Another type of pump you find aboard ship is the rotary pump. A number of types are included in this classification, among which are the gear pump, the screw pump, and the moving vane pump. Unlike the centrifugal pump, which we have discussed, the rotary pump is a positivedisplacement pump. This means that for each revolution of the pump, a fixed volume of fluid is moved regardless of the resistance against which the pump is pushing. As you can see, any blockage in the system could quickly cause damage to the pump or a rupture of the system. You, as a pump operator, must always be sure that the system is properly aligned so a complete flow path exists for fluid flow. Also, because of their positive displacement feature, rotary pumps require a relief valve to protect the pump and piping system. The relief valve lifts at a preset pressure and returns the system liquid either to the suction side of the pump or back to the supply tank or sump.

Rotary pumps are also different from centrifugal pumps in that they are essentially self-priming. As we saw in our discussion of centrifugal pumps, the pump is located below the liquid being pumped; gravity creates a static pressure head which keeps the pump primed. A rotary pump operates within limits with the pump located above the source of supply.

A good example of the principle that makes rotary pumps self-priming is the simple drinking straw. As you suck on the straw, you lower the air pressure inside the straw. Atmospheric pressure on the surface of the liquid surrounding the straw is therefore greater and forces the liquid up the straw. The same conditions basically exist for the gear and screw pump to prime itself.

Figure 9-8.-Gear pump located above the tank.

Rotary pumps are useful for pumping oil and other heavy viscous liquids. In the engine room, rotary pumps are used for handling lube oil and fuel oil and are suitable for handling liquids over a wide range of viscosities.

Rotary pumps are designed with very small clearances between rotating parts and stationary parts to minimize leakage (slippage) from the discharge side back to the suction side. Rotary pumps are designed to operate at relatively slow speeds to maintain these clearances; operation at higher speeds causes erosion and excessive wear, which result in increased clearances with a subsequent decrease in pumping capacity.

Classification of rotary pumps is generally based on the types of rotating element. In the following paragraphs, the main features of some common types of rotary pumps are described.

GEAR. PUMPS.- The simple gear pump has two spur gears that mesh together and revolve in opposite directions. One is the driving gear, and the other is the driven gear. Clearances between the gear teeth (outside diameter of the gear) and the casing and between the end face and the casing are only a few thousandths of an inch. As the gears turn, the gears unmesh and liquid flows into the pockets that are vacated by the meshing gear teeth. This creates the suction that draws the liquid into the pump. The liquid is then carried along in the pockets formed by the gear teeth and the casing. On the discharge side, the liquid is displaced by the meshing of the gears and forced out through the discharge side of the pump.

One example of the use of a gear pump is in the LM2500 engine fuel pump. However, gear pumps are not used extensively on gas turbine ships.

Figure 9-9.-Simple gear pump.

Figure 9-10.-Double-screw, low-pitch pump.

Figure 9-11.-Triple-screw, high-pitch pump.

SCREW PUMPS.- Several different types of screw pumps exist. The differences between the various types are the number of intermeshing screws and the pitch of the screws. Figure 9-10 shows a double-screw, low-pitch pump; and figure 9-11 shows a triple-screw, high-pitch pump. Screw pumps are used aboard ship to pump fuel and lube oil and to supply pressure to the hydraulic system. In the double-screw pump, one rotor is driven by the drive shaft and the other by a set of timing gears. In the triple-screw pump, a central rotor meshes with two idler rotors.

In the screw pump, liquid is trapped and forced through the pump by the action of rotating screws. As the rotor turns, the liquid flows in between the threads at the outer end of each pair of screws. The threads carry the liquid along within the housing to the center of the pump where it is discharged.

Most screw pumps are now equipped with mechanical seals. If the mechanical seal fails, the stuffing box has the capability of accepting two rings of conventional packing for emergency use.

SLIDING VANE PUMPS.- The sliding-vane pump fig 9-12 has a cylindrically bored housing with a suction inlet on one side and a discharge outlet on the other side. A rotor (smaller in diameter than the cylinder) is driven about an axis that is so placed above the center line of the cylinder as to provide minimum clearance between the rotor and cylinder at the top and maximum clearance at the bottom.

The rotor carries vanes (which move in and out as the rotor rotates) to maintain sealed spaces between the rotor and the cylinder wall. The vanes trap liquid on the suction side and carry it to the discharge side, where contraction of the space expels liquid through the discharge line. The vanes slide on slots in the rotor. Vane pumps are used for lube oil service and transfer, tank stripping, bilge, aircraft fueling and defueling and, in general, for handling lighter viscous liquids.

Figure 9-12.-Sliding vane pump.

Figure 9-13.-Eductor.

Jet Pumps

The pumps discussed so far in this chapter have had a variety of moving parts. One type of pump you find in the engine room is the jet pump, usually called an eductor. Figure 9-13 shows an eductor, which has no moving parts. These pumps are used for pumping large quantities of water overboard in such applications as pumping bilges and dewatering compartments. As an engineer, you will think of eductors as part of the main and secondary drainage system; you will also become familiar with them as part of the ship's damage control equipment.

Eductors use a high-velocity jet of seawater to lower the pressure in the chamber around the converging nozzle. Seawater is supplied to the converging nozzle at a relatively low velocity and exits the nozzle at a high velocity. As the seawater leaves the nozzle and passes through the chamber,

Figure 9-14.-Typical eductor system.

air becomes entrained in the jet stream and is pumped out of the chamber. Pressure in the chamber decreases, allowing atmospheric pressure to push the surrounding water into the chamber and mix with the jet stream. The diverging nozzle allows the velocity of the fluid to decrease and the pressure to increase; the discharge pressure is then established.

Figure 9-14 is an example of a typical shipboard eductor system. Note that the eductor discharge piping is below the water line. The swing-check valve above the overboard-discharge valve prevents water from backing up into the system if the system pressure drops below the outside water pressure. To prevent engineering spaces from flooding, you must follow the step-by-step procedures that are posted next to eductor stations.