ENGINEMAN 1 & Ccan be diverted overboard or into the CHT tank.The basic CHT system concept requires that wastedrains be kept separate from soil drains whereverpractical until they reach their respective over-board diverter valves. Downstream of their over-board diverter valves both waste drains and soildrains may be combined into a single drain line.All drains above the waterline may be divertedoverboard by gravity. Drains located below thewaterline cannot be diverted directly overboardand must use the CHT system as an ejectionsystem. In this case, the CHT system must operatecontinuously in all modes.All drain piping is pitched to insure rapid andcomplete drainage. Pitch is 1/4-inch/ft wheneverpossible, but not less than 1/8-inch/ft relative tothe operating trim.Very large tanks may require swash bulkheads todampen movement of the tank contents. The tankbottom slopes approximately 1.5 inches/ft towardthe pump section. All internal surfaces of the tankare coated in accordance with procedures givenin the Naval Ship’s Technical Manual chapter63(9190). Preservation of Ships in Service, forprotecting sanitary tanks, and to prevent corro-sion. Each CHT tank is fitted with a vent to theatmosphere and an overflow to the sea. In addi-tion, a manhole is provided for internalmaintenance. Vents should be positioned to avoidintake of CHT gases into the air compressor orventilation intakes.Transfer ElementGarbage grinder drains connected to the wastedrains are installed with a minimum slope of 3inches/ft. Garbage grinder drains are also pro-vided with a check valve to preclude back-flowfrom the waste drain and a diverter valve to per-mit drainage to either the CHT tank or overboard.When the garbage grinder employs seawater forflushing, the waste piping downstream of the gar-bage grinder is of copper-nickel alloy.Plumbing drains may penetrate watertightbulkheads. Usually, each bulkhead penetrationbelow flooding water level (FWL-1) is providedwith a bulkhead stop valve. The stop valve is around, full-port plug or ball valve. The stop valveis operable at the valve and the damage controldeck. In some installations, diverter valves (3-wayvalves) are used to prevent progressive floodingthroughout the CHT system drains, eliminatingthe need for a bulkhead stop valve.Each tank is equipped with two nonclogmarine sewage pumps connected in parallel. Thepumps may discharge sewage to a tender, barge,shore facility, or directly overboard, dependingon the position of the discharge diverter valve.Each pump is equipped with full-port plug or ballsuction and discharge valves, and a dischargeswing check valve with a hold-open device. Anexplanation of pump characteristics curves is givenin Naval Ships’ Technical Manual, Chapter503(9470), Pumps.System TypesWhere CHT system valves are designated asdamaged control closures, the valve bonnet andhand wheel is labeled SET X-RAY, SET YOKE,or SET ZEBRA, with the direction to be turnedmarked with an arrow. Similar labeling is requiredat the damage control deck box. The damage con-trol labeling is in addition to the CHT classifica-tion and label plate.Two types of CHT systems are installed. Thetype selected for a particular ship depends on theholding tank capacity. Systems with tanks witha capacity of more than 2000 gallons use a com-minutor and aeration system. Smaller systemswith tanks having a capacity of less than 2000gallons use strainers.ComminutorHolding ElementThe CHT tank is usually sized for a 12-hourholding period. Individual ship constraints mayaffect this design objective. Each tank has insidesurfaces which are usually free of structuralmembers such as stiffeners, headers, and brackets.In a comminutor-type system the comminutorlocated in the soil drain or the combined soil andwaste drain serves to macerate solids passing in-to the CHT tank. A bypass is fitted upstream ofthe comminutor. If the comminutor jams orplugs, the bypass provides drainage around thecomminutor and into the tank. If a valve is fit-ted in the bypass, it should always remain open.Isolation valves are fitted directly before and afterthe comminutor to allow for maintenance. Mostinstallations include an access port, or cleanout,8-14
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