A cross section of a tubular-type centrifugal purifier is shown in figure 9-9. This type of purifier consists essentially of a hollow rotor or bowl which rotates at high speeds. The rotor has an opening in the bottom to allow the dirty fuel to enter. It also has two sets of openings at the top to allow the fuel and water to discharge. The bowl, or hollow rotor, of the purifier is connected by a coupling unit to a spindle. The spindle is suspended from a ball bearing assembly. The bowl is belt-driven by an electric motor mounted on the frame of the purifier.

The lower end of the bowl extends into a flexibly mounted guide bushing. The assembly restrains movement of the bottom of the bowl, but it also allows the bowl enough movement to center itself during operation. Inside the bowl is a device consisting of three flat plates that are equally spaced radially. This device is commonly referred to as the THREE-WING DEVICE, or

Figure 9-8.-Flow of fuel through a disc-type purifier (DeLaval).

Figure 9-9.-Flow of fuel through a tubular-type purifier (Sharples).

just the three-wing. The three-wing rotates with the bowl and forces the liquid in the bowl to rotate at the same speed as the bowl. The liquid to be centrifuged is fed, under pressure, into the bottom of the bowl through the feed nozzle. After the bowl has been primed with water, separation is basically the same as it is in the disc-type purifier. Centrifugal force causes clean fuel to assume the innermost position (lowest specific gravity) and the higher density water and dirt are forced outward towards the sides of the bowl. Fuel and water are discharged from separate openings at the top of the bowl. The location of the fuel-water interface within the bowl is determined by the size of a metal ring called a RING DAM, or by the setting of a discharge screw. The ring dam or discharge screw are also located at the top of the bowl. Any solid contamination separated from the liquid remains inside the bowl all around the inner surface. Specific instructions for the operation of a purifier should be obtained from the NAVSEA technical manual that is provided with the unit. The following general information applies to the basic operation of purifiers in naval service. When a purifier is operated as a separator, the bowl must be primed with fresh water before any fuel is admitted to the purifier. The water seals the bowl, and the spinning bowl creates an initial equilibrium of layers of liquid according to specific gravities. If the bowl is not primed, the fuel will be lost through the water discharge ports.

The time required for purification and the out-put of a purifier depends on many factors. Two important factors are the size of the sediment particles and the temperature of the incoming dirty fuel. In order for any purifier to operate at its rated capacity in gallons per hour, the fuel must be heated to a specified temperature. In this way, the viscosity of the fuel is reduced. (The fuel becomes thinner.) A lower viscosity does two things: (1) it lowers the specific gravity, and (2) it enables the fuel more easily to give up any water which may be entrained.

The viscosity of the fuel determines to a great extent the length of time required for purification. The more viscous the fuel, the longer the required time is for the fuel to be subjected to centrifugal force. In other words, decreasing the viscosity of the fuel by heating will speed up the purification process and will increase the capacity of the purifier. To reach a higher temperature, the fuel must pass through a heater. In this way, the fuel will reach the proper temperature in the heater before it enters the purifier bowl.

Proper care of any fuel purifier requires that the bowl be cleaned as required and that all sediment be carefully removed. How often you clean a purifier depends on the amount of foreign matter in the fuel to be purified. If the amount of foreign matter in a fuel is not known, you should shut down the machine and check it. The amount of sediment found in the bowl at this time will indicate how often you should clean the purifier.

Detailed procedures for operating and maintaining purifiers are furnished by NAVSEA technical manuals, PMS, and the Engineering

Operational Sequencing System (EOSS). Care-fully follow these written procedures when you are operating or performing maintenance on purifiers.

It should be obvious from the preceding information that the purpose of the external fuel system is to store and deliver clean fuel to the fuel injection equipment.