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Water Storage Tank

06/19/2009 12:12 AM

Will a water storage tank built in the shape of an arrow head pointing down (with the outlet spout at the point) have a greater gravity flow of water than, say, a cylindrical tank of the same volume?

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#1

Re: Water storage tank.

06/19/2009 1:47 AM

It all depends on the relative height from the top water level to the outlet. (Neglecting features like "smoothness of interior" and such esoteric items.)

I would presume that a tank as you described would be many times higher than a standard "squat" cylindrical tank and so the initial flow rate would be greater.

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#2

Re: Water storage tank.

06/19/2009 3:31 AM

Basically no, it's the head of water (height of the top surface of the water) and the diameter of the outlet which will effect it.
Del

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#3
In reply to #2

Re: Water storage tank.

06/19/2009 3:47 AM

Thank you. The shape would increase the height of the volume I am looking at.

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#4
In reply to #3

Re: Water storage tank.

06/19/2009 4:03 AM

So would adding longer legs to a regular shaped & therefore cheaper tank...

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#11
In reply to #3

Re: Water storage tank.

06/20/2009 2:11 AM

volume of tank does not affect the issue.

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#10
In reply to #2

Re: Water storage tank.

06/20/2009 2:09 AM

That is all that matters - head and diameter of the outlet.

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#5

Re: Water storage tank.

06/19/2009 4:07 AM
  • Pressure does not depend upon tank shape. Pressure depends upon level.
  • Pressure and flowrate are interlinked by the characteristics of the distribution system and the number and flowrate of users drawing from it.
  • For a given flowrate, pressure will reduce quicker in a conical base tank compared to a cylindrical tank, as the level will fall quicker on account of the conical shape. So the flowrate will fall back quicker.
  • For some systems, a rapid drop in system pressure so as to reduce the flowrate in the distribution system may be applicable, in which case a conical-base tank is more suitable. Circumstances in which this may be important might include when there is only a limited flowrate with which to refill the tank.
  • A conical base tank is more difficult to construct than a cylindrical tank.
  • A cylindrical tank will contain more water than a conical base tank of the same height, and may require stronger foundations and supporting structure for the same system pressure.
  • A conical base tank will be taller than a cylindrical tank that contains the same volume. The overall tower height could remain the same if the bottom outlet connection were to be closer to the ground.
  • A conical base tank will be less resilient to temperature swings than a cylindrical tank on account of the greater surface area per unit volume stored.
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#6

Re: Water storage tank.

06/19/2009 8:28 AM

No. It this were the case, you could invent a perpetual motion machine. It sometimes seems like this is the case because very pointed discharge volumes can trade flow for pressure.

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#7

Re: Water Storage Tank

06/19/2009 10:49 PM

Terrace the tank. Wire and shotcrete with xtra base. Head and gravity is freedom. CROM tanks I saw in Florida were interesting and there are some new tiltups that I want to see more on.

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#8

Re: Water Storage Tank

06/19/2009 11:38 PM

The answer to this is more complex than previously indicated. The important thing so far is that flow rate depends on the height of the water level above the outlet.

Assume the conical and cylindrical tanks are the same height h and volume V. (The diameter of the cone will be sqrt 3 times the diameter of the cylinder.) When the cylindrical tank is half full, the water level is 0.5 h. But when the conical tank is half full, the water level will be (h sqrt 2)/2 ~ 0.707 h. Greater height = greater pressure = greater flow. Similarly, under all but completely full and completely empty conditions, the water level in the conical tank will be higher than in the cylinder.

An elevated cylindrical tank (d = h for minimal material) would make more sense. Even when near empty, you would still have a height of water level to create good flow.

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#9

Re: Water Storage Tank

06/20/2009 1:54 AM

No.

The gravity flow will be the same according to water level but they will drain at different rates.

For a cylindical tank and a conical (funnel shaped) tank filled to the same level and having the same drain characteristics the initial flow rate will be the same.
When each is half full the the pressure will be the same.
When each is one eigth full the the pressure will be the same.

The conical tank pressure will reduce at a quicker rate due to the progressively lower volume as it drains.

For the cylindrical tank the discharge characteristic is like that of a fixed capacitor.

For the conical tank the discharge characteristic is like that of a capacitor that reduces its plate dimensions in step with its discharge pressure.

Jon

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#12

Re: Water Storage Tank

06/20/2009 2:28 AM

Hello xperimentor,

Sounds interesting, but the answer is NO.

There should be no advantage as long as the water height is as it would be in an ordinary tank.

If the water lever is much higher, then you may get more pressure.

bb

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#13

Re: Water Storage Tank

06/20/2009 2:37 AM

You say," When each is half full the the pressure will be the same.
When each is one eigth full the the pressure will be the same."

Completely wrong. Just do the arithmetic. The half-full cone will have a higher water level than the half-full cylinder, and so will the eighth-full cone have a higher water level. Therefore the conical version will drain faster.

While you're at it, look around and see how many municipal water towers consist of an elevated tank (various shapes in which the width and height are about equal). This is the correct design concept.

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#14
In reply to #13

Re: Water Storage Tank

06/20/2009 4:00 AM

The answer is still no.

"The half-full cone will have a higher water level than the half-full cylinder, and so will the eighth-full cone have a higher water level."

Yep, wrong as can be. thanks for pointing that out. I was thinking of full height rather than full volume.

Assuming the tanks are the same height: In my example the water levels are at the same height without regard to volume at half height and one eighth height. Therefore the pressure is the same in both cases.

The cylindrical tank's contents are drained at an exponential rate.

The rate of drainage for the conical tank is quite different as the greater volume is disharged at a higher pressure and the lesser volume at a lower pressure. That produces a more linear rate of drainage of the contents.

This has nothing to do with the question but none of our water towers are raised off the ground and they are considerably taller than their width.

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#15
In reply to #14

Re: Water Storage Tank

06/20/2009 4:05 AM

With a liquid such as water with no viscosity then the shape of the tank is irrelevant . Once again only the size of the outlet and the height (depth or head) matters for any given viscosity.

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#16

Re: Water Storage Tank

06/20/2009 4:21 AM

Isn't most of this shape discussion complete bollocks?
Won't any tank have some sort of level switch auto fill system such that the water level will always be near the top?
The extra volume is presumably just as a back up for when the system fails/maintenance/ unusually high demand for short periods etc.

Build it the simplest most cost effective shape as high as is practicable.

<ctrl\G> Grumpy mode off
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#20
In reply to #16

Re: Water Storage Tank

06/20/2009 9:58 AM

Hello Del,

Hope you start feeling your 'old' funny self soon!,

I wish I had the balls to say what you did! This specific question, with all the 'facts' about pressure etc already known, should never have needed to have been asked.

Of course it depends..............if you have a water tower 500 metres high, you would get more pressure from it. Most water thingies are about the same height so that rarely if ever applies.

bb

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#28
In reply to #20

Re: Water Storage Tank

06/21/2009 7:17 AM

Hi Babybear. The facts about pressure may be known - but not neccessarily to all? Consider this, if I had known the answer...I would not have asked the question.

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#27
In reply to #16

Re: Water Storage Tank

06/21/2009 7:09 AM

Hi Del. I should explain that I am not an engineer and have joined CR4 after googling for information about my layman's hypothesis on shape affecting pressure. My purpose in joining was to utilise the knowledge of those who have it - such as yourself - life is too short to study the science of all ventures we undertake. The various posts have rid my ignorance on the matter and assisted me in now building the tank! By the way, I should have given more info - such as that I intend capturing rainwater from the roof to fill the tank and therefore have height restrictions and that there is NO source of power in the rural area I am building the cottage. Thanks for your contribution.

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#31
In reply to #27

Re: Water Storage Tank

06/21/2009 12:45 PM

Sorry, I'm not always that grumpy...
Key for any DIY adventures...keep it simple, keep it cheap (use anything which you can get for free), keep it accessible!
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#17

Re: Water Storage Tank

06/20/2009 7:53 AM

Fastest water tanks were built like this

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#44
In reply to #17

Re: Water Storage Tank

06/25/2009 8:57 AM

If this is a toilet it must have a hell of a flush!

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#18

Re: Water Storage Tank

06/20/2009 8:05 AM

I agree, use ground storage shape of your pleasing and add pumps. No elevated tanks in Florida anymore - pumps are the answer to move liquids anyway.

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#19
In reply to #18

Re: Water Storage Tank

06/20/2009 8:35 AM

What about when you have a large power failure, like in 2003 when a large portion of the East/Central US was without power. Here in Cleveland after a day or two without power there were large areas that could not maintain sufficient pressure to guarantee safe drinking water. For that matter what about Fire Protection. If you have a power failure and need water pressure for fire protection what will you do if the pumps have no power for several days? I think elevated tanks still serve a useful purpose.

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#21
In reply to #19

Re: Water Storage Tank

06/20/2009 10:23 AM

Hello d m rosenberg,

How are you?

If you are talking of 'drinking water' only, the water companies should provide enough by way of 'bottled water'.

If you refer to water pressure in general after a power outage, and it is known no one is going to be 'nanny' and supply water, then you go out and get yourself a good generator and have the domestic water at least, piped to take advantage of the power from the generator.

Perhaps people should have to let the power/water companies know they have a generator, and or, maybe it should be sorted by household earnings, where the less well off are supplied with water and or generators to pump saved underground water... ..................... maybe?

I think small water towers can be 'hidden' amongst trees etc to carry just enough water for one house, so personally, I think that, if they had to, 'private' water towers are the way to go, which would not need power,............ if, and only IF, the power cannot be guaranteed to say, a cut of no longer than a day?

Of course, people with swimming pools could get that water and purify it? If it is used for domestic washing and personal use only, other than drinking.

Just a few of my thoughts. bb

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#46
In reply to #21

Re: Water Storage Tank

06/25/2009 9:04 AM

Daft idea - bottled water is a rip off and an environmental menace. Also plastic add to fossil fuel use.

If the city water supply is NBG then boil it first. Gets rid of the Chlorine.

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#45
In reply to #18

Re: Water Storage Tank

06/25/2009 9:00 AM

Depends what you want the stored water for. If it is part of a sprinkler system then it beats a pump. Pressure tanks are a possibility but they cost big bucks.

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#22

Re: Water Storage Tank

06/20/2009 3:04 PM

Search "hydrostatic paradox"

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#23

Re: Water Storage Tank

06/20/2009 6:16 PM

no unless your head pressure changes or, in other words the elevation of the water.

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#24

Re: Water Storage Tank

06/20/2009 8:40 PM

Thanks to everyone for their replies. I should explain my reason for the question. I am building a cottage in a remote and rural part of the country where there is no power source and running a generator would be expensive and impractical. I intend supplying my water needs with roof captured rainwater (a flat slightly tilted concrete roof) and am therefore restricted with the height of the tank to roof level. I intend building the entire backwall of the cottage from roof to ground as one side of the tank structural housing which would be rectangular shaped reinforced concrete. Inside this housing I intend building a cone shaped tank with a 1 inch outlet to supply the cottage. I thought a cone shape would have a higher pressure than a cylindrical shape even though I would have less volume due to the cone shape. I realise that as the tank drains the pressure will lessen faster than a cylindrical shape but hopefully the rains keep the tank almost full at all times. To enjoy greater pressure (since my height is restricted) I therefore reasoned a cone shape would be better. Remember, I cannot make the tank higher than the roof since it's refill source is the roof. Am I correct in assuming a better pressure level with a cone shape in these circumstances?

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#25
In reply to #24

Re: Water Storage Tank

06/20/2009 10:54 PM

With the majority of the volume nearer the roof top you will satisfy your pressure needs. With consistant rains you will maintain the volume. You will have greater volume and a better sediment collection characteristic if the tank is more like a box that is easier to support.

I recently visited Waiheke Island in North New Zealand (population 8000) that depends on rain to meet all of their water needs as there is almost no water table to draw from. Emergency water from the mainland is quite expensive. Waiheke could teach us a lot about resilient homes - "each house on the island must maintain its own water supply, collecting rainwater in tanks and cisterns, and must install a septic tank and septic field to handle sewage."

I grew up on an Island and we collected rain for uses other than drinking. I carried water for drinking from a spring located down a steep path in the forest.

We cut wood for cooking and heating and used an oil heater in winter.

No Lions and Tigers and Bears. Oh My!

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#26
In reply to #25

Re: Water Storage Tank

06/21/2009 6:55 AM

Thanks for that Kudukdweller. Talking about sediment I also reasoned that I would have a sediment discharge outlet at the lowest point of the tank and my outlet to feed the cottage would be perhaps a foot higher. Sediment should sink to the lowest point and I could disharge the tank for a few seconds every now and again to minimise the collected sediment. I have seen the bottom of an emptied cylindrical tank and it was filled with about a foot of accumulated sediment. Do you think the conical shape would work for clearing sediment every now and again as I propose to do?

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#32
In reply to #26

Re: Water Storage Tank

06/21/2009 3:02 PM

Yes, the shape would not allow settling and send it below the outlet. Good plan, less maintenance. You would also be able to see what is getting into the water.

I was thinking of more volume.

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#36
In reply to #24

Re: Water Storage Tank

06/23/2009 2:07 AM

Hi xperimentor,

Let me start by saying that I have actually built my own bungalow in the country with a rain storage water tank. My tank however did not have to be as high as my roof because I used an electric pump to pump the water to a temporary storage tank in my attic. I have drawn a quick diagram below which I think is a reasonable representation of your plans. Let me give you my tuppence worth:-

1. As others have said, you will not get extra pressure or flowrate with the conical tank. I would therefore advise you build a rectangular tank from either mass concrete or concrete blocks layed on the flat - cheaper and easier to construct. One other possible option might be a precast cylindrical tank if you can find one.

2. Remember that the water may just trickle out of the taps even when the tank is full ( My attic storage tank was at least 3 metres above the tap outlets )

3. Plaster the inside with a waterproof concrete plaster mix.

4. Consider installing a reserve emergency tap near the bottom but above any sediment line - if the water level drops under your lowest inside tap level you will need this.

5. Cover your tank with corrugated iron ( Galvanised ) to stop leaves and debris from entering your tank.

6. Put some form of simple filter ( A box with stones, gravel and sand from bottom to top is quite effective) between the eaveshoot and the tank to prevent leaves and debris from running into the tank - this will substantially reduce sediment too.

7. Also put some form of wire mesh into the eaveshoot which provides additional filtering and prevent the eaveshoots from blockage.

8. Make sure your tank has a run-off or overflow facility in case of too much rain.

9. Put a gap between the cottage wall and tank wall to allow you to walk between ( or maybe enough to drive a car between? ). Do not share the wall of your cottage with the tank - major dampness problems!!! ( My tank actually sprung a leak despite the waterproof plastering - not a major problem when they are seperate structures)

10. Finally do some form of elementary calculation to ensure that the tank is large enough to hold the amount of water you will need to use during periods of no rain without going below the inside tap level.

Good Luck with the project

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#37
In reply to #36

Re: Water Storage Tank

06/23/2009 9:23 AM

Dear M.P.M.,

Nicely stated- although it took me a moment to figure out "eaveshoot" or gutter as is common here.

I would add a couple of points. A manway, although a somewhat complicate addition to a simple concrete cistern at an elevation to enable periodic access would be useful for maintenance and cleaning.

I would lean to install a moderately shallow conical bottom with a nozzle at the lower vertex connected to a relatively substantial flush-out valve, say 3" or so.

I would also suggest a means to periodically introduce a little household bleach to prevent the water from going septic.

In any case, I hope that you have an alternative supply for drinking and cooking.

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#39
In reply to #37

Re: Water Storage Tank

06/23/2009 1:45 PM

Thanks Flynnstar - I will be utililizing your conical bottom idea in a rectangular tank as suggested by MPM...and yes, I will have filtered well water for drinking and cooking.

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#38
In reply to #36

Re: Water Storage Tank

06/23/2009 1:41 PM

Hi MPM,

Wow! How can I thank you? Your advice and sketch are just what I needed. I am going to go for the rectangular tank with a slight conical bottom for flushing. Once again, thank you.

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#40
In reply to #38

Re: Water Storage Tank

06/23/2009 11:27 PM

Hi xperimentor,

Do you plan on having a hot water cylinder (Perhaps heated from a solid fuel wrap around boiler) and a shower in the cottage?

If so, your minimum height of water in your storage tank will be at an even higher level than I have already indicated.

Is there no possibility of setting up a simple battery operated water pump which can pump the water to a small storage tank on the roof using a float switch? ( I am guessing you will never have a frost problem as you are building this somewhere near Durban in SA ). A standard 12V car battery could be trickle charged by a solar cell or a small wind powered turbine perhaps. Advantage of doing it this way is that your tank can now be much lower in height than your roof ( Less of an eyesore and possibly cheaper construction ) and you will pretty much be guaranteed a reasonable head of water and no dribbling taps or showers during dry spells.

Just another thought for you.

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#41
In reply to #40

Re: Water Storage Tank

06/24/2009 12:00 AM

This 12V pump is worth a look:-

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200330141_200330141

Click on the reviews tab also to see how it is being used on an RV.

Was just thinking you could even charge the 12V battery from your vehicle with jump leads whenever necesssary.

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#29

Re: Water Storage Tank

06/21/2009 8:56 AM

as per the fluid dynamics principles, there is no difference in pressure exerted on the bottom of tank with the same height of water,so if the height of water is same in tanks of different bases the pressure of water is the same on base.

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#30

Re: Water Storage Tank

06/21/2009 9:10 AM

For a nice Sunday challenge, then. Suppose a deranged hunter happens by a cylindrical water tank of height h, and, being bored, whips out his trusty Weatherby 500 and puts a hole in the side at h/8, h/4, 3h/8, h/2, 5h/8, 3h/4, and 7h/8. Which hole will "spout" water the farthest horizontally?

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#33
In reply to #30

Re: Water Storage Tank

06/21/2009 3:08 PM

The bottom one if the tank has liquid in it.

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#34

Re: Water Storage Tank

06/22/2009 10:20 AM

What do you mean by arrow head? As you know all which is important is the height, meanwhile building tanks in odd shape would become so funny!!!!!!!

The only pressure exerting on water is atmospheric not more???????

Funny things

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#35
In reply to #34

Re: Water Storage Tank

06/22/2009 10:34 AM

Arrowhead - triangular shape with point downward to fit into a space between walls.

You forgot about gravity?

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Anonymous Poster
#42

Re: Water Storage Tank

06/24/2009 4:54 PM

No. Shape does not affect the pressure. Only the height and temperature will affect the pressure at the bottom.

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Anonymous Poster
#43

Re: Water Storage Tank

06/25/2009 6:58 AM

The answer depends on the remainder of the system. But under normal domestic conditions the generally given answers would be correct - the initial flow depends on the height of the top surface of the water, and the medium-term maintenance of flow will usually depend on the area of the top surface.

Exceptions:
A conical (round) tank can be worse than a square one for maintaining flow during refill if incoming water creates a vortex in the tank.
The height of the outflow can be significant if the flow levels are very high; the reason is not the shape of the tank but the length of pipe. Usually the total length of piping is sufficient that this is negligible; but you could see an effect if you have too many high-flow systems (e.g. power-showers?) that are tapped-off from the outflow. If this is significant it is likely to cause other problems (such as falling surface and inadequate capacity), so you would be likely to need an oversized tank (or a slave tank). As regards the problem of stealing from other outlets, multiple pipes direct from the tank or a larger diameter section of pipe are better solutions than odd-shaped tanks.

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Anonymous Poster
#47

Re: Water Storage Tank

06/21/2010 8:02 PM

BUILD THE SQUARE/ RECTANGULAR TANK!!!! I'm a water engineer. Zillion reasons why. Thank you and, Your welcome ladies and gentlemen.

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