If the wrong pump is chosen or is worn out, this can increase pumping costs and reduce productivity. In the second of a series of fact sheets, the NSW Department of Primary Industries describes a simple way to work out the pumping costs and energy efficiency of your electric pump.
Tests of irrigation pumps across New South Wales have found that many were not performing efficiently, either because the wrong pump had been chosen for the job, or because the pump was worn.
To contain costs, you need to monitor your energy usage, repair and maintain the pump and work out what your pumping costs are.
When you have determined the operating cost you can perform quick checks to detect any change, and when you have determined the pump efficiency, you can compare it to the manufacturer’s figures to decide when repair or replacement is cost-effective.
Measuring operating costs
One way of tracking pumping costs is to work out how much it costs to pump a megalitre of water. To do this, you need to measure:
- The power consumption rate in kilowatts (kW)
- The flow rate in litres per second (L/s).
Combining these measures with the cost of electricity gives you the pumping cost.
Step 1: Measure the power used
You can measure the power used by reading your electricity meter. Electronic meters are most commonly used but single and multiple meters continue to be used on many farms.
Electronic meters
Electronic meters usually measure and record the electricity used for the main rate, shoulder rate and the off-peak rate in separate registers. The various rates are switched ‘on’ and ‘off’ by the internal clock at the appropriate times.
Electronic meters record your electricity consumption in a time-of-use format. They may also have registers for the date, the time and for testing the display.
Each register has a three figure identification number. For example, the current off-peak kilowatts may be given register number ‘126’. You should check with your local energy authority what the display register numbers are for each of your rates.
The meter scrolls through each register at four to six second intervals.
The register number appears, often in smaller numbers, on the LCD screen (in the diagram, in the top left-hand corner) and may have a short description underneath (for example: 126 = off-peak)
The usage in kilowatt-hours appears in the larger main display. It is usually a six-figure number (for example: 1253.64).
When the time rate that is currently being measured is reached, the number may flash. Record this number. If none of the displays flash, record the readings from all the displays.
Let the pump run for at least 15 minutes before taking the next reading.
In systems that consume large amounts of electricity, there may be a multiplier programmed into the electronics.
If so, it will be noted on the electricity supplier’s bill for this meter as ‘Mult’ or ‘M’ and the display may read to a couple of decimal places. If there is a multiplier, run the pump for at least 30 minutes before taking the second register reading.
If the second reading has not changed, you are reading the wrong register.
Reading an electronic meter
First reading (register 126) = 1253.64 kWh
Second reading (register 126) = 1254.16 kWh
Multiplier stated on power bill = 40
Power usage =
Reading a disc meter
Note the rating figure, the revolutions per kilowatt hour (r/kWh), marked on the electricity meter.
R (r/kWh as marked on meter) = 266.6
Next, with the irrigation system set up in an average position and running, time the spinning horizontal disc on the power meter for at least 10 per cent of R (In this example, R is 266.6, so 10 per cent is about 30 revs).
N (number of disc revolutions) = 30 T (time of test) = 386 seconds
In systems that consume large amounts of electricity, the disc may be geared down so it doesn’t run too fast. If so, you will notice a multiplier ‘M’ is marked on the meter.
M (multiplier as marked on meter) = 40
From this data you can calculate the power usage in kilowatts.
Power usage =
In this example, the pump uses 42kW.
Perform this test regularly, over a season or between seasons, to check the pump’s power consumption. If you find that it takes less time for the same number of disc revolutions than when you first tested the pump, the power use is higher, and you will need to find out why.
This comparison is only possible when the irrigation is set up in the same position as the initial test, with the same number of sprinklers, and with the pumping water level roughly the same.
Multiple disc meters
If there are three meters, for example, one for each phase of a three-phase power supply, measure the three meters individually and add the kW figures together.
Measuring each meter separately gives an accurate answer as rarely are three meters exactly the same. If a very accurate result is needed, you need to monitor the system over all the irrigation positions for one complete cycle.
In this case you need to record the total electricity used, the total hours of use and the total amount pumped over the period.
Step 2: Measure the flow rate (Q)
The second measure needed to calculate pumping cost per megalitre is the flow rate of the system (Q).
The flow rate is the amount (or quantity) of water pumped in a certain amount of time, usually given in litres per second (L/s). It should be measured after the system has had sufficient time from startup to be running normally.
Measure the flow rate by reading your water meter at the pump for preferably the whole irrigation cycle or at least half an hour and dividing the litres pumped by the time in seconds.
Water meter reading at start: 1108.345 kL
Water meter reading after 35 minutes: 1230.145 kL
Q =
Estimating flow rate by discharge
If no water meter is fitted or it is losing accuracy, the flow rate of a spray irrigation system where all the sprinklers are the same model and size can be estimated by measuring the sprinkler discharge. Use several sprinklers: at least one at the start of the line, one in the middle and one at the end.
Record how long each sprinkler takes to fill a container (for example, a 10L bucket or a 20L drum). To find the flow rate of each sprinkler in litres, divide the container volume (in litres) by the time required to fill it (in seconds).
You can then find the average for the sprinklers you measured. To calculate the total flow rate of the system, multiply the average by the number of sprinklers operating.
For example:
First sprinkler takes nine seconds to fill a 10L bucket = 10 ÷ 9 = 1.11L/s
Middle sprinkler takes eight seconds to fill a 10L bucket = 10 ÷ 8 = 1.25L/s
End sprinkler takes seven seconds to fill a 10L bucket = 10 ÷ 7 = 1.43L/s
Average flow = (1.11 + 1.25 + 1.43) ÷ 3
= 1.26L/s
There are 46 sprinklers operating, so the total flow rate is = 1.26 x 46 = 58L/s
Step 3: Calculate the power per megalitre pumped
From the power usage and the flow rate, the kilowatt-hours per megalitre (kWh/ML) for your pump can be calculated.
This is called the ‘calibration’ value (the value used where no water meter is installed and electricity meter readings are read to infer the amount of water used).
Pump calibration (kWh/ML)
= kW ÷ (Q x 0.0036)
= 42 ÷ (58 x 0.0036)
= 201.1kWh/ML
(Note: 0.0036 converts kilowatt-seconds per litre to kilowatt-hours per megalitre.)
Step 4: Calculate the pumping cost
Having calculated the power used to pump a megalitre, if you know the cost per kWh, you can calculate the cost of pumping.
The charges per kWh may be difficult to work out exactly if your supplier has different rates for day or night, weekends, and so on so you should contact your supplier for help to work this out.
Pumping costs
If supply costs 25 cents per kWh then:
Pumping cost = 201 kWh/ML x $0.25
= $50.25 per ML
Measuring pump efficiency
Irrigation pump efficiency is a measure of how well the pump converts electrical energy into useful work to move water.
The aim of careful pump selection and regular pump maintenance is to have the pump performing as efficiently as possible (ie moving the most water for the least energy required). Efficient pump operation minimises running costs per megalitre pumped.
Pump efficiency of 70 per cent to 85 per cent should be achievable in most circumstances. An acceptable minimum for a centrifugal irrigation pump is 65 per cent, and 75 per cent for a turbine pump.
An efficiency figure below these means either the wrong pump was chosen for the job, the pump is worn and needs repair or maintenance is needed.
The key to containing your pumping costs is to regularly monitor your energy usage and check on any significant change that suggests attention is needed.
To calculate pump efficiency, you need to know the flow rate (Q) and the pump pressure, or total head (H or TH) of the system. The pressure and flow that a pump is working at is called the duty or duty point. Pump efficiency varies over the range of possible duties for any specific pump.
When you have calculated the pump duty, you can compare it to the manufacturer’s specifications shown in the pump’s performance curves. The two efficiency figures can then be compared to see if there is room for improvement and therefore possibly a reduction in costs.
Step 5: Determine Total Head
Total Head (TH or H) or Total Dynamic Head (TDH) is the Discharge Head plus the Suction Head.
If you have surface irrigation systems, you can skip this step.
An adequate estimate of total dynamic head for surface systems is the vertical height in metres from source water level to the end of the discharge pipe, or, if the discharge is submerged, to the height of the water above the discharge, that is, water level to water level, plus the losses due to friction in the suction pipe.
- Measure the discharge (or delivery) head
This is the pressure read from the gauge fitted at the pump when the system is at full operating pressure. This reading needs to be converted to equivalent metres of head, this is also sometimes called Pressure Head.
New pumps usually have a pressure gauge installed but they often suffer physical damage quickly. A better method is to fit an access point on the delivery side of the pump where you can temporarily install a pressure gauge whenever you want to take a reading. The gauge can be easily detached when not needed.
A change in the pump operating pressure through the season or across seasons, when irrigating the same block or shift, immediately tells you something has changed.
A sudden reduction usually indicates a new leak or a blockage on the suction side; a gradual reduction usually indicates wear of the impeller or sprinkler nozzles; and an increase usually suggests a blockage somewhere in the system downstream of the pressure gauge.
Pressure can be thought of as equivalent to a pipe of water of a certain height in metres. This is referred to as ‘head’ (H). At sea level, the pressure at the bottom of a pipe of water 10m high is about 100 kilopascals (kPa).
If your pressure gauge reads only in psi, convert to kPa by multiplying by 6.9.
For example: 40 psi = 40 × 6.9 = 276k Pa = 27.6 m head
- Determine the suction head
Suction head is the distance between the centre line of the pump and the source water level, plus losses in the suction pipe if the pump is positioned above the water level. Typical suction head figures for centrifugal pumps are three to five metres.
Most problems with pumps positioned above the water level occur in the suction line, so ensure everything correct. Common problems include blocked inlet or foot-valve or strainer, pipe diameter too small, pipe damaged or crushed, suction height too great, or air trapped at the connection to the pump.
Turbine and axial flow pumps must be submerged to operate, so they usually do not have any suction head.
For example:
Pressure Head = 27.6m
Suction head = 4.0m
Total Head = 31.6m
Another useful figure that can now be calculated is the pumping cost per ML per metre of head. This allows a meaningful comparison between different pump stations.
Pumping cost per ML per metre head: = cost ($/ML) ÷ TH (m)
= $50.25/ML ÷ 31.6m
= $1.59 / ML / m head
Step 6: Determine motor efficiency (Me)
Electric motors have an efficiency value. That is, they lose some of the energy going into them as heat. This energy loss changes with the size of the motor. The table below is a guideline for motors operating at full load.
Submersible motors lose about four per cent more than air-cooled electric motors (for example, where Me is 88 per cent for an air-cooled motor it would be 84 per cent for a submersible). Voltage losses through long electrical cables may also be significant so this should be checked with an electrical engineer.
Step 7: Determine transmission losses (Df)
If the engine is not directly coupled to the pump, there is a loss of energy through the transmission. This loss is taken into account by what is termed the drive factor (Df).
Step 8: Calculate pump efficiency (Pe)
Pe = (Q × H) ÷ (power consumed × Me × Df)
This example includes the data from the previous steps discussed. The drive from the motor to the pump is a V-belt in this case.
Pe ({87a03eb4327cd2ba79570dbcca4066c6d479b8f7279bafdb318e7183d82771cf}) = (Q × H) ÷ (power × Me × Df)
= (58 × 31.6) ÷ (42 × 0.9 × 0.9)
= 1832.8 ÷ 34.02
= 53.9 per cent
Step 9: Calculating potential savings
Most centrifugal pumps are designed to operate with at least 75 per cent efficiency, and most turbine pumps are designed to operate with at least 85 per cent efficiency.
The pump in our example is only about 54 per cent efficient, so how much would be saved by improving the efficiency from 54 per cent to 75 per cent?
Take this example:
If our pumping cost is $50.25 per ML, the improvement is calculated as follows:
Cost saving per ML:
= $50.25 – (50.25 x (54 ÷ 75))
= $50.25 – (50.25 x 0.72)
= $50.25 – 36.18
= $14.07
If 900ML are pumped during a season, the total cost saving is $14.07 × 900 = $12,663.
If impeller wear is the problem and the cost of replacement is $10,000, it would be paid for in less than one season. After that, the savings are all increased profit.
Notice that a reduction in the pump efficiency figure of 21 per cent (75 per cent to 54 per cent) causes an increase in pumping cost of 39 per cent ($36.18/ML to $50.25/ML).
Other factors that affect cost and pump efficiency
There are two other variables affect cost and pump efficiency: pump speed and impeller size.
Pump speed
You must know the pump speed in order to read the pump curves. The curves are usually prepared for specific pump speeds and impeller sizes.
If the pump is directly coupled to the electric motor, the speed is fixed by the speed of the motor: two-pole motors run at 2,900 rev/min and four-pole motors run at 1,440 rev/min. However, because the speed of electric motors varies a little, it would be good to check your motor speed with a rev counter.
If the motor is not directly coupled to the pump, the speed is altered by the gearing ratio of the transmission. Gear drives normally have the ratio stamped on the identification plate.
The ratio for a V-belt and pulley drive can be calculated from the diameter of the pulleys on the motor and the pump (see the diagram below – ensure the pump is stopped before measuring the pulleys).
A complication that can occur when working out the cost and efficiency revolves around Variable Speed Drives (VSD), also known as Variable Frequency Drives (VFD).
VSDs are becoming increasingly popular as their price reduces because of the benefits they offer. These units are added to electric motors and allow the speed to be altered by changing the frequency of the alternating current. They allow electrically driven pumps to have their speed set at exactly what is required for the pump duty and they eliminate the need for throttling the irrigation system using valves.
Savings of one quarter of the usual power consumption are often reported by irrigators, and may be as much as half depending on the situation. For determining the cost and efficiency of a pump, the measurements outlined in this article should be made several times with the pump set at different typical speeds.
Impeller size
Impeller wear has the same effect as a reduction in impeller size so you need to know the size of impeller fitted to your pump to work out which performance curve applies to your pump.
Sometimes the impeller size is stamped on the pump’s ID plate. If not, you need to find out the size by dismantling the pump and measuring it, or asking the person who made the change.
Sometimes an impeller is deliberately reduced in diameter to adjust the pump’s performance and obtain a specific duty.
To give a range of duties, manufacturers may offer impellers of different diameters for the same pump casing. Available impeller sizes are shown on the pump curves.
Power factor
Power factor may also substantially affect your running costs and perhaps the operation of your pump as well.
Measuring performance
Keeping track of your pump’s performance and costs is not difficult. It may save you a lot of money and keep your irrigation system performing properly.
If you identify your pump is operating below the acceptable minimum level, check the internal condition for wear or maintenance and the suitability of the pump for its current duty, or take steps to improve the drive or replace it with a VSD.
