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Construction of Centrifugal Pumps

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Construction of Centrifugal Pumps

The following information applies in general to most of the centrifugal pumps used in naval service. Figure 13-8 shows the parts of a centrifugal pump that is used for the cool-ing water system on a diesel engine. Figure 13-9 shows a cutaway view of a fire and flushing pump. For the relative location of components, refer to figure 13-9 as we continue our discussion of the construction of centrifugal pumps.

The shaft is protected from excessive wear and corrosion by a Monel or corrosion-resistant steel sleeve wherever the shaft comes in contact with the liquid being pumped or with the shaft pack-ing. The advantage of using a shaft sleeve is that it can be replaced more economically than the en-tire shaft.

The impellers are carefully machined and balanced to reduce vibration and wear since they rotate at very high speeds. To prevent corrosion of pumps that handle seawater, the components of these pumps are made of nonferrous materials such as bronze or Monel.

A close radial clearance must be main-tained between the outer hub of the im-peller and that part of the pump casing in which the hub rotates to minimize leakage from the discharge side of the pump casing to the inlet side. Because of the close clear-ances at the hub and the high rotational speed of the impeller, the running surfaces of both the impeller hub and the casing at that point are subject to wear. Wear results from erosion as the liquid passes between the close spacing (clearance) of the wearing rings, from the high-pressure side of the impeller, and back to the low-pressure side of the pump. (See the insert in fig. 13-10.)

Centrifugal pumps are provided with re-placeable wearing rings to eliminate the need for renewing an entire impeller and pump casing because of wear. One ring is attached to each outer hub of the impeller. This ring is called the IMPELLER WEARING RING. The other ring, which is stationary and attached to the casing, is called the CASING WEARING RING.

Figure 13-8.-Exploded view of a centrifugal water pump.

Figure 13-9.-Fire pump (vertical, double-suction impeller).

Figure 13-10.-Stuffing box on a centrifugal pump.

Figure 13-11 illustrates an impeller and casing wearing rings.

Some small pumps with single-suction im-pellers have only a casing wearing ring and no im-peller ring. In this type of pump, the casing wearing ring is fitted into the end plate.

Recirculating lines are installed on some cen-trifugal pumps to prevent the pumps from overheating and becoming vapor bound in case the discharge is entirely shut off or the flow of fluid is stopped for extended periods. Seal pip-ing is installed to cool the shaft and the packing, to lubricate the packing, and to seal the rotating joint between the shaft and the packing against air leakage. A lantern ring spacer is inserted be-tween the rings of the packing in the stuffing box. Seal piping (fig. 13-10) leads the liquid from the discharge side of the pump to the annular space formed by the lantern ring. The web of the ring is perforated so that the water can flow in either direction along the shaft (between the shaft and the packing). Water flinger rings are fitted on the shaft between the packing gland and the pump bearing housing. These flingers prevent water from the stuffing box from flowing along the shaft and entering the bearing housing.

During pump operation, a certain amount of leakage around the shafts and casings normally takes place. This leakage must be controlled for two reasons: (1) to prevent excessive fluid loss from the pump, and (2) to prevent air from enter-ing the area where the pump suction pressure is below atmospheric pressure. The amount of leakage that can occur without limiting pump ef-ficiency determines the type of shaft sealing selected. Shaft sealing systems are found in every

Figure 13-11.-Impeller and wearing rings for a centrifugal pump.

pump. They can vary from simple packing to complicated sealing systems.

Packing is the most common and oldest method of sealing. Leakage is checked by the compression of packing rings that causes the rings to deform and seal around the pump shaft and casing. The packing is lubricated by liquid mov-ing through a lantern ring in the center of the packing. The sealing slows down the rate of leakage. It does not stop it completely since a cer-tain amount of leakage is necessary during operation.

Mechanical seals are rapidly replacing conven-tional packing on centrifugal pumps. A typical mechanical seal is shown in figure 13-12. Some of the reasons for the use of mechanical seals are as follows:

1. Leaking causes bearing failure by con-taminating the oil with water. This is a major problem in engine-mounted water pumps.

2. Properly installed mechanical seals elimi-nate leakoff on idle (vertical) pumps. This design prevents the leak (water) from bypassing the water flinger and entering the lower bearings. Leakoff causes two types of seal leakage:

a. Water contamination of the engine lubrication oil.

b. Loss of treated fresh water which causes scale buildup in the cooling system.

Figure 13-12.-Type-1 mechanical seal.

In regard to the use of mechanical seals, there are two important safety considerations:

1. Flammable liquids must be contained in the system.

2. Pumps in the vicinity of electrical or elec-tronic gear where moisture can be a major problem must have zero leakoff.

Fire pumps and seawater pumps that are pro-vided with mechanical shaft seals may also have cyclone separators. These separators use cen-trifugal force to prevent abrasive material (such as sand) in the seawater from passing between the sealing surfaces of the mechanical seal. (Refer to fig. 13-9.)

As figure 13-9 shows, this abrasive separator has no moving parts. Liquid from the high-pressure side of a pump is directed through tub-ing to the opening in the sides of the separator device, which is offset from the center line. As the liquid enters, it is given a swirling motion (cyclone effect) which causes heavier abrasive materials to be forced to the walls of the center tube, which is shaped to form a venturi. There is an opening at each end of the separator. The opening at the top is for “clean” water, which is directed through tubing to the mechanical seals in the pump. The high-velocity “dirty” water is directed through the bottom of the separator, back to the inlet piping for the pump.

Shaft and thrust bearings support the weight of the impeller and maintain the position of the rotor, both radially and axially. (Radial bearings may be sleeve or ball type. Thrust bearings may be ball or pivoted segmental type.)

The power end of a centrifugal pump may be a steam turbine, an electric motor, or a diesel engine. Pumps used for continuous service can be either turbine or motor driven. Smaller pumps, such as those used for in-port or cruising opera-tions, are generally motor driven. Pumps used for emergency firemain service are generally diesel driven.



   


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