held in place by the gold-wire spring pin. A force of approximately 16 pounds is required to break (or bend) the pin. Should a missile accidentally ignite, the pressure created by rocket motor exhaust acts on the top of the actuation piston. The piston is forced downward and breaks the pin. The lock balls drop into the throat of the nozzle and release the closure piston. The 200-psi freshwater pressure forces the closure piston upward to activate the injection system.