Measuring pumping prices for electric irrigation pumps

by Brenna ShumbamhiniJune 10, 2022


If the incorrect pump is chosen or is worn out, this will increase pumping prices and scale back productiveness. In the second of a series of fact sheets, the NSW Department of Primary Industries describes a easy method to work out the pumping prices and vitality effectivity of your electric pump.
Tests of irrigation pumps throughout New South Wales have found that many weren’t performing efficiently, either as a end result of the mistaken pump had been chosen for the job, or as a outcome of the pump was worn.
To comprise prices, you have to monitor your power usage, repair and keep the pump and work out what your pumping costs are.
When you have decided the working value you possibly can carry out quick checks to detect any change, and when you could have decided the pump efficiency, you can compare it to the manufacturer’s figures to decide when restore or replacement is cost-effective.
Measuring operating costs

One method of tracking pumping prices is to work out how much it prices to pump a megalitre of water. To do this, you have to measure:
The power consumption price 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 price.
Step 1: Measure the ability used

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

Electronic meters normally measure and record the electricity used for the main rate, shoulder rate and the off-peak price in separate registers. The numerous charges are switched ‘on’ and ‘off’ by the inner clock at the appropriate instances.
Electronic meters report 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 three figure identification number. For instance, the current off-peak kilowatts could additionally be given register number ‘126’. digital pressure gauge should examine along with your native vitality 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 number appears, typically in smaller numbers, on the LCD screen (in the diagram, in the top left-hand corner) and will have a brief description beneath (for example: 126 = off-peak)

The utilization in kilowatt-hours seems within the larger primary show. It is often a six-figure number (for example: 1253.64).
When the time rate that’s currently being measured is reached, the quantity might flash. Record this number. If none of the displays flash, report the readings from all of the shows.
Let the pump run for at least 15 minutes before taking the subsequent reading.
In methods that devour massive amounts of electrical energy, there could additionally be a multiplier programmed into the electronics.
If so, it goes to be famous on the electricity supplier’s invoice for this meter as ‘Mult’ or ‘M’ and the show may learn to a couple of decimal places. If there’s a multiplier, run the pump for at least 30 minutes earlier than taking the second register studying.
If the second reading has not modified, you would possibly be studying the wrong register.
Reading an electronic meter

First reading (register 126) = 1253.64 kWh

Second studying (register 126) = 1254.sixteen kWh

Multiplier said on energy invoice = forty

Power utilization =

Reading a disc meter

Note the rating 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 working, time the spinning horizontal disc on the facility meter for a minimum of 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 systems that consume large quantities of electrical energy, the disc may be geared down so it doesn’t run too quick. If so, you will discover a multiplier ‘M’ is marked on the meter.
M (multiplier as marked on meter) = 40

From this knowledge you presumably can calculate the ability utilization in kilowatts.
Power usage =

In this example, the pump uses 42kW.
Perform this check frequently, over a season or between seasons, to verify the pump’s power consumption. If you discover that it takes much less time for a similar variety of disc revolutions than whenever you first tested the pump, the ability use is higher, and you will need to find out why.
This comparability is only possible when the irrigation is set up in the same place as the initial check, with the same variety of sprinklers, and with the pumping water degree roughly the identical.
Multiple disc meters

If there are three meters, for example, one for every section of a three-phase power supply, measure the three meters individually and add the kW figures collectively.
Measuring each meter individually gives an accurate answer as rarely are three meters precisely the identical. If a really correct result is needed, you need to monitor the system over all of the irrigation positions for one full cycle.
In this case you have to document the entire electricity used, the whole hours of use and the whole quantity pumped over the period.
Step 2: Measure the circulate price (Q)

The second measure needed to calculate pumping value per megalitre is the circulate price of the system (Q).
The circulate rate is the amount (or quantity) of water pumped in a sure period of time, often given in litres per second (L/s). It should be measured after the system has had enough time from startup to be running usually.
Measure the flow fee by studying your water meter at the pump for preferably the whole irrigation cycle or no much 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 flow price by discharge

If no water meter is fitted or it’s dropping accuracy, the move rate of a twig irrigation system where all of the sprinklers are the same model and size can be estimated by measuring the sprinkler discharge. Use a number of sprinklers: no much less than one initially of the line, one in the center and one at the finish.
Record how lengthy each sprinkler takes to fill a container (for instance, a 10L bucket or a 20L drum). To find the flow fee of each sprinkler in litres, divide the container quantity (in litres) by the time required to fill it (in seconds).
You can then find the typical for the sprinklers you measured. To calculate the whole circulate rate of the system, multiply the common by the number 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) ÷ three

= 1.26L/s

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

Step 3: Calculate the power per megalitre pumped

From the ability utilization and the move price, the kilowatt-hours per megalitre (kWh/ML) in your pump may be calculated.
This is recognized as the ‘calibration’ worth (the worth used where no water meter is installed and electrical energy meter readings are read to deduce the amount of water used).
Pump calibration (kWh/ML)

= kW ÷ (Q x zero.0036)

= forty two ÷ (58 x 0.0036)

= 201.1kWh/ML

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

Step four: Calculate the pumping value

Having calculated the ability used to pump a megalitre, if you understand the fee per kWh, you’ll be able to calculate the cost of pumping.
The charges per kWh may be troublesome to work out exactly if your provider has completely different rates for day or night, weekends, and so forth so you should contact your provider for help to work this out.
Pumping costs

If provide costs 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 energy into helpful work to move water.
The goal of cautious pump choice and common pump maintenance is to have the pump performing as efficiently as attainable (ie shifting essentially the most water for the least power required). Efficient pump operation minimises working prices per megalitre pumped.
Pump effectivity 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 75 per cent for a turbine pump.
An effectivity figure below these means either the mistaken pump was chosen for the job, the pump is worn and needs repair or maintenance is needed.
The key to containing your pumping prices is to frequently monitor your energy utilization and verify on any significant change that means attention is needed.
To calculate digital pressure gauge , you have to know the move fee (Q) and the pump stress, or whole head (H or TH) of the system. The pressure and move that a pump is working at is called the obligation or responsibility point. Pump effectivity varies over the vary of attainable duties for any particular pump.
An adequate estimate of complete dynamic head for floor techniques is the vertical height in metres from source water stage 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 degree, plus the losses as a outcome of friction in 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 operating strain. This studying needs to be transformed to equivalent metres of head, this is additionally typically referred to as Pressure Head.
New pumps usually have a strain gauge installed but they usually undergo physical damage quickly. A higher technique is to fit an entry level on the delivery facet of the pump where you probably can quickly set up a strain gauge everytime you wish to take a studying. The gauge may be easily indifferent when not wanted.
A change within the pump working stress through the season or across seasons, when irrigating the identical block or shift, instantly tells you one thing has changed.
A sudden discount usually signifies a model new leak or a blockage on the suction side; a gradual reduction normally signifies put on of the impeller or sprinkler nozzles; and a rise usually suggests a blockage someplace in the system downstream of the stress gauge.
pressure gauge can be thought of as equal to a pipe of water of a certain peak in metres. This is known as ‘head’ (H). At sea level, the stress at the backside of a pipe of water 10m excessive is about 100 kilopascals (kPa).
If your stress gauge reads solely 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 supply water degree, plus losses within 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 degree occur within the suction line, so ensure everything right. Common issues embrace blocked inlet or foot-valve or strainer, pipe diameter too small, pipe damaged or crushed, suction height too great, or air trapped on the connection to the pump.
Turbine and axial flow pumps have to be submerged to function, in order that they often wouldn’t have any suction head.
For instance:
Pressure Head = 27.6m

Suction head = 4.0m

Total Head = 31.6m

Another useful figure that may now be calculated is the pumping price per ML per metre of head. This allows a significant comparability between completely different pump stations.
Pumping cost per ML per metre head: = value ($/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 value. That is, they lose a few of the energy going into them as warmth. This vitality loss changes with the size of the motor. The desk below is a guideline for motors operating 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 84 per cent for a submersible). Voltage losses through lengthy electrical cables can 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 lack of energy through the transmission. This loss is taken under consideration by what is termed the drive issue (Df).
Step 8: Calculate pump efficiency (Pe)

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

This example consists of the information 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

= fifty three.9 per cent

Step 9: Calculating potential financial savings

Most centrifugal pumps are designed to operate with at least 75 per cent efficiency, and most turbine pumps are designed to function with a minimal of eighty five per cent efficiency.
The pump in our instance is only about 54 per cent environment friendly, so how a lot could be saved by enhancing the effectivity from 54 per cent to 75 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 zero.72)

= $50.25 – 36.18

= $14.07

If 900ML are pumped throughout a season, the total cost saving is $14.07 × 900 = $12,663.
If impeller put on is the problem and the worth of substitute is $10,000, it might be paid for in lower than one season. After that, the financial savings are all elevated revenue.
Notice that a discount within the pump effectivity figure of 21 per cent (75 per cent to fifty four per cent) causes a rise in pumping value of 39 per cent ($36.18/ML to $50.25/ML).
Other factors that affect value and pump efficiency

There are two other variables affect price and pump effectivity: pump speed and impeller measurement.
Pump pace

You must know the pump pace to find a way to read the pump curves. The curves are often prepared for particular pump speeds and impeller sizes.
If the pump is immediately coupled to the electric motor, the pace is fastened 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 electric motors varies a little, it will be good to examine your motor pace with a rev counter.
If the motor just 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 can be calculated from the diameter of the pulleys on the motor and the pump (see the diagram below – make certain the pump is stopped earlier than measuring the pulleys).
A complication that can occur when understanding the fee and efficiency revolves around Variable Speed Drives (VSD), also called Variable Frequency Drives (VFD).
VSDs are becoming more and more well-liked as their worth reduces due to the benefits they provide. These models are added to electric motors and permit the velocity to be altered by changing the frequency of the alternating current. They permit electrically driven pumps to have their velocity set at precisely what is required for the pump obligation they usually remove the necessity for throttling the irrigation system using valves.
Savings of one quarter of the usual power consumption are sometimes reported by irrigators, and may be as much as half relying on the state of affairs. For determining the fee and effectivity of a pump, the measurements outlined on this article should be made several occasions with the pump set at completely different typical speeds.
Impeller measurement

Impeller wear has the identical impact as a discount in impeller dimension so you want to know the size of impeller fitted to your pump to work out which performance curve applies to your pump.
Sometimes the impeller dimension 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 who made the change.
Sometimes an impeller is deliberately lowered in diameter to regulate the pump’s efficiency and acquire a selected duty.
To give a range of duties, producers might provide impellers of different diameters for a similar pump casing. Available impeller sizes are shown on the pump curves.
Power factor

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

Keeping track of your pump’s performance and prices just isn’t troublesome. It could save you a lot of money and keep your irrigation system performing properly.
If you establish your pump is operating beneath the appropriate minimum level, verify the inner condition for wear or maintenance and the suitability of the pump for its current duty, or take steps to enhance the drive or replace it with a VSD.