DEWATERING

One of the special hazards of fire fighting aboard ship is that it is possible to sink the ship while putting out the fire. A standard Navy 2 1/2-inch nozzle operating at 100 psi nozzle pressure can discharge almost a ton of water per minute. It is of vital importance to remember that all water pumped into the ship must be drained or pumped out again; this must be done rapidly enough to prevent a critical impairment of the ship's stability. Remember, flooding water has the same effect on stability, whether it enters the ship by accident or by intent. The effect of water used for fire fighting can be just as disastrous as the effects of water entering through holes in the hull. This is especially true when you are fighting fires in spaces that are high in the ship. In either case, flooding must be controlled promptly and efficiently.

Dewatering should be done according to the ship's stability and loading diagram (DC diagram No. 1). Loose water (that is, water with a free surface) and water located high in the ship should be removed first. Compartments that are solidly flooded and are low in the ship are generally dewatered last. An exception is when the flooding is sufficiently off-center to cause a serious list. Compartments must always be dewatered in a sequence that will contribute to the overall stability of the ship. For example, a ship could be capsized if solidly flooded compartments low in the ship were dewatered while water still remained in partially flooded compartments high in the ship.

Facilities for dewatering compartments consist of fixed drainage systems and portable equipment such as electric submersible pumps and eductors. The P-250 pump, described in chapter 5 of this training manual, may also be used for dewatering.

Submersible Pumps

The portable submersible pump (fig. 6-41) used aboard naval ships is a centrifugal pump driven by a water-jacketed constant speed ac or dc electric motor. The latest design of submersible pump is rated to deliver 140 gallons per minute against a maximum head of 70 feet. When the head is reduced to 50 feet, the output rises to 180 gallons per minute. Basket strainers are always used with submersible pumps when flood water is being pumped.

To dewater a compartment with a submersible pump, lower the pump into the water using the

Figure 6-41.-Ac portable electric submersible pump.

attached nylon handling line and lead the 2 1/2-inch discharge hose to the nearest point of discharge. The amount of flooding water taken from a flooded space increases as the discharge head decreases. Therefore, dewatering is accomplished most efficiently if the water is discharged at the lowest practicable point and if the discharge hose is short and free from kinks. When it is necessary to dewater against a high discharge head, you can use two submersible pumps in tandem (series) as shown in figure 6-42. The pump at the lower level lifts water to the suction side of the pump at the higher level. A multiple outlet box is provided for making the necessary electrical connections.

When using a submersible pump, always lower it and raise it by the nylon handling line and NOT by the electric cable. Handling the pump by the electric cable could break the watertight seal where the cable enters the housing. The handling line is secured to the pump housing through an eye installed for that purpose. It may be married to the power cable (tied together),

Figure 6-42.-Tandem connections for submersible pumps.

provided considerable slack is left in the cable at the pump end.

Submersible pumps are not designed to pump gasoline or heavy oil. Since the pumped liquid circulates around the motor as a coolant, gasoline can leak into the motor and cause an explosion. If you use a submersible pump to pump heavy oil, the motor will burn out because the viscous liquid will impose a heavy load on the motor. Also, a heavy, viscous liquid will not dissipate heat rapidly enough to keep the motor cool.

Use the following questions as a checklist to make sure that submersible pumps are in good operating condition and ready to be used:

. Is the handling line properly secured to the eye of the pump?

. Is the motor casing watertight? This point should be checked by testing the pump with air pressure. Do not wait until the pump is needed for emergency operations; check it out ahead of time. (Have an Electrician's Mate assist you in this check.)

. Are the foot valves equipped with washers and gaskets?

. Is a strainer mounted on the pump? A basket strainer can easily be made from number 3 mesh screen wire.

. Is a spare basket strainer kept at each repair party locker?

. Is the discharge fitting cap in place?

. Is the cable stuffing tube properly packed? (Have an Electrician's Mate check the cable stuffing tube.)

. Is a portable multiple outlet box provided at each repair party locker?

. Is a 75-foot jumper cable available at each repair party locker for use in case of local power failure? . Are the necessary spanner wrenches

provided for removing and replacing strainers and for making hose connections?

Observe the following safety precautions when the pump is being placed in operation and when it is actually in use:

. Keep the handling line, the electric cable, and the discharge hose clear so that the pump can be removed quickly. . Keep the hose free of kinks.

. Always use a basket strainer.

. Keep the suction end of the pump or the end of the suction hose in water while the pump is operating.

l Keep the strainer clean and free of debris.

l NEVER use an electric submersible pump or any other apparatus that might produce a spark if you are pumping where explosive vapors may be present. . Ensure that the discharge flow is unrestricted.

. A qualified electrician is the only person of an emergency repair party authorized to energize or de-energize an electric submersible pump.