Most operational fleet ships of sufficient size will be equipped with CHT systems. This system is designed to accept soil drains from water closets and urinals and waste drains from showers, laun-dries, and galleys. As the name of the system im-plies, sewage collection, holding, and transfer are three functional elements which constitute the CHT system.

Collection Element

The collection element consists of soil and waste drains with diverter valves. Depending on the position of the diverter valves, the soil or waste can be diverted overboard or into the CHT tank. The basic CHT system concept requires that waste drains be kept separate from soil drains wherever practical until they reach their respective over-board diverter valves. Downstream of their over-board diverter valves both waste drains and soil drains may be combined into a single drain line. All drains above the waterline may be diverted overboard by gravity. Drains located below the waterline cannot be diverted directly overboard and must use the CHT system as an ejection system. In this case, the CHT system must operate continuously in all modes. All drain piping is pitched to insure rapid and complete drainage. Pitch is 1/4-inch/ft whenever possible, but not less than 1/8-inch/ft relative to the operating trim.

Garbage grinder drains connected to the waste drains are installed with a minimum slope of 3 inches/ft. Garbage grinder drains are also pro-vided with a check valve to preclude back-flow from the waste drain and a diverter valve to per-mit drainage to either the CHT tank or overboard. When the garbage grinder employs seawater for flushing, the waste piping downstream of the gar-bage grinder is of copper-nickel alloy.

Plumbing drains may penetrate watertight bulkheads. Usually, each bulkhead penetration below flooding water level (FWL-1) is provided with a bulkhead stop valve. The stop valve is a round, full-port plug or ball valve. The stop valve is operable at the valve and the damage control deck. In some installations, diverter valves (3-way valves) are used to prevent progressive flooding throughout the CHT system drains, eliminating the need for a bulkhead stop valve.

Where CHT system valves are designated as damaged control closures, the valve bonnet and hand wheel is labeled SET X-RAY, SET YOKE, or SET ZEBRA, with the direction to be turned marked with an arrow. Similar labeling is required at the damage control deck box. The damage con-trol labeling is in addition to the CHT classifica-tion and label plate.

Holding Element

The CHT tank is usually sized for a 12-hour holding period. Individual ship constraints may affect this design objective. Each tank has inside surfaces which are usually free of structural members such as stiffeners, headers, and brackets. Very large tanks may require swash bulkheads to dampen movement of the tank contents. The tank bottom slopes approximately 1.5 inches/ft toward the pump section. All internal surfaces of the tank are coated in accordance with procedures given in the Naval Ships Technical Manual chapter 63(9190). Preservation of Ships in Service, for protecting sanitary tanks, and to prevent corro-sion. Each CHT tank is fitted with a vent to the atmosphere and an overflow to the sea. In addi-tion, a manhole is provided for internal maintenance. Vents should be positioned to avoid intake of CHT gases into the air compressor or ventilation intakes.