ENGINEMAN  1  &  C 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. 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 Ship’s 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. Transfer Element 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. Each  tank  is  equipped  with  two  nonclog marine sewage pumps connected in parallel. The pumps may discharge sewage to a tender, barge, shore facility, or directly overboard, depending on the position of the discharge diverter valve. Each pump is equipped with full-port plug or ball suction  and  discharge  valves,  and  a  discharge swing check valve with a hold-open device. An explanation of pump characteristics curves is given in   Naval   Ships’   Technical   Manual,   Chapter 503(9470),  Pumps. System Types 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. Two types of CHT systems are installed. The type selected for a particular ship depends on the holding tank capacity. Systems with tanks with a capacity of more than 2000 gallons use a com- minutor  and  aeration  system.  Smaller  systems with tanks having a capacity of less than 2000 gallons use strainers. Comminutor 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. In a comminutor-type system the comminutor located in the soil drain or the combined soil and waste drain serves to macerate solids passing in- to the CHT tank. A bypass is fitted upstream of the  comminutor.  If  the  comminutor  jams  or plugs, the bypass provides drainage around the comminutor 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 after the  comminutor  to  allow  for  maintenance.  Most installations  include  an  access  port,  or  cleanout, 8-14