LebGeeks

duke-of-bytes

Member

water collection calculation

hello
i am trying to see if a rain gutter can fill a tank but i am extremely rusty about the equations and calculations
i have a rain gutter connected to 4 or 5 inch pvc tube running down to a really big tank ( more than 50 000 litres ) from the bottom (not the top) without any pump
relying on the pressure of the water in the gutter and pipe and according to my understand , rain will only fill the tank as high as the diameter of the tube or will the rain water be enough to fill it ?
is this correct? can you point me toward the equation needed to prove this ?

Johnaudi

Member

Re: water collection calculation

How big is the rain gutter (to prevent overflowing)? What surface are you collecting the rain from and throwing it into the gutter?

It will fill the tank for as high the rain gutters' pipe goes (if you assume the pressure loss is negligible), if your rain gutter is, say 15m high, it will fill the water tank for whatever volume 15m of water is equivalent to inside the tank.

4 inch is a good diameter used to prevent debris from clogging the pipes, it would be preferable to include a filter at the beginning of the gutter though. (also known as a grill)

On another side note, if you're interested in the math and getting detailed, I would suggest checking out Bernouilli's equations in fluid mechanics (in order to get a better insight on the speed, pressure loss, etc). It will get you more accurate results, but in this case, you cannot get the energy lost by friction and whether your flow is turbulent or not without proper testing beforehand.

All of this is assuming your water tank is open from the top (not allowing pressure build-up from the inside)

Good luck :)

Last edited by Johnaudi (June 28 2020)

duke-of-bytes

Member

Re: water collection calculation

How big is the rain gutter (to prevent overflowing)? What surface are you collecting the rain from and throwing it into the gutter?

It will fill the tank for as high the rain gutters' pipe goes (if you assume the pressure loss is negligible), if your rain gutter is, say 15m high, it will fill the water tank for whatever volume 15m of water is equivalent to inside the tank.

4 inch is a good diameter used to prevent debris from clogging the pipes, it would be preferable to include a filter at the beginning of the gutter though. (also known as a grill)

On another side note, if you're interested in the math and getting detailed, I would suggest checking out Bernouilli's equations in fluid mechanics (in order to get a better insight on the speed, pressure loss, etc). It will get you more accurate results, but in this case, you cannot get the energy lost by friction and whether your flow is turbulent or not without proper testing beforehand.

All of this is assuming your water tank is open from the top (not allowing pressure build-up from the inside)

Good luck :)

Consider their is an air valve on the top
The gutter is as high as the top of the tank ..but the tank is huge ! Will it fill ? Wouldnt need any pump ? The tube is on the bottom side

Johnaudi

Member

Re: water collection calculation

Although I'm still a student in this field, I would say that (by theory) it should be able to fill it (given enough security factors taken into account, preventing sudden reverse current flow, etc).
I would rather wait for someone of expertise to give us his advice though, I'm just stating what I've been taught at uni haha.

Check out the following: https://www.pseau.org/outils/ouvrages/a … ing_en.pdf

Last edited by Johnaudi (June 28 2020)

VincentKeyboard

Member

Re: water collection calculation

Supposedly according to what I remember from university physics classes, water dripping into the rain getter and down the pipe will fill the tank up to the same altitude as the getter. After that, any additional rain will just flow out from the getter instead of going into the pipe.

Edit: I think what I just posted does not take into account that the tank is closed from the top and other factors such as air pressure.

Last edited by VincentKeyboard (June 28 2020)

DNA

Member

Re: water collection calculation

It is simple suppose you get 800mm rainfall in lebanon a year, to fill a 50,000L tank you need
Gutter area= 50m3 / 0.8m = 62.5m2.
that is around 8m x 8m gutter to fill your tank every year.

eWizzard

Member

Re: water collection calculation

Consider their is an air valve on the top

If it's a manual valve that you have to open and close, then make sure you open it when the rain starts so that the tank can fill with water. Otherwise, a closed valve means that the pressure within the tank will start rising as soon as the bottom pipe is filled with water. When the pressure within the tank is greater than atmospheric pressure, the tank will not fill.

If it's a pressure relief valve, most likely it will have a rating tens (if not hundreds) of times greater than atmospheric pressure, which means that it's effectively a permanently closed valve in this scenario, and the tank won't fill beyond a certain point.

Btw, if you haven't done so already, I strongly recommend reading guidance on use of rainwater tanks as there are many hazards you need to be aware of and know how to address.

Last edited by eWizzard (June 29 2020)

duke-of-bytes

Member

Re: water collection calculation

Consider their is an air valve on the top

If it's a manual valve that you have to open and close, then make sure you open it when the rain starts so that the tank can fill with water. Otherwise, a closed valve means that the pressure within the tank will start rising as soon as the bottom pipe is filled with water. When the pressure within the tank is greater than atmospheric pressure, the tank will not fill.

If it's a pressure relief valve, most likely it will have a rating tens (if not hundreds) of times greater than atmospheric pressure, which means that it's effectively a permanently closed valve in this scenario, and the tank won't fill beyond a certain point.

Btw, if you haven't done so already, I strongly recommend reading guidance on use of rainwater tanks as there are many hazards you need to be aware of and know how to address.

my only issue was the physics with filling the tank or not , if filled it will be used for agriculture and mainly housework (cleaning) thank you for the pdf file , very important details

DNA

Member

Re: water collection calculation

what physics are we talking about there is no physics in such thing its as simple as it gets. build a rain collector hook it to the tank and you are done, i wonder why would you attach it below not above the tank which will be much simpler

duke-of-bytes

Member

Re: water collection calculation

what physics are we talking about there is no physics in such thing its as simple as it gets. build a rain collector hook it to the tank and you are done, i wonder why would you attach it below not above the tank which will be much simpler

the main question : would the water pressure in the rain collector and tube be enough to fill the tank ( we are talking petrol storage tank size )
the answer was as long as there is no counter air pressure ( tank not fully closed ) the water will rise to the height of the tube

eWizzard

Member

Re: water collection calculation

my only issue was the physics with filling the tank or not , if filled it will be used for agriculture and mainly housework (cleaning) thank you for the pdf file , very important details

Either way, you have to safely disinfect the water, because otherwise it's going to have significant bacterial growth and mold, which could make you ill and bring diseases to your plants. My guess is you'll need a water purification station with particle filter, charcoal filter, and maybe even a UV lamp.

Johnaudi

Member

Re: water collection calculation

How far away is the gutter from the tank? How high is each?

DNA

Member

Re: water collection calculation

what physics are we talking about there is no physics in such thing its as simple as it gets. build a rain collector hook it to the tank and you are done, i wonder why would you attach it below not above the tank which will be much simpler

the main question : would the water pressure in the rain collector and tube be enough to fill the tank ( we are talking petrol storage tank size )
the answer was as long as there is no counter air pressure ( tank not fully closed ) the water will rise to the height of the tube

Offcourse it will i never though as this would be a question, the main issue with filling bottom up is the pipe should withstand the pressure lowest point being highest pressure but seeing that the pipe is already there then it is built to specs so just hook your collector and dont think about it more