AGRICULTURE
Measuring pumping costs for electric irrigation pumps
by Brenna ShumbamhiniJune 10, 2022
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If the incorrect pump is chosen or is worn out, this can enhance pumping prices and cut back productiveness. In the second of a series of truth sheets, the NSW Department of Primary Industries describes a easy method to work out the pumping costs and vitality effectivity of your electrical pump.
Tests of irrigation pumps across New South Wales have found that many were not performing efficiently, either because the mistaken pump had been chosen for the job, or as a result of the pump was worn.
To comprise prices, you need to monitor your vitality usage, repair and maintain the pump and work out what your pumping costs are.
When you have determined the operating value you probably can perform quick checks to detect any change, and when you’ve decided the pump efficiency, you can examine it to the manufacturer’s figures to determine when repair or replacement is cost-effective.
Measuring operating prices
One way of tracking pumping costs is to work out how much it costs to pump a megalitre of water. To do this, you have to measure:
The power consumption fee in kilowatts (kW)
The move fee in litres per second (L/s).
Combining these measures with the value of electrical energy offers you the pumping value.
Step 1: Measure the power used
You can measure the ability utilized by reading your electricity meter. Electronic meters are mostly used however single and multiple meters proceed for use on many farms.
Electronic meters
Electronic meters often measure and report the electricity used for the primary fee, shoulder fee and the off-peak rate in separate registers. The varied rates are switched ‘on’ and ‘off’ by the interior clock at the applicable times.
Electronic meters document your electrical energy consumption in a time-of-use format. They may have registers for the date, the time and for testing the display.
Each register has a 3 determine identification quantity. For instance, the current off-peak kilowatts could also be given register quantity ‘126’. You ought to check with your local vitality authority what the show register numbers are for each of your charges.
The meter scrolls through each register at four to six second intervals.
The register quantity seems, usually in smaller numbers, on the LCD display (in the diagram, within the high left-hand corner) and may have a brief description beneath (for instance: 126 = off-peak)
The usage in kilowatt-hours seems within the bigger major show. It is usually a six-figure number (for instance: 1253.64).
When the time fee that’s currently being measured is reached, the number might flash. Record this quantity. If none of the shows flash, document the readings from all the shows.
Let the pump run for no less than 15 minutes before taking the next reading.
In techniques that eat large amounts of electricity, there could also be a multiplier programmed into the electronics.
If so, it will be noted on the electrical energy supplier’s invoice for this meter as ‘Mult’ or ‘M’ and the display might read to a couple of decimal locations. If there’s a multiplier, run the pump for no much less than half-hour before taking the second register reading.
If the second studying has not modified, you are reading the incorrect register.
Reading an electronic meter
First reading (register 126) = 1253.64 kWh
Second reading (register 126) = 1254.16 kWh
Multiplier said on power bill = 40
Power usage =
Reading a disc meter
Note the score figure, the revolutions per kilowatt hour (r/kWh), marked on the electrical energy meter.
R (r/kWh as marked on meter) = 266.6
Next, with the irrigation system set up in a mean place and working, time the spinning horizontal disc on the ability 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 techniques that consume giant amounts of electrical energy, the disc may be geared down so it doesn’t run too fast. If so, you’ll discover a multiplier ‘M’ is marked on the meter.
M (multiplier as marked on meter) = forty
From this data you probably can calculate the facility usage in kilowatts.
digital pressure gauge =
In this example, the pump uses 42kW.
Perform this take a look at often, over a season or between seasons, to check the pump’s energy consumption. If you discover that it takes less time for the same variety of disc revolutions than whenever you first examined the pump, the ability use is greater, and you’ll need to seek out out why.
This comparison is just possible when the irrigation is ready up in the identical place as the preliminary take a look at, with the same number of sprinklers, and with the pumping water stage roughly the identical.
Multiple disc meters
If there are three meters, for instance, one for every part of a three-phase power supply, measure the three meters individually and add the kW figures collectively.
Measuring each meter separately gives an correct reply as not often are three meters exactly the same. If a very correct result’s wanted, you have to monitor the system over all the irrigation positions for one full cycle.
In this case you should record the whole electricity used, the total hours of use and the total quantity pumped over the interval.
Step 2: Measure the move price (Q)
The second measure needed to calculate pumping value per megalitre is the flow fee of the system (Q).
The circulate fee is the quantity (or quantity) of water pumped in a sure period of time, often given in litres per second (L/s). It ought to be measured after the system has had enough time from startup to be working normally.
Measure the flow price by studying your water meter on the pump for ideally the whole irrigation cycle or no much less than half an hour and dividing the litres pumped by the time in seconds.
Water meter studying at begin: 1108.345 kL
Water meter studying after 35 minutes: 1230.one hundred forty five kL
Q =
Estimating flow fee by discharge
If no water meter is fitted or it’s losing accuracy, the circulate fee of a sprig irrigation system where all the sprinklers are the same mannequin and size could be estimated by measuring the sprinkler discharge. Use several sprinklers: no less than one firstly of the road, one within the center and one at the finish.
Record how long each sprinkler takes to fill a container (for example, a 10L bucket or a 20L drum). To find the circulate price of every sprinkler in litres, divide the container quantity (in litres) by the point required to fill it (in seconds).
You can then discover the average for the sprinklers you measured. To calculate the whole circulate fee of the system, multiply the typical by the number of sprinklers working.
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 circulate = (1.11 + 1.25 + 1.43) ÷ 3
= 1.26L/s
There are 46 sprinklers operating, so the entire circulate rate is = 1.26 x forty six = 58L/s
Step 3: Calculate the power per megalitre pumped
From the power utilization and the circulate price, the kilowatt-hours per megalitre (kWh/ML) for your pump may be calculated.
This known as the ‘calibration’ worth (the worth used the place no water meter is installed and electrical energy meter readings are read to infer the quantity of water used).
Pump calibration (kWh/ML)
= kW ÷ (Q x zero.0036)
= forty two ÷ (58 x zero.0036)
= 201.1kWh/ML
(Note: 0.0036 converts kilowatt-seconds per litre to kilowatt-hours per megalitre.)
Step four: Calculate the pumping cost
Having calculated the facility used to pump a megalitre, if you know the cost per kWh, you presumably can calculate the value of pumping.
The charges per kWh could also be troublesome to work out exactly if your provider has different rates for day or night time, weekends, and so on so you must contact your supplier for assist 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 properly the pump converts electrical power into helpful work to maneuver water.
The aim of careful pump choice and common pump maintenance is to have the pump performing as effectively as possible (ie shifting the most water for the least power required). Efficient pump operation minimises running costs per megalitre pumped.
Pump effectivity of 70 per cent to eighty five per cent must be achievable in most circumstances. An acceptable minimal for a centrifugal irrigation pump is sixty five per cent, and 75 per cent for a turbine pump.
An efficiency figure below these means either the mistaken pump was chosen for the job, the pump is worn and needs repair or upkeep is required.
The key to containing your pumping prices is to often monitor your power utilization and verify on any significant change that means attention is required.
To calculate pump effectivity, you should know the circulate rate (Q) and the pump strain, or complete head (H or TH) of the system. The pressure and flow that a pump is working at is called the obligation or duty level. Pump efficiency varies over the vary of attainable duties for any specific pump.
An sufficient estimate of total dynamic head for surface techniques is the vertical peak in metres from supply water level to the end of the discharge pipe, or, if the discharge is submerged, to the peak of the water above the discharge, that is, water level to water level, plus the losses because of friction in the suction pipe.
Measure the discharge (or delivery) head
This is the strain read from the gauge fitted at the pump when the system is at full working pressure. This studying needs to be converted to equivalent metres of head, that is additionally generally known as Pressure Head.
New pumps often have a pressure gauge put in but they often suffer bodily injury shortly. A better methodology is to fit an access level on the delivery facet of the pump the place you can briefly set up a strain gauge whenever you wish to take a reading. The gauge can be easily detached when not wanted.
A change within the pump operating strain by way of the season or across seasons, when irrigating the identical block or shift, instantly tells you something has modified.
A sudden reduction often signifies a new leak or a blockage on the suction aspect; a gradual discount often indicates put on of the impeller or sprinkler nozzles; and an increase often suggests a blockage someplace in the system downstream of the pressure gauge.
Pressure may be considered equivalent to a pipe of water of a sure peak in metres. This is referred to as ‘head’ (H). At sea degree, the stress on the bottom of a pipe of water 10m high is about one hundred kilopascals (kPa).
If your pressure gauge reads only in psi, convert to kPa by multiplying by 6.9.
For instance: 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 stage, plus losses in the suction pipe if the pump is positioned above the water stage. Typical suction head figures for centrifugal pumps are three to 5 metres.
Most problems with pumps positioned above the water level occur in the suction line, so guarantee everything right. Common problems embody blocked inlet or foot-valve or strainer, pipe diameter too small, pipe damaged or crushed, suction height too nice, or air trapped on the connection to the pump.
Turbine and axial circulate pumps must be submerged to operate, so that they often wouldn’t have any suction head.
For instance:
Pressure Head = 27.6m
Suction head = four.0m
Total Head = 31.6m
Another helpful determine that can now be calculated is the pumping value per ML per metre of head. This permits a meaningful comparability between completely different pump stations.
Pumping value 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 a variety of the power going into them as warmth. This power loss modifications with the scale of the motor. The desk under is a suggestion for motors working at full load.
Submersible motors lose about four per cent more than air-cooled electrical motors (for instance, where Me is 88 per cent for an air-cooled motor it would be eighty four per cent for a submersible). Voltage losses via lengthy electrical cables may also be vital so this must be checked with an electrical engineer.
Step 7: Determine transmission losses (Df)
If the engine isn’t directly coupled to the pump, there’s a lack of power via the transmission. This loss is taken into consideration by what’s termed the drive issue (Df).
Step 8: Calculate pump effectivity (Pe)
Pe = (Q × H) ÷ (power consumed × Me × Df)
This example contains the info from the earlier steps mentioned. The drive from the motor to the pump is a V-belt on this case.
Pe (87a03eb4327cd2ba79570dbcca4066c6d479b8f7279bafdb318e7183d82771cf) = (Q × H) ÷ (power × Me × Df)
= (58 × 31.6) ÷ (42 × zero.9 × zero.9)
= 1832.eight ÷ 34.02
= 53.9 per cent
Step 9: Calculating potential financial savings
Most centrifugal pumps are designed to function with at least seventy five per cent efficiency, and most turbine pumps are designed to operate with no less than 85 per cent effectivity.
The pump in our instance is just about 54 per cent environment friendly, so how a lot can be saved by improving the efficiency from 54 per cent to seventy five per cent?
Take this instance:
If our pumping value is $50.25 per ML, the development 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 price saving is $14.07 × 900 = $12,663.
If impeller wear is the problem and the value of substitute is $10,000, it might be paid for in less than one season. After that, the financial savings are all increased revenue.
Notice that a reduction within the pump effectivity determine of 21 per cent (75 per cent to fifty four per cent) causes an increase in pumping value of 39 per cent ($36.18/ML to $50.25/ML).
Other elements that affect price and pump effectivity
There are two other variables have an result on value and pump effectivity: pump speed and impeller size.
Pump velocity
You must know the pump velocity in order to read the pump curves. The curves are usually prepared for particular pump speeds and impeller sizes.
If the pump is directly coupled to the electrical motor, the velocity is mounted 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, as a outcome of the velocity of electrical motors varies a little, it might be good to verify your motor velocity with a rev counter.
If digital pressure gauge isn’t instantly coupled to the pump, the pace is altered by the gearing ratio of the transmission. Gear drives usually 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 – make sure the pump is stopped earlier than measuring the pulleys).
A complication that may occur when understanding the cost and efficiency revolves round Variable Speed Drives (VSD), also referred to as Variable Frequency Drives (VFD).
VSDs are becoming more and more well-liked as their value reduces due to the benefits they provide. These models are added to electrical motors and allow the pace to be altered by altering the frequency of the alternating current. They enable electrically pushed pumps to have their speed set at precisely what’s required for the pump responsibility and so they get rid of the need for throttling the irrigation system utilizing valves.
Savings of 1 quarter of the usual power consumption are often reported by irrigators, and may be as much as half depending on the scenario. For figuring out the price and efficiency of a pump, the measurements outlined on this article ought to be made several times with the pump set at completely different typical speeds.
Impeller dimension
Impeller wear has the same effect as a reduction in impeller measurement so you need to know the size of impeller fitted to your pump to work out which efficiency curve applies to your pump.
Sometimes the impeller size is stamped on the pump’s ID plate. If not, you want to discover out the scale by dismantling the pump and measuring it, or asking the one that made the change.
Sometimes an impeller is deliberately lowered in diameter to adjust the pump’s efficiency and procure a particular duty.
To give a spread of duties, manufacturers might offer impellers of various diameters for a similar pump casing. Available impeller sizes are proven on the pump curves.
Power issue
Power factor may also substantially affect your running prices and perhaps the operation of your pump as well.
Measuring performance
Keeping track of your pump’s efficiency and costs isn’t troublesome. It might save you a lot of money and maintain your irrigation system performing properly.
If pressure gauge 10 bar determine your pump is working below the suitable minimal degree, verify the interior situation for wear or maintenance and the suitability of the pump for its current responsibility, or take steps to enhance the drive or exchange it with a VSD.
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