3.2.10 Multi stage pump

Multi stage pump

A multi stage pump is designed with the purpose to increase differential head (between pump inlet and pump outlet) efficiently. Large suction heads are divided up by several series connected impellers. Friction losses between the sides of the impeller and pump casing increase proportionally to the fifth power of the impeller diameter, whereas the delivery head increases are quadrically proportional to the impeller diameter. It is therefore necessary to use a multi stage pump in order to avoid unacceptably low efficiency.

Small multistage pumps always consist of radial segments. Each segment comprises an impeller. The segments are shaped with guide vanes, which convert the kinetic energy imparted by the impeller into pressure energy, and transfer channels to the next impeller figure 3.210a.

Various delivery heads are achieved by varying the number of stages. In order to maintain good efficiency it is necessary, for a given delivery head, to increase the number of stages as the volume of flow decreases, see figure 3.210b.

Figure 3.210b Duty ranges at 2.900 r/min for small multi-stage pumps. The figures 1 to 13 refer to number of pump stages.

Large multi stage pumps have a very wide range of applications, notably mine drainage, boiler feed and process industries. They are also used extensively in the refining and petrochemical industry. The design and construction being determined by the specific functions and components (see figure 3.210c and 3.210d):

• Balancing thrust
• Type of bearing — criteria and methods of lubrication
• Clearance between stationary and rotating elements
• Various types of pump casing. Horizontally split pump casing and barrel pump casing.

For moderate pressure increase or small flows it is possible to absorb the thrust directly in the thrust bearings. However, it is generally necessary to balance out the thrust. The most common method of balancing thrust is by opposing the direction of the impellers, balance discs or by means of balance drums.

The classic high pressure pump is equipped with many stages, sometimes even up to 40 stages. The problems of critical speed and clearance have led to the limitation of hydraulic constructions aimed at maximum efficiency, i.e. many stages in favour of more reliable designs having a maximum of 6 to 8 stages. It is often required to run at higher speeds than those afforded by electric motors, this necessitates the use of steam turbine or geared up electric
motors. The speed is generally 5000-8000 r/min. Recent development trends are towards single stage pumps using a construction designed to give maximum reliability with a specified replacement time (disassembly/assembly) for the complete rotor system, of less than 8 hours.

Figure 3.210c Feed water pump-thrust balancing by means of balancing drum.

Figure 3.210d Multi-stage pump for process industry — thrust balancing by means of opposed impellers. Axially split pump casing.

The process industry today favors the single stage high speed pumps as shown in figure 3.54 operating at speeds of up to 40,000 r/min, despite somewhat reduced efficiency. The reason being that this type of pump is more robust and less liable to suffer from operational interruptions. A high speed single stage pump can run dry for a longer period of time. Such single stage pumps can be used for delivery heads of up to 1500 m at powers of approx. 500 kW.