Terminology

To understand centrifugal pumps and their operation, we must understand the terminology associated with centrifugal pumps:

Base plate—The foundation under a pump. It usually extends far enough to support the drive unit. The base plate is often referred to as the pump frame.

Bearings—Devices used to reduce friction and to allow the shaft to rotate easily. Bearings may be sleeve, roller, or ball.

• Radial (line) bearing—In a single-suction pump, it is the one closest to the pump. It rides free in its own section and takes up and down stresses.

• Thrust bearing—In a single-suction pump, it is the bearing located nearest the motor, farthest from the impeller. It takes up the major thrust of the shaft, which is opposite from the discharge direction.

Note: In most cases, where pump and motor are constructed on a common shaft (no coupling), the bearings will be part of the motor assembly.

Casing—The housing surrounding the rotating element of the pump. In the majority of centrifugal pumps, this casing can also be called the volute.

• Split casing—A pump casing that is manufactured in two pieces fastened together by means of bolts. Split casing pumps may be vertically split (perpendicular to the shaft direction) or horizontally split (parallel to the shaft direction).

Coupling—Device to join the pump shaft to the motor shaft. If the pump and motor are constructed on a common shaft, the assembly is referred to as a close-coupled arrangement.

Extended shaft—For a pump constructed on one shaft that must be connected to the motor by a coupling.

Frame—The housing that supports the pump bearing assemblies. In an end-suction pump, it may also be the support for the pump casing and the rotating element.

Impeller—The rotating element in the pump that actually transfers the energy from the drive unit to the liquid. Depending on the pump application, the impeller may be open, semi-open, or closed. It may also be single or double suction.

Impeller eye—The center of the impeller, the area that is subject to lower pressures due to the rapid movement of the liquid to the outer edge of the casing.

Priming—Filling the casing and impeller with liquid. If this area is not completely full of liquid, the centrifugal pump will not pump efficiently.

Seals—Devices used to stop the leakage of air into the inside of the casing around the shaft.

• Gland—Also known as the packing gland, it is a metal assembly that is designed to apply even pressure to the packing to compress it tightly around the shaft.

• Lantern ring—Also known as the seal cage, it is positioned between the rings of packing in the stuffing box to allow the introduction of a lubricant (water, oil, or grease) onto the surface of the shaft to reduce the friction between the packing and the rotating shaft.

• Mechanical seal—A device consisting of a stationary element, a rotating element, and a spring to supply force to hold the two elements together; may be either single or double units.

• Packing—Material that is placed around the pump shaft to seal the shaft opening in the casing and prevent air leakage into the casing.

• Stuffing box—The assembly located around the shaft at the rear of the casing. It holds the packing and lantern ring.

Shaft—The rigid steel rod that transmits the energy from the motor to the pump impeller. Shafts may be either vertical or horizontal.

Shaft sleeve—A piece of metal tubing placed over the shaft to protect the shaft as it passes through the packing or seal area. In some cases, the sleeve may also help to position the impeller on the shaft.

Shroud—The metal plate that is used to either support the impeller vanes (open or semi-open impeller) or enclose the vanes of the impeller (closed impeller).

Discharge

Discharge

Figure 10.8 Cross-sectional diagram showing the features of a centrifugal pump.

Shut-off head—The head or pressure at which the centrifugal pump will stop discharging. It is also the pressure developed by the pump when it is operated against a closed discharge valve. This is also known as a cut-off head.

Slinger ring—A device to prevent pumped liquids from traveling along the shaft and entering the bearing assembly. A slinger ring is also called a deflector.

Wearing rings—Devices that are installed on stationary or moving parts within the pump casing to protect the casing and the impeller from wear due to the movement of liquid through points of small clearances.

• Casing ring—A wearing ring installed in the casing of the pump. A casing ring is also known as the suction head ring.

• Impeller ring—A wearing ring installed directly on the impeller.

• Stuffing box cover ring—A wearing ring installed at the impeller in an end-suction pump to maintain the impeller clearances and to prevent casing wear.

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