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Pumping help

(23 posts) (13 voices)
  • Started 10 years ago by Warnsey
  • Latest reply from manofaus

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  • Water Pumps
  1. Warnsey

    Warnsey
    Member

    Hi Everyone,

    Well I have been in my new place for a few months now and I am loving off-grid living. There is one item that I am continually having trouble with and that is water pumping.

    I have a 90'000L in ground tank with a DAB .4kw pump with pressure tank (one of the smallest I could find) on top. The problem is that because the pump sits on top of the tank it has to suck water from near the bottom which is nearly 2m suction. This mean the pump has to work extra hard and draws 600W of power, which is fine this time of the year but in winter with less power it makes life harder.

    I did think of putting a small header tank up the hill. But this gets very expensive to pump to a new tank, solar pump (or petrol) to get from main tank to header and buy a tank and stand to gravity feed down.

    I was looking at 12V dc variable pump but I'm not sure if it will have the lift.

    Any ideas are welcomed.

    Cheers

    Posted Tuesday 3 Jan 2012 @ 11:21:55 am from IP #
  2. Bushwalker

    Bushwalker
    Member

    Actually, your pump may take water from 2m down, but the 'work' is only lifting the water the distance between the top of the water in the tank and the pump, plus the resistance in the pipe. Pressurising the pressure tank is additional work.

    Posted Tuesday 3 Jan 2012 @ 12:03:11 pm from IP #
  3. scott drew

    scott drew
    Member

    Bushwalker is right. Assuming your tank is full at the moment, it will be pulling water up from the level of the water. You may well find that the pump has to work much harder when the water level is lower at some point in the future.

    Can you relocate the pump? Perhaps dig it into a below ground pump enclosure next to the tank? This will reduce the lift.

    Posted Wednesday 4 Jan 2012 @ 10:10:41 am from IP #
  4. sun2steam

    sun2steam
    Member

    The work required stays always the same. The work is proportional to the difference in height between the initial water level and the final water level. The position of the pump does not affect the energy required to pump water, if the line has been primed. Putting the pump into a lower position would not help. However there are some other side effects that can be useful. A pump below water level does not need to be primed and does not need to be designed to be able to start up dry.

    It gets a bit more complicated as pressure losses in pipes also have to be taken into account. Here a smaller pump actually helps. if the flow rate is lower, so will be the pressure losses.

    Pumping into a pressure vessel is also consuming more energy, as more work needs to be done to compress the gas in the pressure vessel. The same is true with an open header tank 'on the hill'. As more lift is required, the corresponding energy to do the work is correspondingly larger.

    Posted Wednesday 4 Jan 2012 @ 12:22:07 pm from IP #
  5. rockabye

    rockabye
    Member

    I used a Flojet DC 24V pressure pump of 18lpm capacity on a 22000 litre tank for many years and combined it with a 20l pressure tank for our 4 b/r 1 bathroom house. It was used to replace a conventional AC pump. Although successful they are more expensive. I would use 24V over 12V if possible. Mine had a separate solar backed 40Ah battery bank to keep it running.

    Posted Wednesday 4 Jan 2012 @ 7:06:19 pm from IP #
  6. Russell Moore

    Russell Moore
    Member

    I would also use the largest diameter pipes suitable for the pump, larger diameter pipes reduce internal hydrodynamic resistance.

    Posted Thursday 5 Jan 2012 @ 1:07:21 am from IP #
  7. scott drew

    scott drew
    Member

    Sun2steam, I'll have to disagree with you here. 'The work required stays the same.'? I don't agree with that statement.

    If you get a pump and draught from a level equal to the level of the water it will pump to 'X' height.

    If you get that same pump and elevate it 3m from the water level and run it at the same revs and configuration as the above example it will not pump the water to the same level as the above example. That is because the pump must work harder to 'pull' the water up the 3m, it does not have the same amount of energy to pass onto the water and can not pump it as high as the first example.

    Similarly, if you get that same pump and elevate it to say 7m from the water level and run it at the same revs and configuration as the first example it will pump the water to a point significantly lower than the first two examples. This is because more energy from the pump is expended pulling water into the pump and it can not send it on as far.

    If you have your pump lower than the water level it will better than if the pump is above water level.

    Posted Thursday 5 Jan 2012 @ 8:44:51 am from IP #
  8. sun2steam

    sun2steam
    Member

    Scott, how do you explain this in terms of physics?

    I agree, that at 3 m above water level and even more at 7m above water level can be less performance of the pump. (The initial example was just 2m.) This has nothing to do with 'pushing' and 'pulling'.
    It relates to the fact that there is a maximum of 1 bar (at 3m 0.7 bar, at 7m just 0.3 bar driving force caused by atmospheric pressure available to fill the pump. This can cause flow restrictions if the suction line is too narrow. It can also cause cavitation and power loss by degassing of the liquid.

    At the outlet of the pump, depending on the design and power of the pump the driving force can be higher.

    However these effects are hardly visible with pumps with only a couple of m head height.

    Posted Thursday 5 Jan 2012 @ 9:20:01 am from IP #
  9. Diver

    Diver
    Member

    Suction lift is a different animal to the pump's rated head. This is why submersible pumps are commonly found in below ground water sources. You need to check the pump's Q-H curve to check the rated flow V's head.

    Warnsey correctly referred to 'lift' in his opening post.

    "I agree, that at 3 m above water level and even more at 7m above water level can be less performance of the pump".

    "Can" is a poor choice of word. "Will" is more appropiate.

    Posted Thursday 5 Jan 2012 @ 9:37:47 am from IP #
  10. onrbikes

    onrbikes
    Member

    You may find that a good quality 12V pump would easily pump to a header tank.

    We too have a 100,000l tank with a small 12V Sureflow pump. Mind you it sits near the bottom of the holding tank.
    It's hooked up to a small deepcycle battery powered by 120W panel without a regulator.

    This then pumps to a 4500L header tank via 1.5" polypipe that has a homemade gauge to tell the water level. The total height (head) is about 12M.
    We turn it on every 3-4 days for a few hours. This has been going on for over 10 years and has had no problem.
    If you had a similar setup should look at maybe a small non return valve to keep the water in the suction end.

    Posted Saturday 7 Jan 2012 @ 7:35:39 am from IP #
  11. scott drew

    scott drew
    Member

    Sun2steam, I can't explain it in terms of physics because I am not a physicist! I am a fire fighter and in practical terms this is what happens. You have explained it in terms of physics utilising the figures of .7 bar and .3 bar at the various heights. You have given two reasons why a lower output of the pump will occur, flow restrictions and cavitation. These two reasons will end up giving a pump lower output performance. I just used plain English to describe the barometric pressure acting on a body of water and forcing it up into a tube which contains a lower pressure as ‘pulling’ and the energised water coming out the other end as being ‘pushed’. So essentially we both agree, you utilise physics and I utilise experience. I just tried to explain my point to Warnsey in plain English rather than utilising physics.

    Either way, physics or practical plain English, a pump will perform better and give a better output if it can utilise the 1 bar of atmospheric pressure pushing down on the body of water to its advantage. IE if the pump is lower than the water level it is not required to work to get the water into the pump and it can energise the water to its maximum capacity. Conversely, if the pump is required to form a vacuum to allow atmospheric pressure to push the water up into the pump it stands to reason that the pump has to work to get the water into it then it can’t energise the water the same amount as before.

    Warnsey asked for suggestions. I suggested if it was possible to get the pump below the water level it would work better. Others have suggested header tank and submersible pumps. I was just providing a suggestion and trying to explain it in terms anyone can understand.

    Posted Wednesday 11 Jan 2012 @ 12:34:03 pm from IP #
  12. photonthief

    photonthief
    Member

    Hello Warnsey,

    Could you tell us the exact model of DAB pump, as well as what you use the pumped water for? Does this pump provide all your household needs, such as sinks, showers etc indoors, as well as outdoor taps for garden watering etc? Do you find the existing DAB pump easily provides for your pressure and flow requirements?

    Also, how did you measure the '600W' power consumption of the pump? That sounds about right, BTW. From memory, the small DAB pumps use a 0.44kW motor, or maybe yours is a 0.4kW as you state, either way the actual power consumption will be something on the order of 600W when the efficiency of the motor is acounted for.

    Unfortunately, the power consumption of your pump will not (for any practical purposes) be reduced if you re-locate the pump at at or below the bottom of the tank. Bushwalker, Scott & Sun2steam are correct. Apart from easier priming, relocating the pump will not make any useful difference, either to the discharge pressure (at a given height), or to the power consumed by the motor, and I would be happy to bet a considerable sum of money upon it. In either case, the mechanical work required to lift the water from the water level in the tank, to the height at which it is discharged, is the same. There is no such thing as 'obtaining useful work from atmospheric pressure'. If that was possible, we wouldn't need power stations or solar panels or windmills to obtain our energy, just use atmospheric pressure! Of course, that is impossible.

    My suggestion would be to think carefully about whether a smaller pump would meet your requirements, thus the questions in my first paragraph. If slightly less flow and/or delivery pressure would be satisfactory, I suggest you consider a smaller pump such as the Onga RivaFlow TF30, which can be had for ~$150.

    Specs for this pump are:

    250 Watts
    29m head @ zero flow
    23m head @ 10 l/min
    17m head @ 20 l/min
    8m head @ 30 l/min

    What do you think?

    Posted Thursday 12 Jan 2012 @ 3:38:05 am from IP #
  13. Diver

    Diver
    Member

    photonthief said:

    Apart from easier priming, relocating the pump will not make any useful difference, either to the discharge pressure (at a given height), or to the power consumed by the motor,

    I could not find the suction head performance curves for the TF30 but I have linked the charts for the Onga GPP40, a similar pump. If you look at the suction head litres per minute (lpm) and the rated head/lpm and by way of example, you will see that with a 3 metre suction head, lpm is 28 lpm but the rated head less 3 metres is 56 lpm.

    Relocating the pump to reduce the suction head's impost will make a considerable difference and allow the pump to operate to its optimum efficiency.

    http://www.onga.com.au/objectlibrary/817?filename=GPP40%20-%20Brochure.pdf

    Another poster also posted "Pumping into a pressure vessel is also consuming more energy, as more work needs to be done to compress the gas in the pressure vessel".

    This is irrelevant when compared to the energy savings and other benefits that pressure vessels offer which include;

    Fewer pump high energy demand start ups - and yes, pumps DO NOT operate with an unvariable voltage demand.
    The pump operates with peak efficiency provided that the pump is correctly matched to the pressure vessel.
    Extended pump life.

    Using a pressure vessel uses much less energy in the overall scheme of things and readers should not be mislead into believing otherwise.

    Posted Saturday 14 Jan 2012 @ 6:12:37 am from IP #
  14. photonthief

    photonthief
    Member

    Diver said:
    I could not find the suction head performance curves for the TF30 but I have linked the charts for the Onga GPP40, a similar pump. If you look at the suction head litres per minute (lpm) and the rated head/lpm and by way of example, you will see that with a 3 metre suction head, lpm is 28 lpm but the rated head less 3 metres is 56 lpm.

    Relocating the pump to reduce the suction head's impost will make a considerable difference and allow the pump to operate to its optimum efficiency.

    http://www.onga.com.au/objectlibrary/817?filename=GPP40%20-%20Brochure.pdf

    Firstly, many thanks for finding the data for the Onga GPP40 pump, which is exactly what is needed to anwer the question as to if and how much pump performance is reduced due to a suction head. Theoretical predictions are all very well, but measured data in the real world is what ultimately matters.

    However, what I find is that the Onga data confirms what I said previously.

    Firstly, let me reiterate what I said previously, but in more detail. For small suction heads (say <2m, where nasty effects such as cavitation should not come into play), conservation of energy tells us that the TOTAL pump head, defined as the difference in pressure between inlet and outlet ports, should not be a function of whether the pump inlet is above or below the surface of the water being pumped, but should only be a function of the flowrate. To put that another way, for the situation described by Warnsey, the pressure and flow available at any given discharge height should not depend on whether the pump is located just above the top of his 2m high tank, or at the base of the tank.

    Now to the Onga data, which appears to confirm what physics predicts. The Onga graph of head vs Flow is plotted for zero meters of suction head, so a good first step is to see if the tabulated point (0m suction, 35 lpm, 140kPa) really does lie on the graph. Onga are not helping us by mixing pressure units of meters head and kPa, but the conversion is 1kPa=0.102meters. Thus, 140kPa is 14.3kPa. Look on the graph, and observe that the point (35 lpm, 14.3kPa) does indeed lie on the graph. Excellent.

    Next, we need to think about what the tabulated suction data actually means. The tabulated data, at a variety of suction depths, is all taken at a fixed 'operating pressure' of 140kPa (14.3m). 'Operating pressure' as specified in the table presumably means the gauge pressure measured at the outlet port, which corresponds to the vertical head above the pump body. To this must be added the vertical suction head, being the vertical height from the surface of the water being pumped to the pump inlet on the body of the pump. What the table is showing us, is that as the total head is increased by way of increased suction head, the available flowrate decreases. That is exactly what we should expect. Furthermore, if the 'physics' prediction is correct, the flowrate should depend only on the TOTAL head, being the height from the surface of the water being pumped, to the height at which the water is discharged. For example, the flowrate should be the same for a suction head of 3m plus a +ve head of 14.3m (total head 17.3m), compared to a suction head of 0m plus a +ve head of 17.3m (total head 17.3m). The Onga data shows that this is indeed the case.

    Case1.
    Suction head = 3m
    Positive head = 14.3m (140kPa)
    Flow = 28 lpm (from the tabulated data, for a suction depth of 3m)

    Case2.
    Suction head = 0m
    Positive head = 17.3m
    Flow ~ 28 lpm (from the graph, check for yourself)

    The physics, and what I said previously, are just fine.

    I'm sure some of the confusion can be traced to the term 'suction head's impost'. There is no 'suction head impost', in Warnsy's situation, except of course for cavitation and friction loss in the inlet pipework, which are both negligible for a 2m suction height. Let me explain when there is a 'suction head impost' (from the suction height), and when there is not.

    Case1 - Impost from increased suction height.
    Consider the tabulated suction data for the Onga pump, where it is observed that the available flow decreases with increasing suction height. The physical situation here, is that the height from the pump body to the point of discharge is kept constant, and the suction height is increased by raising the level of the pump above the height of the water. The decreased flowrate at large suction depth is totally expected, because this results in a greater overall height from the surface of the water, to the point of discharge.

    Case2 - No significant impost from increased suction height.
    This is the case pertaining to the original posting by Warnsey, where the overall height from the surface of the water to the point of discharge remains constant, regardless of whether the pump is placed at the bottom of the tank, or at the top.

    Re the benefits of a pressure vessel on the pump outlet, I agree completely, and may talk more about it in another posting.

    Posted Monday 16 Jan 2012 @ 3:08:41 am from IP #
  15. scott drew

    scott drew
    Member

    Oh My Goodness Photonthief... nice essay.

    Posted Friday 20 Jan 2012 @ 6:20:49 am from IP #
  16. Eco

    Eco
    Member

    I have read about "true" constant pressure pumps that have electronic controllers that adjust the speed of the impeller to maintain a constant water pressure over a large flow range. (http://www.irrigationwarehouse.com.au/category312_1.htm - quite expensive pumps.) If these work the way they say then a large pressure tank would not do much good - though they shouldn't lead to more energy consumption. Some manufacturers seem to claim constant pressure but don't state what that pressure is and still quote different flow rates for different pressures so I doubt they are true constant pressure. (I think in these cases the manufacturers are using "constant pressure" to mean continuously providing pressure, not necessarily at the one pressure.) If the pump switches on at a certain pressure and turns off at a higher pressure then a pressure tank will help with efficiency at most flow rates.

    Agree that low suction heads do not effect efficiency.

    To save energy use the lowest pressure, lowest wattage pump to suit the usage. Don't need to pump the pressure up to 35 metres if 15 metres will do the job. Use a pressure tank (if not a true constant pressure pump).

    Posted Friday 20 Jan 2012 @ 11:00:20 am from IP #
  17. Bushwalker

    Bushwalker
    Member

    Watched one of these in action at a relative's house. Quite uncanny. The motor would spin at whatever revs were required to maintain the pressure. If you let a tap drip, you could hear the motor spinning very slowly, and it would accelerate as soon as you open the tap up.

    I guess, theoretically, there is an energy saving in running without a pressure tank, but the price of admission probably negates that for many years.

    Cool gadgets though...

    Posted Friday 20 Jan 2012 @ 11:31:18 am from IP #
  18. Greg

    Greg
    Member

    Forum thread on pressure tanks -> http://www.ata.org.au/forums/topic/2128

    Posted Friday 20 Jan 2012 @ 8:07:02 pm from IP #
  19. Farmer

    Farmer
    Member

    Hello Warnsey and contributors,

    This is my first time at contributing to a forum. I have been reading this one and considering the contributions for about a week. My experience is in irrigation design for agriculture and horticulture and my production plant nursery.

    My thoughts on where the discussion has headed are:

    - the suction lift does not matter as long as suction conditions are within the specified range for the pump. This has to do with the net positive suction head available and the net positive suction head required by the particular pump over the range of operating conditions.

    - the initial post Is essentially about providing a household water supply with the least amount of energy as energy supply may be a limiting factor. This is therefor so a question of how much water volume and pressure is required and the efficiency of converting electric power to potential pressure energy then the pressure to flow at the taps. The energy is now essentially velocity energy and less pressure energy.

    - an important consideration in this water supply system is the pump hydraulic efficiency and to a lesser extent the motor efficiency and transmission efficiency to the pump. The most important efficiency in this situation is the pump hydraulic efficiency. Small centrifugal pumps typically operate somewhere between 40% and 70% hydraulic efficiency. A positive displacement type pump will typically operate at about 90% hydraulic efficiency. The Shurflo and Flojet pumps mentioned are positive displacement type pumps using a diaphram and valves. Other types include helical rotor and piston pumps. An important characteristic of this type of pump is efficiency stays essentially the same variable speeds. This is why they are well suited to direct solar powered installations. So this type of pump can halve your total pumping energy requirement.
    Warnsey, your consideration of a variable 12v pump, which is most likely to be a diaphram pump is the most hydraulically efficient practical option. This is a good option.

    - the next issue is one of peak demand verses total demand. This applies to both water demand and therefor so to electric power demand.
    Consider the amount of power to get a useable amount of water from a tap. The power required in watts is fluid density x standard acceleration due to gravity x pressure in metres head of water x flow rate in cubic metres per second. As an example for one tap, 1000 x 9.81 x 8 x 0.001 = about 80 watts. Allow for say three outlets operating at once, gives 240 watts peak power in the water. Now apply motor,transmission and hydraulic efficiency to this and for three taps we now need 270 watts for a diaphram pump and 600 watts for a centrifugal pump.

    The use of a header tank can readily supply peak water demands while keeping the peak power demand much lower.

    - The total amount of energy needed to keep your water flowing depends on how long the taps or other outlets are on for. The pump power required is the total divided by the duration the pump will run to supply enough water for your household. Warnsey, your consideration of a header tank is also a good one as it acts as an efficient store of energy and could be designed to supply high flow rates to your house while keeping pump flow rates low. The use of a positive displacement pump into a pressure tank is also an option however the pump will need to be much larger to directly supply the peak demand after the stored water in the pressure tank is depleted. This larger pump would operate for much less time of course.

    I am finding this whole thing very difficult to explain without the aid of diagrams and I can't work out how to scroll back over what I have written above the size of the posting window, using an iPad.

    A few more points in summary
    - use a high efficiency pump which operates at its Best Efficiency Point most, if not all, of the time. For this job a diaphram pump or helical rotor pump is best.

    - keep hydraulic friction losses low in your whole system. Use the pipe friction loss tables do determine the pipe size required. These tables usually also show water velocity values for the various flows and pipe size. If you keep velocity lower than 0.5 metres per second you will be keeping energy losses low.

    - you do not need more than 10 metres of vertical head,( pressure). Our house operates well at 6 metres. If you need more for outdoor use, say a sprinkler or hose use your existing pump directly to the outlet. A tank with 3 to 7 days supply would be good.

    - you could use a tank level float switch and relay and timer to automate filling your header tank during good solar periods.

    - the header tank can be sized to provide continuous water supply during low or no electricity supply or pump breakdowns.

    - if bush fire is possible consider the fire resistance of the tank and pipelines

    Hope this helps a bit

    -

    Posted Saturday 21 Jan 2012 @ 12:00:21 pm from IP #
  20. Farmer

    Farmer
    Member

    Sorry,
    The above power calculation is incorrect for one outlet.

    The required flow rate to say wash your hands, wash up or shower with reasonable flow is in the order of 0.1 to 0.2 litres per second. The rate I used in the example is 1.0 l/s. The theoretical power required for one outlet should be, 1000 x 9.81 x 8 x 0.00001 = about 8 watts. Even at 0.2 l/s per outlet, with say 3 outlets( tap, shower, and washing machine) the flow rate could be in the order of 0.6 l/s so the power required is in the order of 50 watts.

    You can play with more or less pressure and more or less flow rate. Just for fun, look at the data from the example photonthief has used for a similar pump the Warnsey's. Power required to do the duty is 1000 x 9.81 x 17.3 x 0.000466 = say 80 watts. It is not accurate to compare this to the power required by the pump as there is no data on the power draw at this mid range duty.
    The maximum power draw is at maximum flow rate which is at the lowest pressure. The lowest power required for a centrifugal pump is at zero flow and so at it highest pressure. The motor for these pumps are selected to be non overloadable when pumping against zero head.
    All this really just shows how much power is lost with small centrifugal pump pressure systems through efficiency losses, hydraulic friction losses and excess pressure than required.

    I believe very significant energy savings can be gained from Warnsey's initial consideration of a positive displacement pump and header tank.
    A variable speed 12V setup is of no advantage with a header tank.

    My thoughts on variable speed "constant pressure" pumps:
    - they will still operate through a very wide range of hydraulic efficiency points each time they start and run
    - they start every time you want water, usually there is no pressure tank to supply a small quantity of water.
    - more expensive, more sensors, more control circuits = less reliable
    - energy savings claimed are not often realised

    Posted Sunday 22 Jan 2012 @ 6:07:27 am from IP #
  21. Warnsey

    Warnsey
    Member

    wow. what a thread. Thanks for all the info and feedback. To be honest i'm still not sure which way to go but all this info has given me some fantastic insights and areas to research.

    Posted Thursday 26 Jan 2012 @ 7:49:08 am from IP #
  22. Farmer

    Farmer
    Member

    Hi Warnsey
    If you need some specific help send me a private message
    Farmer

    Posted Thursday 26 Jan 2012 @ 9:19:40 am from IP #
  23. manofaus

    manofaus
    Member

    seeing as you are living off grid I would assume that you have a bank of batteries for storage? If that is the case, you could possibly use the excess capacity of your grid at certain times of the day to pump your water from your tank to the header tank, or even just turn it on in the morning so that your batteries will have a chance to reach peak capacity during the day.

    Posted Sunday 3 Jun 2012 @ 7:49:16 am from IP #

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