SEAWATER STRAINERS

pressed air to an appropriate pressure from the ship’s low-pressure air system. In some systems, a hose is used to pressurize the tank through a quick-disconnect valve. In other systems, a permanent pipe installation is connected to the expansion tank through a pressure- relief valve and an air shutoff valve. The ship’s low-pressure air system is used to charge the pressure tank, and then it is secured to pre- vent a possible flood back of coolant into the low- pressure air system. The relief valve protects the tank and the distilled-water system from being overpres- surized. The sight glass and the low-level alarm switch function the same as those on the gravity ex- pansion tank. In both types of expansion tanks, the bottom of the tank is connected by piping to the return main of the secondary cooling system. Changes in coolant volume cause the coolant to flow into or out of the reservoir, as necessary, to maintain a stable return-line pressure. Makeup water (distilled water) is added to the expansion tank through the funnel on the top of the tank. A funnel cap is provided for the funnel to pre- vent dirt from entering the system. When you fill the pressurized expansion tank, you must first isolate the tank from the cooling system and the air supply before you vent the air pressure off through the vent pipe at the top of the tank. The makeup water can be obtained directly from the ship’s evaporators and preferably when the ship is making boiler-feed water, because the water is double distilled. At no time should potable (drinking) water or treated boiler-feed water be used in any electronic cooling systems. After the water is drawn from the ship’s evapo- rators, it should be transported only in a clean, capped container. You should take a sample of the water from the container and have it tested for chloride by the ship’s water test facility before any of the water is used in the cooling system. The maximum permis- sible level of chloride is 0.065 ppm (equivalent parts per million). The supply system provides an alternate source of makeup water. The expansion tank sight glass is your best indi- cation of a coolant leak in the secondary cooling system. When the system uses excessive makeup water, you should inspect the whole secondary sys- tem, including the telltale drains on the heat ex- changer, to locate the source of the leak. A small drip can amount to several gallons of water a day. On the pressurized expansion tank, a very small air leak (in- dicated by a pressure drop on a tank gage) can be located by brushing on a leak detector (a thick, clear, soapy liquid, such as concentrated liquid dishwashing soap) over the suspected area of the leak. The escap- ing air causes bubbles to form in the leak detector. SEAWATER STRAINERS Strainers are used in the seawater cooling system to remove debris and sea life, which could clog the pressure and flow-control device (orifice) and/or the tubes of the heat exchanger. The two types of in-line seawater strainers most commonly used in weapons cooling systems are the simplex (single) and duplex (double) basket strainers. The simplex basket strainer, shown in figure 2-8, has a Y-pattern body. (Some simplex strainers have a small drain on the cover to allow you to drain the water off before removing the cover.) The basket is removed periodically for cleaning and inspecting for deterioration. This type of strainer requires that the seawater be secured before you clean the basket. 2-10