Type of Centrifugal Pumps
Centrifugal pumps can be either horizontal or vertical. The horizontal pump requires a firm foundation for its mounting, whereas the vertical pump can be attached to the piping with a minimum of support beneath the pump. The horizontal pump is a more rugged pump which will resist most vibration present.
Vertical pumps are commonly used in process plants in locations where vibration is not a problem. Another advantage of the vertical pump is that the alignment between the motor and pump is much easier to maintain than that of the horizontal pump.
Horizontally or Axially Split
The top and bottom halves of the casing join together along a line parallel to the shaft. The main advantage is that it can be opened up, so that the internal parts can be seen, without disturbing it too much. See Figure 6.
FIGURE 6 CENTREHUNG, HORIZONTALLY SPLIT CASING PUMP
Vertical or Radial Split
The casing is split at 90° to the shaft where it can be attached to the pump body. The main advantage is that the internal parts can be removed for repair, while the volute is left in place. See Figure 7.
FIGURE 7 OVERHUNG VERTICALLY SPLIT CASING PUMP
Volute type pumps are also described by the way the impeller is positioned. The three common types are as follows:
The impeller shaft is supported on bearings at one end, with the impeller mounted at the other. See Figure 8.
Between Bearings Type
The impeller is mounted centrally on the shaft, and supported at either side on bearings. See Figure 9.
Close Coupled Type
This type of pump has the impeller mounted on the end of the motor shaft. It requires no coupling to link it to the motor. See Figure 10.
Pumps with more than one impeller are called multistage pumps. Each impeller is referred to as a stage. A pump with 5 impellers is a 5 stage pump. There are three common types of multi-stage pumps:
The submersible pump is an integral pump-motor unit in a sealed enclosure. In a well, the pump is inserted inside the casing. An electric cable runs from the surface to the motor. These pumps are used for lifting water or oil from any depth. The pump capacity is limited by the size of the casing. For example, a submersible pump which will fit inside an 8 inch casing will deliver a maximum flow rate of about 300 gallons per minute. An electric power source is required at the surface to operate the pumps. See Figure 11.
Can-type vertical pumps are used to lift liquids from sub-surface sources such as sumps and caissons. The pump driver is located at or above the liquid surface, and the shaft extends from the driver to the pump, which may be located some distance below the liquid surface. See Figure 12.
Both the can and submersible pumps can have up to 60 stages or impellers, depending upon the depth at which the pump is set. If one impeller develops a pressure rise of 43.5 psig and a total pressure rise of 1305 psig is required to lift the liquid to the surface then the number of stages will equal:
Horizontal multistage pumps are used in process plants and oil pipelines where the pump must increase the liquid pressure several hundred psig. There is no theoretical limit to the number of impellers in a horizontal pump, but more than 8 are seldom used. See Figure 13.
HORIZONTAL MULTISTAGE PUMP
SINGLE-STAGE CENTRIFUGAL PUMP