The PIA’s Australian Pump Technical Handbook is a cornerstone text for the Australian pump industry and, in our opinion, a must have for anyone who deals with pumps on a regular basis. In this ongoing series, we feature abridged chapters from the classic book to showcase the various areas covered and to reacquaint readers with the technical aspects of pumps. In this issue, we explore the defining characteristics of centrifugal pumps.
As covered in the previous edition, Let’s get classy (Pump Industry, Issue 10, May 2015), a centrifugal pump is a rotodynamic pump in which a shaft-mounted impeller rotates within the pump casing, thus accelerating the pumped fluid radially outward. Therefore, the two main components of a centrifugal pump are:
- The rotating element, consisting of an impeller on a shaft.
- The stationary casing.
These components can come in a range of different types and centrifugal pumps come in various different configurations, depending on the requirements of the application for which the pump is required.
The impeller’s rotary motion imparts energy to the pumped liquid in the form of velocity, some of which is then converted to pressure within the impeller passages.
The different types of impellers used include:
Radial flow impellers
The liquid enters the impeller axially and discharges radially, changing the direction of the flow 90°. These impellers can be single suction (where the fluid enters the impeller from one side) or double suction (where fluid enters from both sides).
Axial flow impellers
The liquid both enters and discharges from the impeller axially (i.e. the flow does not change direction). Single suction only.
Mixed flow impellers
The liquid enters the impeller axially and discharges in both radial and axial directions. Can be single or double suction.
These are basically axial flow impellers with extended vanes. They act as an in-line booster for the main impeller when suction conditions are poor.
The conversion of velocity head to pressure head in the pump casing either occurs via a volute casing or by a stationary diffuser.
These are the most common type of casing. The cross sectional area of the volute increases from the cutwater (which directs the liquid to the discharge) to the inner end of the discharge cone, resulting in a near constant average water velocity. Most of the conversion from velocity head to pressure head occurs in the discharge cone.
Volute casings are available in both single and double volute designs. In some situations, especially when the pump is operating at reduced flows, uneven pressure distribution around the impeller can cause undesirable radial loading on the shaft. A double volute design reduces this.
A diffuser fits inside the pump casing and directs the flow smoothly into the discharge pipe (or to the next impeller in the case of a multi-stage pump). The diffusers multiple vanes form radially diverging water passages around the periphery of the impeller and recover much of the total head.
A diffuser’s vanes can be radial or axial. Radial vanes are used for most applications. However, axial vanes may be used in cases where the outside diameter of the casing must be reduced or the clearance between the impeller and the diffuser vanes must be increased (e.g. to reduce vane tip erosion in very high speed pumps.
Shaft and bearings
The shaft transmits the driving torque to the impeller. With the support of bearings, the shaft also locates the impeller within the casing.
Single stage overhung impeller pumps usually operate in the stiff shaft mode (with no shaft bending) below the first shaft critical speed (the lowest natural resonant frequency of vibration of the pump rotor).
Multistage pumps often operate in the flexible shaft mode, above the first critical speed. In these pumps the hydrodynamic support and damping afforded by the clearances usually guarantees satisfactory ‘wet’ operation.
What is axial thrust?
The combined unbalanced forces within a pump that act on the impeller and shaft end in the axial direction are known as axial thrust.
Contributors to axial thrust include suction pressure and ‘hydraulically imbalanced’ impellers (caused by an unopposed build up of pressure at the impeller back shroud). In smaller pumps where the suction diameter is similar to the shaft diameter the impact of this pressure is usually minimal. However in larger and/or higher head pumps axial thrust can be much greater and steps must often be taken to combat it.
Balancing axial thrust in centrifugal pumps
The most common way to combat axial thrust involves the use of a ‘hydraulically balanced’ impeller with a back ring and balance holes, which reduces the pressure at the rear of the impeller by allowing leakage through to the suction side. Another method is to employ impeller rear (pump out) vanes.
Multistage pumps may have opposed impellers to balance axial thrust or may use a balancing device such as a balance disc or balance drum.
Single stage pump with double inlet impeller
While a pump of this type should theoretically be in complete axial balance, slight casting differences can cause slight differences in the flow pattern of each impeller entry (eye), resulting in residual axial thrust. A ball bearing of the combined radial/thrust type is used to take this thrust.
Single stage overhung impeller
This impeller design involves a wear ring on the back impeller that simulates an additional ‘eye’ at the rear of the impeller. Near complete axial balance is achieved via ‘balance holes’ in the back shroud, which allow leakage to the suction side.
An unbalanced load towards the driver, caused by suction pressure acting on the shaft area through the stuffing box, may generate some residual axial thrust. However, this is usually small and can be taken by fitting a thrust bearing.
Multistage horizontal pump with balance disc
In this case, the unbalanced axial thrust is approximately equal to the pump differential pressure acting on the annular area of the impeller back shroud. The balance disc rotates with the shaft and automatically adjusts the gap so that the pressure in the inner balance chamber opposes the axial thrust of the impellers.
Multistage pump with balance drum
The area of the balance drum is similar to the impeller unbalanced area but slightly undersize to ensure a residual unbalanced load to keep the shaft in tension. The axial load is taken by a thrust bearing. The thrust bearing can be of the ball type but a tilting pad thrust bearing may be used for high speed or high thrust load applications.
What is radial thrust?
When a single volute pump is operated at best efficiency flow rate the velocities, and therefore pressures, acting on the impeller are uniform around the volute. When the pump is operated at a flow rate other than best efficiency point (BEP) the pressures are no longer uniform, resulting in radial thrust.
The magnitude of radial thrust is determined by the total head, impeller diameter and impeller width. High head pumps with large impeller diameters will experience higher radial thrust. If pumps of this type are frequently operated below BEP, bearing life may be reduced and large shaft deflections may eventually result in shaft failure.
Combatting radial thrust
There are a number of options to combat radial thrust in this situation. One option is the use of an oversized shaft with larger bearings. However, this can substantially increase the cost of the pump.
Another solution for high head applications is to use a double volute casing. This divides the flow into two almost equal streams through the use of two cutwaters 180° apart. Although the volute pressure inequalities remain, the two opposing radial forces almost cancel each other out, significantly reducing radial thrust. Diffuser type casings also virtually eliminate radial thrust in a similar way.
Further information and detailed diagrams and schematics can be found in the Australian Pump Technical Handbook, available from the PIA website (pumps.asn.au/publications). In the next edition of Pump Industry we will explore the concept of specific speed.