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 continue exploring energy efficiency in pumping systems, focusing on drives and controls.
Drives and controls
There are several major classes of driver that are commonly used in pump applications:
• Electric motors
• Internal combustion engines
• Steam turbines
• Compressed air
• Hydraulic motors
Electric motors are probably the most economical and common driver for pumps, and it is generally not complicated to select a motor as a driver.
To select the most energy efficient electric motor, the following should to be considered:
• The motor should not be oversized – electric motors meeting International Standards come in specific ratings (i.e. 1.1kW, 2.2kW, 3kW, etc).
Normally a motor would be selected in a standard-size up from the maximum kW absorbed by the pump, for example, for 13.4kW absorbed, a 15kW motor would be selected. However, if the pump kW absorbed is less than five per cent below a standard motor size, the next size up should be selected
• The motor efficiency from several manufacturers should be checked in order to maximise the motor efficiency relative to the capital cost. This is particularly important for larger motors where a one per cent efficiency improvement can reduce the lifecycle cost of the pumpset by an amount that could make a cheaper but less efficient motor look expensive
• The motor should be MEPS Hi Efficiency certified Electric motors come in all shapes and sizes, and can be categorised into two main types:
• Standard motors
• Special motors
Standard motors are available as DC motors in low power sizes and AC induction motors. In Australia, the most common type of standard motor found in pumps are AC induction motors.
AC induction motors
Some fundamental points that should be considered when applying a squirrel cage induction motor to a pumping applications, include:
• Rating voltage – the motor winding should be chosen to suit the power supply available
• Standard voltages – these include 240V, 415V, 690V, 1,000V, 3.3kV, 6.6kV, 11kV and 33kV. It should be remembered that the higher the voltage, the lower the current for the same power rating
• Rated speed – the motor windings should be configured to suit the rotational speed of the motor. Standard speeds include: 2-pole (nominally 3,000rpm); 4-pole (nominally 1,500rpm); 6-pole (nominall 1,000rpm); and 8-pole (nominally 750rpm). Pump performance curves are often based on these speeds
• Rated torque – it should be established how much torque an application needs, and a motor with a rated power that suits the starting and running requirements of the pump should be selected.
This is particularly important for many positive displacement pump applications, but less so generally for rotodynamic pumps which don’t have high starting torque requirements
Most pump applications simply need the pump to start and stop as required to meet the demand from the user. There are a variety of commercially available motor starting methods, which are summarised in Table 1.
The preferred starting method would be determined by the conditions of service for the pump and local restrictions on starting currents for electric driven machines. The different options for starting devices should be reviewed carefully to ensure the most appropriate one is selected.
Variable frequency drive (VFD) motor control
For motors with a speed control facility, the methods for selecting the required speed can range from fully manual to fully automated. Many VFD systems utilise PLC controllers to achieve a particular control philosophy.
Common control philosophies include:
• Constant pressure (for booster systems, etc)
• Constant level (for sumps, etc)
• Surge minimisation
Typical VFD control systems have been developed to accept signals for a variety of condition monitoring devices:
• From a pressure signal which allows a VFD to maintain a set system delivery pressure irrespective of what the system flow demand is, for example, irrigation systems where the pressure requirement is constant but flow varies depending on the size of the area being irrigated
• From a level sensor which allows a VFD to maintain a set level in a tank or sump irrespective of how the flow varies to or from the tank or sump, for example, a sewage sump with variable in-flow
• From a flow meter to allow a VFD to alter the pump speed up or down to maintain a set system flow, for example, in a process situation where a steady flow must be maintained for process purposes
• From a temperature sensor allowing a VFD to maintain a set system temperature by varying the flow up or down, for example, in a heating system where flow is increased as the temperature falls below setting, and decreased as it rises above setting
Using a VFD can also help maximise pumping system efficiency, particularly when the system demands a variety of duty points.
However, there are some important considerations:
• Any VFD has its own internal losses, which should be taken into account
• Electric motors must be suitable for projected VFD waveforms and not overheat at low rotational speeds due to poor fan performances
• Special electrical installations methods should be adopted, i.e. screened cable
• Equipment may have a reduced operational life due to unexpected factors
When used correctly, VFDs can save a considerable amount of power.
However, there are circumstances where VFDs are inappropriate, effectively turning them into an expensive soft starter which spends the majority of its life running at full speed.
• They should not be used as a safety net for poor system design
• A proper cost versus benefit analysis should be conducted to justify the additional investment
Internal combustion engines
Internal combustion engines are normally used to drive pumps in situations where there is no electric power available or as a standby option (black start) where electric power failure cannot be tolerated.
One of the advantages of using an engine to drive a pump is that it can run over a range of speeds often by manual or remote adjustment, therefore allowing the pump to run at the speed that optimises its efficiency for a particular duty.
This minimises the engine size, however, it needs to be balanced with the engine’s fuel consumption. When selecting an engine with the best combined efficiency, it should be remembered that engines have ratings which are based on whether it runs continuously – 24 hours a day – or intermittently – eight hours a day maximum.
At its continuous rating, an engine’s power is limited to a lower amount than when rated intermittently. This means that a pumpset that is operated intermittently can possibly use a smaller engine than one that must run continuously.
Once it has been established whether a continuous or intermittent engine is required, an engine can be selected for the rating for the power required to run the pump. However, the selected engine should not be oversized.
Once the engine power and speed have been selected, it is recommended that several engine makes are considered to determine the most fuel efficient engine for the application.
While these considerations should allow the selection of the most energy efficient engine to drive the selected pump, however, it should be noted that engine maintenance is generally higher than for the same size electric motor.
Steam turbines are often used in plants where steam is generated as part of the main process. The are commonly found in power station, process plants and refineries (petrochemical, sugar, etc), and provide a high speed of rotation either for direct drive or through a reduction gearbox, making it relatively economical to operate the pumps energy- wise, which would probably not be the case if one generated steam to power the pumps alone.
If steam is available at an economic cost, it can be an energy efficient option provided that all the ancillary costs are properly taken into account.
As a driver, compressed air is used most commonly to operate air operated diaphragm pumps and sump pumps in the construction and mining industries.
While compressed air is appropriate in certain circumstances, it is an expensive and generally inefficient way to power pumps because, as well as the energy used to drive the pump, energy has to be expended to compress the air.
In addition to high energy consumption, the nature of compressors, air motors, etc. generally require a higher than normal maintenance. For energy conscious pump users, compressed air should only be used when no other practical alternative is available.
Hydraulic motors and rams
As a drive method, hydraulic equipment can be used in two ways:
• Hydraulic cylinders on single mechanical diaphragm pumps
• Using hydraulic motors to drive pumps (mainly submersible pumps)
Both methods can be used to drive other types of centrifugal and positive displacement pumps. The type of drive requires a hydraulic power unit to provide oil flow and pressure, this means that they have similar inefficiencies to a compressed air system.
Hydraulic motors and rams generally require higher maintenance than electric drives. For pump users that are energy conscious, they should try to avoid using hydraulics to drive a pump, however, this type of rive does have benefits in specialised applications.
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 continue to explore energy efficiency in pumping systems, looking at lifecycle costs.