Water storage and irrigation are an important aspect of Farming. Storing Water when it is available and using it is needed with least energy is a must for dry lands. Assuming wind pump will be installed by next month and the summer is fast approaching, I need to plan for water storage. The pumped water and rain water need to be stored so that it can be used for all days with less energy. Since water tank would consume resources, there cannot be mistake in design and should be built with least cost.
The below doubts were always in my mind. Some of it had no answers in internet. Consulting an engineer will only add to more confusion. So I ventured to find that out myself through home experiments. I used a plastic container as water tank, punctured holes, inserted think plastic tubes and started my experiment. Used my cell phone camera and pictured the below using black chart paper for background on my balcony. The below questions were to be answered.
1. How to create water pressure using gravity ?
2. Will the elevation of water tank above the ground level impact water pressure ?
3. Can the water pressure from storage tank be used for sprinkler ?
4. Will there be reduction in pressure due to coiling of tubes
Conclusions:
1. Water pressure is the highest at the bottom of any water tank. This is due to weight of water pressing from above. This pressure can be used to our advantage. Once water is uplifted from ground level to an elevated storage tank, this pressure created by gravitational force or weight of water can be used to our advantage. This also means that a taller and leaner water tank and a shallow wider tank may produce the same pressure at the bottom, given the volume of water being same. This is governed by formula P = gh where P is the water pressure, g is the acceleration due to gravity and h is the depth of the water at that point. Here pressure P stands for pressure at any unit of water inside the tank and is same in all directions.
2. Water pressure increases slowly with elevation of water tank. This experiment was very tricky. I failed in first attempt to understand the concept. I am still not convinced, but searched internet and repeated experiment with longer pipe. then I have to believe it works. The problem is everyone explains pressure increases as you move the water tank up further. But no one explains why? Suppose if I move the water tank to sky high. Will I get infinite pressure at the bottom?
From Newton's Second law of Motion:
Force = mass x accelaration
Force = volume x density x gravity
Force = height x area x density x gravity
Now force on per unit of area is pressue. Therefore P=F/a
From the above equation P = height x density x gravity
Here density of water and gravitational force remains the same. The only variable is height. Therefore as height increases so does pressure. Theoretically one must get additional 0.43 psi (pounds per square inch) of pressure for every foot moved up.
When I used a long and short tube under the tank to find the pressure, the pressure was almost same on both tubes. However when I repeated the experiment with further long tubes I could observe there was difference in fountain height of water - pressure.
So water tanks need to be built at an elevation to stay above the highest level on the farm at the least. At the most it can be at any height. There will be gain on pressure if you construct a water tank at 10 meter or 20 meter elevation from the ground. The cost however would keep going up. There will also be challenges of constructing a large tank at that height and of pumping water to that height.
I am going to use windpump to pull the water up-to 10 to 15 feet above the ground level. And the tank has to be a large reservoir to store water for windless days. Hence water tank if I construct should be at an elevation of 10 feet above the highest point to be irrigated on the farm and large enough. Increasing the height would be advantageous, but need to decide on construction material to be used etc... This point is still open.
3. The pressure created by water tank is not enough to operate Sprinkler or a fountain. Smaller sprinkler can be operated, but not the ones that require high pressure. Therefore using gravity we can use drip irrigation effectively. Small portable sprinklers can also be used for vegetable beds if enough pressure comes from a larger tank. I saw the one shown here in my neighboring garden. Very beautiful idea and gives confidence how we can fabricate one ourselves without asking 'companies' to do 'survey and recommend' a suitable 'solution' to our 'problems'. Earlier I used to think that I will keep a water tank at 30 feet and generate enough pressure to run a sprinkler. The learning is showing otherwise.
4. Speed of water is reducing when it passes through coiled pipes due to friction. It is therefore important to create straight lines to enable smooth flow of water for long distance. Otherwise for drip irrigation, nothing to panic. The pressure from water tank is sufficient to push the water to the corner of any pipe as the decrease of pressure due to viscosity matter only when water is 'flowing'. A moving water may experience pressure dip. But a water which fills a tube slowly will get compressed further by incoming water to be expelled through drip outlets.
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| My experimental setup on Cloth Hanger |
The below doubts were always in my mind. Some of it had no answers in internet. Consulting an engineer will only add to more confusion. So I ventured to find that out myself through home experiments. I used a plastic container as water tank, punctured holes, inserted think plastic tubes and started my experiment. Used my cell phone camera and pictured the below using black chart paper for background on my balcony. The below questions were to be answered.
1. How to create water pressure using gravity ?
2. Will the elevation of water tank above the ground level impact water pressure ?
3. Can the water pressure from storage tank be used for sprinkler ?
4. Will there be reduction in pressure due to coiling of tubes
 |
| Very tricky experiment |
Conclusions:
1. Water pressure is the highest at the bottom of any water tank. This is due to weight of water pressing from above. This pressure can be used to our advantage. Once water is uplifted from ground level to an elevated storage tank, this pressure created by gravitational force or weight of water can be used to our advantage. This also means that a taller and leaner water tank and a shallow wider tank may produce the same pressure at the bottom, given the volume of water being same. This is governed by formula P = gh where P is the water pressure, g is the acceleration due to gravity and h is the depth of the water at that point. Here pressure P stands for pressure at any unit of water inside the tank and is same in all directions.
2. Water pressure increases slowly with elevation of water tank. This experiment was very tricky. I failed in first attempt to understand the concept. I am still not convinced, but searched internet and repeated experiment with longer pipe. then I have to believe it works. The problem is everyone explains pressure increases as you move the water tank up further. But no one explains why? Suppose if I move the water tank to sky high. Will I get infinite pressure at the bottom?
From Newton's Second law of Motion:
Force = mass x accelaration
Force = volume x density x gravity
Force = height x area x density x gravity
Now force on per unit of area is pressue. Therefore P=F/a
From the above equation P = height x density x gravity
Here density of water and gravitational force remains the same. The only variable is height. Therefore as height increases so does pressure. Theoretically one must get additional 0.43 psi (pounds per square inch) of pressure for every foot moved up.
When I used a long and short tube under the tank to find the pressure, the pressure was almost same on both tubes. However when I repeated the experiment with further long tubes I could observe there was difference in fountain height of water - pressure.
So water tanks need to be built at an elevation to stay above the highest level on the farm at the least. At the most it can be at any height. There will be gain on pressure if you construct a water tank at 10 meter or 20 meter elevation from the ground. The cost however would keep going up. There will also be challenges of constructing a large tank at that height and of pumping water to that height.
I am going to use windpump to pull the water up-to 10 to 15 feet above the ground level. And the tank has to be a large reservoir to store water for windless days. Hence water tank if I construct should be at an elevation of 10 feet above the highest point to be irrigated on the farm and large enough. Increasing the height would be advantageous, but need to decide on construction material to be used etc... This point is still open.
 |
| An indigenous portable sprinkler |
4. Speed of water is reducing when it passes through coiled pipes due to friction. It is therefore important to create straight lines to enable smooth flow of water for long distance. Otherwise for drip irrigation, nothing to panic. The pressure from water tank is sufficient to push the water to the corner of any pipe as the decrease of pressure due to viscosity matter only when water is 'flowing'. A moving water may experience pressure dip. But a water which fills a tube slowly will get compressed further by incoming water to be expelled through drip outlets.
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