AGRICULTURE
Measuring pumping costs for electrical irrigation pumps
by Brenna ShumbamhiniJune 10, 2022
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If the wrong pump is chosen or is worn out, this could increase pumping costs and scale back productivity. In the second of a collection of truth sheets, the NSW Department of Primary Industries describes a easy method to work out the pumping costs and energy efficiency of your electrical pump.
Tests of irrigation pumps throughout New South Wales have discovered that many weren’t performing effectively, both because the incorrect pump had been chosen for the job, or because the pump was worn.
To contain costs, you should monitor your energy utilization, restore and maintain the pump and work out what your pumping prices are.
When you’ve determined the operating cost you’ll find a way to perform quick checks to detect any change, and when you’ve decided the pump efficiency, you’ll find a way to evaluate it to the manufacturer’s figures to decide when restore or alternative is cost-effective.
Measuring operating prices
One method of tracking pumping prices is to work out how a lot it costs to pump a megalitre of water. To do that, you have to measure:
The energy consumption rate in kilowatts (kW)
The circulate rate in litres per second (L/s).
Combining these measures with the value of electrical energy gives you the pumping value.
Step 1: Measure the power used
You can measure the ability utilized by studying your electrical energy meter. Electronic meters are most commonly used but single and a quantity of meters proceed for use on many farms.
Electronic meters
Electronic meters often measure and record the electricity used for the principle fee, shoulder fee and the off-peak rate in separate registers. The varied charges are switched ‘on’ and ‘off’ by the internal clock on the acceptable times.
Electronic meters document your electricity 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 three figure identification number. For example, the present off-peak kilowatts could additionally be given register quantity ‘126’. You should examine together with your native power authority what the display register numbers are for each of your rates.
The meter scrolls by way of each register at 4 to 6 second intervals.
The register number appears, usually in smaller numbers, on the LCD display (in the diagram, within the top left-hand corner) and may have a brief description beneath (for instance: 126 = off-peak)
The usage in kilowatt-hours appears in the larger primary display. It is often a six-figure quantity (for instance: 1253.64).
When the time fee that’s currently being measured is reached, the quantity could flash. Record this number. If none of the shows flash, record the readings from all of the displays.
Let the pump run for no less than quarter-hour earlier than taking the subsequent studying.
In methods that consume massive quantities of electricity, there may be a multiplier programmed into the electronics.
If so, will in all probability be noted on the electrical energy supplier’s bill for this meter as ‘Mult’ or ‘M’ and the display may read to a couple of decimal locations. If there is a multiplier, run the pump for no much less than half-hour before taking the second register studying.
If the second reading has not changed, you might be studying the mistaken register.
Reading an electronic meter
First studying (register 126) = 1253.64 kWh
Second reading (register 126) = 1254.16 kWh
Multiplier stated on power invoice = forty
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 arrange in a mean position and running, 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 systems that eat massive amounts of electricity, the disc may be geared down so it doesn’t run too quick. If so, you’ll discover a multiplier ‘M’ is marked on the meter.
M (multiplier as marked on meter) = 40
From this knowledge you can calculate the ability usage in kilowatts.
Power usage =
In this instance, the pump makes use of 42kW.
Perform this test regularly, over a season or between seasons, to verify the pump’s energy consumption. If you discover that it takes much less time for the same variety of disc revolutions than when you first examined the pump, the facility use is larger, and you’ll need to find out why.
This comparability is just potential when the irrigation is set up in the identical place as the preliminary take a look at, with the same variety of sprinklers, and with the pumping water degree roughly the same.
Multiple disc meters
If there are three meters, for instance, one for each part of a three-phase power supply, measure the three meters individually and add the kW figures together.
Measuring each meter separately offers an correct reply as not often are three meters precisely the same. If a very correct result is wanted, you have to monitor the system over all of the irrigation positions for one complete cycle.
In this case you need to report the whole electrical energy used, the whole hours of use and the whole amount pumped over the period.
Step 2: Measure the flow rate (Q)
The second measure wanted to calculate pumping cost per megalitre is the flow price of the system (Q).
The circulate fee is the amount (or quantity) of water pumped in a certain amount of time, normally given in litres per second (L/s). It ought to be measured after the system has had enough time from startup to be operating normally.
Measure the circulate rate by reading your water meter at the pump for ideally the entire irrigation cycle or no less than half an hour and dividing the litres pumped by the point in seconds.
Water meter studying at start: 1108.345 kL
Water meter studying after 35 minutes: 1230.a hundred forty five kL
Q =
Estimating circulate rate by discharge
If no water meter is fitted or it’s losing accuracy, the circulate price of a sprig irrigation system the place all of the sprinklers are the identical model and measurement may be estimated by measuring the sprinkler discharge. Use a quantity of sprinklers: no less than one firstly of the road, one in the middle and one at the end.
Record how lengthy every sprinkler takes to fill a container (for instance, a 10L bucket or a 20L drum). To find the move rate of every sprinkler in litres, divide the container volume (in litres) by the time required to fill it (in seconds).
pressure gauge can then find the typical for the sprinklers you measured. To calculate the total move price of the system, multiply the common by the variety of sprinklers operating.
For instance:
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.eleven + 1.25 + 1.43) ÷ three
= 1.26L/s
There are 46 sprinklers working, so the total circulate price is = 1.26 x 46 = 58L/s
Step three: Calculate the ability per megalitre pumped
From the power usage and the move fee, the kilowatt-hours per megalitre (kWh/ML) on your pump may be calculated.
This is identified as the ‘calibration’ worth (the worth used the place no water meter is installed and electrical energy meter readings are learn 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 4: Calculate the pumping price
Having calculated the ability used to pump a megalitre, if you know the price per kWh, you can calculate the cost of pumping.
The charges per kWh may be tough to work out exactly if your supplier has totally different charges for day or night, weekends, and so forth so you should contact your provider for assist to work this out.
Pumping prices
If supply prices 25 cents per kWh then:
Pumping cost = 201 kWh/ML x $0.25
= $50.25 per ML
Measuring pump effectivity
Irrigation pump effectivity is a measure of how properly the pump converts electrical energy into helpful work to maneuver water.
The aim of careful pump choice and regular pump upkeep is to have the pump performing as efficiently as possible (ie moving the most water for the least vitality required). Efficient pump operation minimises working costs per megalitre pumped.
Pump efficiency of 70 per cent to 85 per cent ought to be achievable in most circumstances. An acceptable minimum for a centrifugal irrigation pump is sixty five per cent, and seventy five per cent for a turbine pump.
An effectivity figure beneath these means both 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 costs is to regularly monitor your vitality utilization and verify on any important change that suggests consideration is needed.
To calculate pump efficiency, you should know the move price (Q) and the pump strain, or complete head (H or TH) of the system. The strain and circulate that a pump is working at is identified as the responsibility or duty level. Pump effectivity varies over the vary of possible duties for any particular pump.
An enough estimate of complete dynamic head for surface techniques is the vertical height in metres from source water stage 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 stage, plus the losses because of friction within the suction pipe.
Measure the discharge (or delivery) head
This is the stress learn from the gauge fitted on the pump when the system is at full operating strain. This studying needs to be transformed to equal metres of head, that is additionally sometimes referred to as Pressure Head.
New pumps often have a stress gauge put in but they usually endure physical injury shortly. A higher method is to suit an entry level on the supply aspect of the pump the place you possibly can briefly install a strain gauge whenever you want to take a reading. The gauge could be easily indifferent when not needed.
A change in the pump operating pressure via the season or throughout seasons, when irrigating the identical block or shift, instantly tells you something has modified.
A sudden reduction normally signifies a new leak or a blockage on the suction side; a gradual discount often indicates wear of the impeller or sprinkler nozzles; and a rise usually suggests a blockage someplace within the system downstream of the stress gauge.
Pressure could be regarded as equal to a pipe of water of a certain peak in metres. This is referred to as ‘head’ (H). At sea stage, the pressure on the bottom of a pipe of water 10m high is about one hundred kilopascals (kPa).
If your strain gauge reads only in psi, convert to kPa by multiplying by 6.9.
For example: forty psi = forty × 6.9 = 276k Pa = 27.6 m head
Determine the suction head
Suction head is the gap between the centre line of the pump and the supply 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 degree occur within the suction line, so ensure every little thing appropriate. Common problems embrace blocked inlet or foot-valve or strainer, pipe diameter too small, pipe broken or crushed, suction height too nice, or air trapped on the connection to the pump.
Turbine and axial move pumps must be submerged to operate, so they often don’t have any suction head.
For instance:
Pressure Head = 27.6m
Suction head = 4.0m
Total Head = 31.6m
Another useful determine that may now be calculated is the pumping cost per ML per metre of head. This permits a significant comparability between totally different pump stations.
Pumping cost per ML per metre head: = value ($/ML) ÷ TH (m)
= $50.25/ML ÷ 31.6m
= $1.59 / ML / m head
Step 6: Determine motor efficiency (Me)
Electric motors have an effectivity worth. That is, they lose a few of the power going into them as warmth. This power loss adjustments with the dimensions of the motor. The table beneath is a tenet for motors operating at full load.
Submersible motors lose about 4 per cent more than air-cooled electrical motors (for example, the place Me is 88 per cent for an air-cooled motor it would be 84 per cent for a submersible). Voltage losses via lengthy electrical cables may also be significant so this should be checked with an electrical engineer.
Step 7: Determine transmission losses (Df)
If the engine isn’t directly coupled to the pump, there is a loss of power through the transmission. This loss is taken into consideration by what’s termed the drive issue (Df).
Step eight: Calculate pump effectivity (Pe)
Pe = (Q × H) ÷ (power consumed × Me × Df)
This instance consists of the info from the previous 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 × 0.9 × zero.9)
= 1832.eight ÷ 34.02
= 53.9 per cent
Step 9: Calculating potential financial savings
Most centrifugal pumps are designed to operate with no much less than seventy five per cent efficiency, and most turbine pumps are designed to function with at least eighty five per cent effectivity.
The pump in our example is simply about fifty four per cent environment friendly, so how a lot can be saved by improving the efficiency from 54 per cent to 75 per cent?
Take this instance:
If our pumping cost 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 zero.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 issue and the worth of replacement is $10,000, it will be paid for in less than one season. After that, the financial savings are all increased revenue.
Notice that a reduction in the pump efficiency determine of 21 per cent (75 per cent to fifty four per cent) causes a rise in pumping cost of 39 per cent ($36.18/ML to $50.25/ML).
Other elements that have an result on cost and pump effectivity
There are two other variables affect value and pump efficiency: pump speed and impeller size.
Pump pace
You must know the pump velocity to find a way to learn the pump curves. The curves are often prepared for particular pump speeds and impeller sizes.
If the pump is immediately coupled to the electrical motor, the pace is fixed by the pace 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 velocity of electric motors varies a little, it will be good to verify your motor speed with a rev counter.
If the motor isn’t instantly coupled to the pump, the speed 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 may 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 happen when understanding the cost and efficiency revolves around Variable Speed Drives (VSD), also called Variable Frequency Drives (VFD).
VSDs have gotten increasingly popular as their worth reduces due to the benefits they offer. These models are added to electric motors and allow the speed to be altered by changing the frequency of the alternating present. They enable electrically driven pumps to have their velocity set at precisely what is required for the pump responsibility they usually get rid of the necessity for throttling the irrigation system using valves.
Savings of 1 quarter of the standard power consumption are often reported by irrigators, and may be as much as half relying on the situation. For figuring out the price and efficiency of a pump, the measurements outlined on this article ought to be made several instances with the pump set at totally different typical speeds.
Impeller size
Impeller wear has the same impact as a reduction in impeller size so you should know the dimensions of impeller fitted to your pump to work out which performance curve applies to your pump.
Sometimes the impeller measurement is stamped on the pump’s ID plate. If not, you have to discover out the dimensions by dismantling the pump and measuring it, or asking the one who made the change.
Sometimes an impeller is deliberately reduced in diameter to adjust the pump’s performance and acquire a selected responsibility.
To give a variety of duties, producers may provide impellers of various diameters for a similar pump casing. Available impeller sizes are proven on the pump curves.
Power factor
Power issue may also substantially have an result on your running prices and perhaps the operation of your pump as well.
Measuring efficiency
Keeping observe of your pump’s performance and costs is not difficult. It may prevent some huge cash and hold your irrigation system performing correctly.
If you identify your pump is working below the appropriate minimum stage, check the interior situation for put on or upkeep and the suitability of the pump for its present obligation, or take steps to improve the drive or substitute it with a VSD.
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