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

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

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, to permit removal of foreign objects which may jam or plug the comminutor. The components of the comminutor-type system, shown in figure 8-5, include:

1. The CHT tank. The capacity of each tank usually is more than 2000 gallons.

2. The CHT pump set; one pump set per tank. A pump set consists of two motor-driven pumps, two suction plug or ball valves, two discharge plug or ball valves, two discharge check valves (with hold-open device), a pump controller, a high level alarm, and an appropriate number of liquid level sensors.

Figure 8-5.Comminutor type CHT system.

3. The comminutor. One comminutor is located in each soil drain or combined drain enter-ing each tank.

4. The aeration supply system.

5. The firemain flushing connections and spray cleaning nozzle for tank washdown.

6. The piping, valves, and fittings.

Strainer

The strainer-type system incorporates an overflow strainer within the CHT tank and an in-flow strainer mounted on the discharge side of each pump. The drain collection piping directs sewage flow through the overflow strainer where liquids may overflow into the CHT tank if the inflow strainer or the pumps become clogged. Solid and liquid wastes flow through the ball or plug valve and the check valves until they reach the pump discharge piping. At this junction, the sewage flow passes through the inflow strainer where large solids are collected, then through the pumps, and back into the CHT tank. The inflow strainer limits the flow of solids, but liquids are allowed to pass through the pump into the tank. Each time the pump operates, its inflow strainer is cleaned by the reverse flow of liquid being pumped from the tank. The strainer-type system components, shown in figure 8-6, include:

1. The CHT tank. The capacity of each tank usually is less than 2000 gallons.

2. The CHT pump set; one pump set per tank. A pump set consists of two motor-driven pumps, two suction plug or ball valves, two discharge plug or ball valves, two discharge check valves (with hold-open device), a pump controller, a high level alarm, and an appropriate number of liquid level sensors.

3. The Firemain flushing connections and spray cleaning nozzle for tank washdowns.

4. The piping, valves, and fittings.