Figure 10-12.-Hydraulic lift.Figure 10-13.-Submarine special ballast tank (safety tank).compressed air. Sufficient air is left trapped in the tanksto prevent the seawater from reentering.We use other tanks, such as variable ballast tanksand special ballast tanks (for example, the negative tank,safety tank, and bow buoyancy tank), either forcontrolling trim or stability or for emergency weight-compensating purposes. The variable ballast tanks haveno direct connection to the sea. Therefore, we mustpump water into or out of them. The negative tank andthe safety tank can open to the sea through large floodvalves. These valves, as well as the vent valves for themain ballast tanks and those for the safety and negativetanks, are all hydraulically operated.The vents and flood valves are outside the pressurehull, so some means of remote control is needed to openand close them from within the submarine. We usehydraulic pumps, lines, and rams for this purpose. Oilpumped through tubing running through the pressurehull actuates the valve’s operating mechanisms byexerting pressure on and moving a piston in a hydrauliccylinder. Operating the valves by a hydraulic systemfrom a control room is easier and simpler than doing soby a mechanical system of gears, shafts, and levers. Thehydraulic lines can be readily led around corners andobstructions, and a minimum of moving parts isrequired.Figure 10-13 is a schematic sketch of the safetytank-one of the special ballast tanks in a submarine.The main vent and the flood valves of this tank operate10-9
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