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 how centrifugal pumps can be applied within pumping systems.
Centrifugal pumps are one of the most commonly used pump types and are utilised widely throughout various industries. This versatile category of pump can be employed in various configurations and comes in a large variety of types to suit different applications.
Operating multiple centrifugal pumps
Often, multiple centrifugal pumps are operated in parallel or in a series in order to enhance system flexibility, provide greater pump redundancy or to provide the additional head or flow rate capacity needed to meet a system’s requirements.
When centrifugal pumps are connected in a series, the discharge of one unit leads to the suction of the next. In other words, two similar centrifugal pumps in a series operate much like a twostage pump.
Each pump imparts energy to the liquid being pumped. Therefore, the total head generated at any quantity is the sum of the head of the individual pumps at that quantity.
When operating centrifugal pumps in a series, a number of factors must be considered. These include:
- Ensuring that the casing of the second (or higher stage) pump is rated for the higher pressure. For example, stronger material, ribbing or extra bolting may be required
- Making sure the stuffing box of the second stage pump is designed for the high suction pressure. In some cases, a mechanical seal may be necessary
- Ensuring that all pumps are filled with fluid during startup and operation, and that the second pump is started after the first is running.
Two pumps are operating in parallel when they are connected to a common discharge and share the same suction conditions. The total head is the same across each pump, but the quantity (or flow rate) is additive.
Some factors to keep in mind are:
- Due to the increased friction with multiple pumps, two pumps operating in parallel will deliver less than twice the flow rates of an individual pump operating alone in the same system. The increase in quantity obtained by operating two pumps in parallel is determined by the shape of the system resistance curve. Therefore, if there is considerable friction in a system, two pumps in parallel may only deliver slightly more than one pump operating by itself
- One pump operating alone will operate at a higher flow rate than if it were working in parallel with another pump. In other words, it will be operating further out the curve, possibly with greater power requirements. Therefore, when a pump is designed for parallel duty, also ensure that the driver is adequately rated for solo operation.
Characteristic curves – stable or unstable?
A stable characteristic curve has maximum head at zero flow and a negative gradient as flow increases. An unstable characteristic curve is one where the maximum head occurs elsewhere than at shut off.
Although not preferred, it is possible to achieve satisfactory parallel operation of centrifugal pumps with unstable characteristics.
However, the shut off head of the combined pumps must be greater than the head at which the first pump is operating by an amount greater or equal to the opening resistance of the closed non-return valve.
On the other hand, pumps that need to be throttled back to very low rates of delivery (e.g.. boiler feed pumps) must have stable characteristic curves and stable characteristics remain generally preferable for parallel operation.
Fundamental laws exist which can be used to predict changes in pump performance with variations in speed or impeller diameter.
The mathematical relationships between flow rate, head, power and speed that enable the development of performance curves corresponding to particular speeds or impeller diameters from standard performance curves are known as affinity laws (full affinity law equations and examples are available in the Australian Pump Technical Handbook).
For variations in speed with a constant impeller diameter, the following rules apply:
- Pump flow rate varies directly with speed
- Pump head varies with the square of the speed
- Power absorbed varies with the cube of the speed.
In the affinity law equations, it is assumed that the pump efficiency remains constant. In practice, the efficiency is generally slightly lower at lower speeds as friction and drag constitute a larger proportion of hydraulic power.
Impeller diameter variation
By reducing the impeller diameter, the head and flow rate characteristic of a centrifugal pump can be permanently reduced, without reducing speed.
The reduction in flow rate and head achieved depends on the type of impeller. This is because, apart from changes in peripheral velocity, there will also be changes in the length and overlap of the vanes, the width of the impeller at exit, and also in the discharge angle, in many cases.
It should be noted that the affinity relationships are less accurate for impeller variation than for speed variation.
If the pump’s net positive suction head available (NPSHA) is less than the net positive suction head required (NPSHR) the pumped liquid will vapourise in the region of the impeller eye (where the local pressure is less than the vapour pressure of the liquid).
In extreme cases this can cause vapour lock, where fluid cannot enter the impeller.
However, more usually, the fluid progresses through the impeller to a region of higher pressure where the vapour cavities implode. This implosion can generate enough force to damage the impeller vane.
This is known as cavitation.
Cavitation can be caused by excessive suction lift, insufficient NPSHA or operating a pump at too high a speed. The effects of cavitation include:
- Pitting of material surfaces due to the hammering action of the imploding vapour cavities
- Significant reduction in performance
- Noise (usually like gravel going through the pump).
Severe cavitation usually results in excessive noise, vibration and damage to the pump, whereas mild cavitation may cause only a small reduction in pump efficiency and moderate wear of pump components.
Cavitation for rotodynamic pumps generally occurs in the eye of the impeller. In reciprocating pumps, it usually occurs at the beginning of each stroke within the pump barrel or cylinder. In rotary pumps, cavitation will occur if by the end of the opening phase a rotating pocket is not filled with liquid.
In multistage pumps, cavitation normally only affects the first stage.
Minimum continuous flow
Although it is recommended to operate a centrifugal pump within its practical range, there are some circumstances where pumps can operate below the minimum practical flow (i.e. near or below minimum continuous flow, towards valve shut head).
If there is insufficient liquid passing through a pump, its temperature will rise to the point where the liquid will boil. This has the potential to cause serious internal damage to the pump, including seal or bearing failure.
To set a minimum continuous flow for a centrifugal pump it is necessary to calculate the temperature increase at low flows to the point where enough liquid is flowing through the pump to remove enough heat to prevent the internal temperature for rising.
Further information and detailed diagrams, equations and schematics can be found in the Australian Pump Technical Handbook, available from the PIA website. In the next edition of Pump Industry, we explore the characteristics of positive displacement pumps.