Measuring pumping prices for electric irrigation pumps

by Brenna ShumbamhiniJune 10, 2022


If the mistaken pump is chosen or is worn out, this could increase pumping costs and reduce productiveness. In the second of a collection of reality sheets, the NSW Department of Primary Industries describes a simple method to work out the pumping costs and power efficiency of your electric pump.
Tests of irrigation pumps across New South Wales have found that many weren’t performing effectively, both as a end result of the incorrect pump had been chosen for the job, or as a result of the pump was worn.
To comprise costs, you should monitor your energy utilization, repair and maintain the pump and work out what your pumping prices are.
When you have determined the working price you’ll find a way to perform fast checks to detect any change, and when you have decided the pump effectivity, you presumably can evaluate it to the manufacturer’s figures to decide when restore or alternative is cost-effective.
Measuring working costs

One means of tracking pumping prices is to work out how much it prices to pump a megalitre of water. To do that, you have to measure:
The power consumption fee in kilowatts (kW)

The circulate rate in litres per second (L/s).
Combining these measures with the cost of electricity provides you the pumping cost.
Step 1: Measure the power used

You can measure the ability utilized by reading your electricity meter. Electronic meters are most commonly used however single and a number of meters proceed for use on many farms.
Electronic meters

Electronic meters normally measure and document the electrical energy used for the principle price, shoulder fee and the off-peak rate in separate registers. The varied rates are switched ‘on’ and ‘off’ by the interior clock on the appropriate occasions.
Electronic meters document your electricity consumption in a time-of-use format. They may also have registers for the date, the time and for testing the show.
Each register has a three determine identification quantity. For example, the present off-peak kilowatts may be given register number ‘126’. You should verify along with your local power authority what the display register numbers are for each of your charges.
The meter scrolls via every register at 4 to 6 second intervals.
The register number seems, typically in smaller numbers, on the LCD screen (in the diagram, in the prime left-hand corner) and may have a brief description underneath (for instance: 126 = off-peak)

The usage in kilowatt-hours appears within the bigger main show. It is often a six-figure quantity (for example: 1253.64).
When the time fee that is presently being measured is reached, the quantity might flash. Record this number. If none of the displays flash, document the readings from all of the displays.
Let the pump run for no much less than 15 minutes before taking the next studying.
In systems that eat giant amounts of electricity, there could additionally be a multiplier programmed into the electronics.
If so, will probably be famous on the electricity supplier’s bill for this meter as ‘Mult’ or ‘M’ and the display might read to a few decimal places. If there is a multiplier, run the pump for no less than 30 minutes before taking the second register studying.
If the second reading has not changed, you are reading the wrong register.
Reading an digital meter

First studying (register 126) = 1253.64 kWh

Second reading (register 126) = 1254.16 kWh

Multiplier said on energy bill = forty

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 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 instance, 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 methods that consume 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 data you’ll have the ability to calculate the power usage in kilowatts.
Power utilization =

In this instance, the pump makes use of 42kW.
Perform this check regularly, over a season or between seasons, to examine the pump’s energy consumption. If you find that it takes less time for a similar number of disc revolutions than whenever you first tested the pump, the facility use is higher, and you’ll need to search out out why.
This comparison is simply possible when the irrigation is set up in the same place as the initial test, with the identical variety of sprinklers, and with the pumping water degree roughly the identical.
Multiple disc meters

If there are three meters, for example, one for each section of a three-phase power supply, measure the three meters individually and add the kW figures collectively.
Measuring each meter individually offers an correct reply as hardly ever are three meters exactly the same. If a very correct result is wanted, you have to monitor the system over all the irrigation positions for one full cycle.
In this case you have to report the whole electrical energy used, the entire hours of use and the whole quantity pumped over the period.
Step 2: Measure the flow rate (Q)

The second measure wanted to calculate pumping price per megalitre is the circulate price of the system (Q).
The move rate is the quantity (or quantity) of water pumped in a sure amount of time, usually given in litres per second (L/s). It must be measured after the system has had enough time from startup to be operating usually.
Measure the flow fee by studying your water meter on the pump for ideally the whole irrigation cycle or no less than half an hour and dividing the litres pumped by the point in seconds.
Water meter reading at start: 1108.345 kL

Water meter reading after 35 minutes: 1230.145 kL

Q =

Estimating circulate rate by discharge

If no water meter is fitted or it is losing accuracy, the flow fee of a spray irrigation system the place all the sprinklers are the same mannequin and dimension could be estimated by measuring the sprinkler discharge. Use several sprinklers: at least one at the start of the road, one within the middle and one on the end.
Record how long every sprinkler takes to fill a container (for instance, a 10L bucket or a 20L drum). To discover the flow fee of every 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 entire flow fee of the system, multiply the typical by the number of sprinklers operating.
For instance:
First sprinkler takes 9 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 forty six sprinklers working, so the whole move fee is = 1.26 x 46 = 58L/s

Step 3: Calculate the facility per megalitre pumped

From the power utilization and the flow rate, the kilowatt-hours per megalitre (kWh/ML) in your pump can be calculated.
This is identified as the ‘calibration’ value (the value used the place no water meter is put in and electricity meter readings are learn to infer the amount of water used).
Pump calibration (kWh/ML)

= kW ÷ (Q x 0.0036)

= forty two ÷ (58 x 0.0036)

= 201.1kWh/ML

(Note: 0.0036 converts kilowatt-seconds per litre to kilowatt-hours per megalitre.)

Step four: Calculate the pumping price

Having calculated the facility used to pump a megalitre, if you know the price per kWh, you’ll be able to calculate the value of pumping.
The charges per kWh could additionally be troublesome to work out exactly if your supplier has completely different rates for day or night time, weekends, and so on so you must contact your supplier for help 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 nicely the pump converts electrical vitality into useful work to maneuver water.
ไดอะแฟรม of cautious pump choice and regular pump maintenance is to have the pump performing as effectively as possible (ie transferring essentially the most water for the least vitality required). Efficient pump operation minimises running costs per megalitre pumped.
Pump effectivity of 70 per cent to eighty five per cent ought to be achievable in most circumstances. An acceptable minimum for a centrifugal irrigation pump is sixty five per cent, and 75 per cent for a turbine pump.
An effectivity figure below these means either the wrong pump was chosen for the job, the pump is worn and wishes restore or maintenance is needed.
The key to containing your pumping costs is to regularly monitor your energy utilization and examine on any important change that implies consideration is needed.
To calculate pump efficiency, you want to know the circulate rate (Q) and the pump strain, or complete head (H or TH) of the system. The stress and flow that a pump is working at is identified as the responsibility or responsibility level. Pump efficiency varies over the range of attainable duties for any specific pump.
An adequate estimate of complete dynamic head for floor systems is the vertical peak in metres from supply water degree to the tip of the discharge pipe, or, if the discharge is submerged, to the height of the water above the discharge, that’s, water degree to water stage, plus the losses due to friction within the suction pipe.
Measure the discharge (or delivery) head

This is the pressure learn from the gauge fitted on the pump when the system is at full operating strain. This reading must be transformed to equal metres of head, this is also typically known as Pressure Head.
New pumps normally have a strain gauge installed however they usually suffer bodily harm rapidly. A higher method is to fit an entry level on the supply aspect of the pump the place you can briefly set up a strain gauge whenever you wish to take a studying. The gauge could be simply detached when not needed.
A change in the pump working stress by way of the season or throughout seasons, when irrigating the same block or shift, instantly tells you one thing has changed.
A sudden discount normally signifies a new leak or a blockage on the suction aspect; a gradual discount usually signifies wear of the impeller or sprinkler nozzles; and an increase normally suggests a blockage somewhere within the system downstream of the strain gauge.
Pressure can be regarded as equal to a pipe of water of a certain top in metres. This is known as ‘head’ (H). At sea level, the pressure at the bottom of a pipe of water 10m excessive is about one hundred kilopascals (kPa).
If your pressure gauge reads solely in psi, convert to kPa by multiplying by 6.9.
For instance: 40 psi = forty × 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 degree. Typical suction head figures for centrifugal pumps are three to five metres.
Most issues with pumps positioned above the water degree occur in the suction line, so guarantee every thing right. Common issues embody blocked inlet or foot-valve or strainer, pipe diameter too small, pipe broken or crushed, suction top too nice, or air trapped on the connection to the pump.
Turbine and axial circulate pumps must be submerged to operate, so they often don’t have any suction head.
For example:
Pressure Head = 27.6m

Suction head = four.0m

Total Head = 31.6m

Another helpful figure that may now be calculated is the pumping cost per ML per metre of head. This allows a meaningful comparison between completely different pump stations.
Pumping price 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 effectivity value. That is, they lose a few of the energy going into them as heat. This energy loss adjustments with the dimensions of the motor. The table under is a guideline for motors working at full load.
Submersible motors lose about four per cent greater than air-cooled electrical motors (for example, the place Me is 88 per cent for an air-cooled motor it might be 84 per cent for a submersible). Voltage losses via lengthy electrical cables may also be vital so this should be checked with an electrical engineer.
Step 7: Determine transmission losses (Df)

If the engine is not instantly coupled to the pump, there is a loss of power by way of the transmission. This loss is taken into account 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 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 × zero.9 × zero.9)

= 1832.eight ÷ 34.02

= fifty three.9 per cent

Step 9: Calculating potential financial savings

Most centrifugal pumps are designed to operate with a minimal of seventy five per cent effectivity, and most turbine pumps are designed to operate with a minimum of 85 per cent efficiency.
The pump in our instance is only about fifty four per cent efficient, so how much can be saved by enhancing the effectivity from fifty four per cent to 75 per cent?

Take this example:
If our pumping value 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 zero.72)

= $50.25 – 36.18

= $14.07

If 900ML are pumped during a season, the whole value saving is $14.07 × 900 = $12,663.
If impeller put on is the issue and the worth of replacement is $10,000, it would be paid for in lower than one season. After that, the savings are all elevated profit.
Notice that a discount in the pump efficiency determine of 21 per cent (75 per cent to fifty four per cent) causes an increase in pumping price of 39 per cent ($36.18/ML to $50.25/ML).
Other components that affect value and pump effectivity

There are two different variables have an result on value and pump efficiency: pump speed and impeller size.
Pump pace

You must know the pump pace in order to read the pump curves. The curves are usually ready for particular pump speeds and impeller sizes.
If the pump is immediately coupled to the electric 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 speed of electric motors varies slightly, it will be good to check your motor pace with a rev counter.
If the motor isn’t immediately coupled to the pump, the velocity 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 could 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 may occur when understanding the fee and effectivity revolves around Variable Speed Drives (VSD), also called Variable Frequency Drives (VFD).
VSDs are becoming increasingly in style as their price reduces due to the benefits they provide. These units are added to electrical motors and allow the pace to be altered by altering the frequency of the alternating present. They allow electrically pushed pumps to have their speed set at exactly what is required for the pump responsibility and so they remove the necessity for throttling the irrigation system using valves.
Savings of 1 quarter of the standard energy consumption are often reported by irrigators, and could additionally be as much as half depending on the state of affairs. For figuring out the price and effectivity of a pump, the measurements outlined on this article ought to be made a number of times with the pump set at different typical speeds.
Impeller dimension

Impeller wear has the identical effect as a discount in impeller size so you have 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 have to find out the dimensions by dismantling the pump and measuring it, or asking the person who made the change.
Sometimes an impeller is intentionally reduced in diameter to adjust the pump’s performance and procure a particular responsibility.
To give a range of duties, manufacturers could offer impellers of various diameters for a similar pump casing. Available impeller sizes are proven on the pump curves.
Power issue

Power issue can also considerably affect your working prices and perhaps the operation of your pump as nicely.
Measuring efficiency

Keeping track of your pump’s performance and prices isn’t difficult. It may save you a lot of money and keep your irrigation system performing correctly.
If you establish your pump is working beneath the appropriate minimum degree, check the inner situation for put on or upkeep and the suitability of the pump for its present responsibility, or take steps to enhance the drive or substitute it with a VSD.