Measuring pumping costs for electrical irrigation pumps


Measuring pumping costs for electrical irrigation pumps

by Brenna ShumbamhiniJune 10, 2022


If the incorrect pump is chosen or is worn out, this can increase pumping costs and reduce productivity. In the second of a series of reality sheets, the NSW Department of Primary Industries describes a simple method to work out the pumping costs and power effectivity of your electrical pump.
Tests of irrigation pumps throughout New South Wales have found that many weren’t performing efficiently, either as a outcome of the wrong pump had been chosen for the job, or as a outcome of the pump was worn.
To comprise prices, you have to monitor your vitality usage, repair and preserve the pump and work out what your pumping prices are.
When you have decided the operating cost you probably can carry out quick checks to detect any change, and when you have decided the pump efficiency, you presumably can examine it to the manufacturer’s figures to resolve when restore or alternative is cost-effective.
Measuring working costs

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

The flow price in litres per second (L/s).
Combining these measures with the worth of electrical energy gives you the pumping cost.
Step 1: Measure the power used

You can measure the power utilized by studying your electrical energy meter. Electronic meters are most commonly used but single and multiple meters proceed to be used on many farms.
Electronic meters

Electronic meters normally measure and record the electricity used for the principle rate, shoulder rate and the off-peak fee in separate registers. The various charges are switched ‘on’ and ‘off’ by the inner clock on the acceptable instances.
Electronic meters record your electricity consumption in a time-of-use format. They can also have registers for the date, the time and for testing the display.
Each register has a 3 figure identification quantity. For example, the present off-peak kilowatts may be given register number ‘126’. You should verify together with your local energy authority what the display register numbers are for each of your charges.
The meter scrolls through every register at four to 6 second intervals.
The register quantity appears, typically in smaller numbers, on the LCD display screen (in the diagram, within the high left-hand corner) and may have a short description beneath (for example: 126 = off-peak)

The usage in kilowatt-hours seems within the bigger major show. It is usually a six-figure quantity (for example: 1253.64).
When the time price that’s currently being measured is reached, the number may flash. Record this number. If not one of the displays flash, report the readings from all the shows.
Let the pump run for no much less than quarter-hour before taking the next reading.
In systems that consume giant quantities of electrical energy, there could additionally be a multiplier programmed into the electronics.
If so, will probably be famous on the electrical energy supplier’s invoice for this meter as ‘Mult’ or ‘M’ and the show may read to a couple of decimal places. If there’s a multiplier, run the pump for no much less than 30 minutes earlier than taking the second register reading.
If the second reading has not modified, you’re reading the wrong register.
Reading an digital meter

First reading (register 126) = 1253.sixty four kWh

Second reading (register 126) = 1254.sixteen kWh

Multiplier acknowledged on energy bill = forty

Power usage =

Reading a disc meter

Note the score 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 a mean place and working, time the spinning horizontal disc on the ability meter for no less than 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 electrical energy, the disc could additionally 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 possibly can calculate the power utilization in kilowatts.
Power usage =

In this example, the pump uses 42kW.
Perform this take a look at frequently, over a season or between seasons, to check the pump’s power consumption. If you find that it takes much less time for a similar number of disc revolutions than whenever you first tested the pump, the power use is greater, and you’ll need to search out out why.
This comparison is only potential when the irrigation is ready up in the identical position because the preliminary 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 instance, one for each part of a three-phase power supply, measure the three meters individually and add the kW figures collectively.
Measuring each meter separately offers an correct reply as not often are three meters exactly the identical. If a really correct result is wanted, you need to monitor the system over all the irrigation positions for one complete cycle.
In this case you have to document the whole electrical energy used, the whole hours of use and the entire quantity pumped over the interval.
Step 2: Measure the move price (Q)

The second measure needed to calculate pumping cost per megalitre is the flow price of the system (Q).
The circulate price is the amount (or quantity) of water pumped in a sure amount of time, usually given in litres per second (L/s). It should be measured after the system has had adequate time from startup to be operating normally.
Measure the move price by studying your water meter at 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 studying at begin: 1108.345 kL

Water meter studying after 35 minutes: 1230.a hundred forty five kL

Q =

Estimating flow rate by discharge

If no water meter is fitted or it’s dropping accuracy, the flow rate of a sprig irrigation system where all the sprinklers are the identical mannequin and dimension could be estimated by measuring the sprinkler discharge. Use several sprinklers: at least one firstly of the road, one in the center and one on the finish.
Record how lengthy each sprinkler takes to fill a container (for example, a 10L bucket or a 20L drum). To find the flow fee of every sprinkler in litres, divide the container volume (in litres) by the point required to fill it (in seconds).
You can then find the typical for the sprinklers you measured. To calculate the total move fee of the system, multiply the typical by the variety of sprinklers working.
For example:
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 move = (1.11 + 1.25 + 1.43) ÷ 3

= 1.26L/s

There are 46 sprinklers working, so the total move price is = 1.26 x forty six = 58L/s

Step 3: Calculate the power per megalitre pumped

From the ability usage and the move fee, the kilowatt-hours per megalitre (kWh/ML) on your pump can be calculated.
This is called the ‘calibration’ worth (the value used where 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 0.0036)

= forty two ÷ (58 x zero.0036)

= 201.1kWh/ML

(Note: zero.0036 converts kilowatt-seconds per litre to kilowatt-hours per megalitre.)

Step four: Calculate the pumping cost

Having calculated the power used to pump a megalitre, if you realize the fee per kWh, you’ll have the ability to calculate the price of pumping.
The charges per kWh may be difficult to work out precisely in case your provider has totally different rates for day or night, weekends, and so forth so you need to contact your provider for assist to work this out.
Pumping prices

If provide costs 25 cents per kWh then:
Pumping price = 201 kWh/ML x $0.25

= $50.25 per ML

Measuring pump effectivity

Irrigation pump efficiency is a measure of how nicely the pump converts electrical vitality into useful work to maneuver water.
The aim of cautious pump selection and common pump maintenance is to have the pump performing as efficiently as potential (ie shifting the most water for the least power required). Efficient pump operation minimises operating prices 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 65 per cent, and seventy five per cent for a turbine pump.
An effectivity figure below these means both the incorrect pump was chosen for the job, the pump is worn and needs repair or upkeep is needed.
The key to containing your pumping prices is to frequently monitor your power utilization and check on any vital change that implies attention is required.
To calculate pump efficiency, you should know the move price (Q) and the pump pressure, or whole head (H or TH) of the system. The pressure and circulate that a pump is working at is identified as the obligation or responsibility level. Pump efficiency varies over the vary of potential duties for any particular pump.
An sufficient estimate of total dynamic head for floor techniques is the vertical height in metres from supply water degree to the end of the discharge pipe, or, if the discharge is submerged, to the peak of the water above the discharge, that’s, water stage to water level, plus the losses as a outcome of friction within the suction pipe.
Measure the discharge (or delivery) head

This is the stress read from the gauge fitted at the pump when the system is at full working pressure. This reading must be transformed to equal metres of head, that is additionally sometimes known as Pressure Head.
New pumps usually have a pressure gauge installed however they often undergo bodily injury shortly. A better methodology is to suit an entry point on the delivery aspect of the pump where you’ll be able to quickly set up a strain gauge whenever you want to take a studying. The gauge may be easily detached when not wanted.
A change within the pump operating strain by way of the season or across seasons, when irrigating the same block or shift, immediately tells you something has changed.
A sudden reduction normally signifies a new leak or a blockage on the suction side; a gradual discount normally indicates wear of the impeller or sprinkler nozzles; and a rise usually suggests a blockage someplace in the system downstream of the stress gauge.
Pressure can be regarded as equivalent to a pipe of water of a certain height in metres. This is known as ‘head’ (H). At sea degree, the stress at the backside of a pipe of water 10m excessive is about 100 kilopascals (kPa).
If your stress gauge reads only in psi, convert to kPa by multiplying by 6.9.
For instance: forty psi = 40 × 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 degree, 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 5 metres.
Most issues with pumps positioned above the water stage happen within the suction line, so guarantee every thing appropriate. Common problems embody 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 circulate pumps should be submerged to operate, in order that they normally don’t 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 price per ML per metre of head. This permits a significant comparison between totally different pump stations.
Pumping value per ML per metre head: = price ($/ML) ÷ TH (m)

= $50.25/ML ÷ 31.6m

= $1.fifty nine / ML / m head

Step 6: Determine motor effectivity (Me)

Electric motors have an effectivity worth. That is, they lose some of the energy going into them as warmth. This vitality loss adjustments with the dimensions of the motor. The desk beneath is a suggestion for motors operating at full load.
Submersible motors lose about four per cent more than air-cooled electrical motors (for example, where Me is 88 per cent for an air-cooled motor it might be 84 per cent for a submersible). Voltage losses via long electrical cables can also be vital so this ought to be checked with an electrical engineer.
Step 7: Determine transmission losses (Df)

If the engine just isn’t instantly coupled to the pump, there’s a lack of power by way of the transmission. This loss is taken into account by what’s termed the drive factor (Df).
Step eight: Calculate pump effectivity (Pe)

Pe = (Q × H) ÷ (power consumed × Me × Df)

This example contains the information from the previous steps mentioned. 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 × zero.9)

= 1832.eight ÷ 34.02

= fifty three.9 per cent

Step 9: Calculating potential financial savings

Most centrifugal pumps are designed to function with a minimal of 75 per cent effectivity, and most turbine pumps are designed to operate with no much less than 85 per cent efficiency.
The pump in our example is simply about 54 per cent environment friendly, so how a lot would be saved by enhancing the effectivity 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 whole value saving is $14.07 × 900 = $12,663.
If impeller put on is the problem and the value of replacement is $10,000, it will be paid for in lower than one season. After that, the financial savings are all increased profit.
Notice that a discount in the pump effectivity determine of 21 per cent (75 per cent to fifty four per cent) causes an increase in pumping cost of 39 per cent ($36.18/ML to $50.25/ML).
Other elements that affect value and pump efficiency

There are two other variables affect value and pump efficiency: pump pace and impeller size.
Pump velocity

You should know the pump velocity in order to read the pump curves. The curves are usually ready for specific pump speeds and impeller sizes.
If the pump is directly coupled to the electric motor, the speed is mounted by the velocity 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 pace of electrical motors varies a little, it might be good to check your motor speed with a rev counter.
If the motor just isn’t instantly 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 may be calculated from the diameter of the pulleys on the motor and the pump (see the diagram beneath – ensure the pump is stopped before measuring the pulleys).
A complication that can occur when working out the price and efficiency revolves round Variable Speed Drives (VSD), also called Variable Frequency Drives (VFD).
VSDs are becoming increasingly popular as their value reduces due to the benefits they provide. These items are added to electric motors and allow the pace to be altered by altering the frequency of the alternating present. They enable electrically pushed pumps to have their speed set at precisely what is required for the pump obligation they usually get rid of the need for throttling the irrigation system using valves.
Savings of 1 quarter of the similar old power 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 efficiency of a pump, the measurements outlined in this article must be made several times with the pump set at different typical speeds.
Impeller dimension

Impeller wear has the identical effect as a reduction in impeller size so you have to 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 find out the dimensions by dismantling the pump and measuring it, or asking the one that made the change.
Sometimes an impeller is intentionally reduced in diameter to adjust the pump’s efficiency and acquire a selected duty.
To give a range of duties, manufacturers might provide impellers of different diameters for a similar pump casing. Available impeller sizes are proven on the pump curves.
Power issue

Power factor may considerably have an effect on your running costs and perhaps the operation of your pump as nicely.
Measuring performance

Keeping track of your pump’s performance and prices is not troublesome. It could prevent some big cash and hold your irrigation system performing correctly.
If you identify your pump is operating below the appropriate minimal stage, examine the interior situation for put on or upkeep and the suitability of the pump for its present obligation, or take steps to enhance the drive or exchange it with a VSD.

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