Ship's service generators furnish electricity for the service of the ship. Aboard most steamdriven ships of the Navy, these generators are driven by turbines. Large ships may have as many as six or eight ship's service generators and from one to three emergency diesel-driven alternators.

New cruisers and destroyers have three gasturbine-driven ship's service generators and smaller diesel-driven emergency generators. These generators are located in three different compartments and separated by at least 15 percent of the length between perpendiculars to make sure they survive.

Figure 12-4.-750-kW turbine generator set.

The type of ship's service generator commonly used aboard ships in the Navy is shown in figure 12-4.1Although generator sets (turbogenerators) are built differently, all have the same arrangement of major parts.

Turbines used for driving the ship's service generators differ from other auxiliary turbines; they usually operate on superheated steam. The service generator turbine exhausts to a separate auxiliary condenser that has its own circulating pumps, condensate pumps, and air ejectors. Cooling water for the condenser is provided by the auxiliary circulating pump through separate injection and overboard valves.

Superheated steam is supplied to the ship's service generator turbine from either the main steam line or a special turbogenerator line that leads directly from the boiler. Aboard some ships, the turbine-in the event of condenser casualty-may be discharged directly to the atmosphere or to the main condenser when the main plant is in operation.

The ship's service generator must supply electricity at a constant voltage and frequency (hertz), which requires the turbine to run at a constant speed even when loads vary. Constant speed is maintained by a speed-regulating governor. The turbine also has overspeed and back-pressure trips, which automatically close the throttle if the turbine exceeds acceptable operating conditions. A manual trip is used to close the throttle quickly if there is damage to the turbine or to the generator. The shaft glands of the ship's service generator turbine are supplied with glandsealing steam. The system is similar to that used for main propulsion turbines. Other auxiliary turbines in naval use are seldom, if ever, provided with gland-sealing systems.