Water pressure?

Question

A rancher friend of mine asked me this question today and I thought it was too complex for me to answer as I am a carpenter and this does not come up in my line of work:
If a water supply containing 10 acre ft of water is 100 feet above the land to be irrigated and has a slope of 45 degrees, a regulated 12 in pipe is inserted at the bottom of the reservoir as the pipe gets smaller the pressure becomes greater but the volume gets smaller. The question is what is the greatest size pipe at the bottom of the grade that will give 125 psi. the slope is 300 feet long and at the bottom the pipe will be attached to a sprinkler system the pressure cannot exceed 125 psi. This entire system is gravity feed.

the single 12 inch pipe will be reduced to supply several smaller sticks of sprinkler pipes the first sprinkler head will be approx 300 feet from the source

Answer 1

Your premise is not quite correct.

The pressure does not become greater as the pipe gets smaller.

If no water is flowing, the pressure is the same, regardless of the size of the pipe, and is equal to the water head (the change in elevation times the weight of the water), in this case 100 ft of head (thats about 45 psig). No matter what you do, with only gravity for power, and only 100 feet of drop from source to use, you will not get more than 45 psig at the use point.

You will actually get a little less than that or a lot less than that depending on the size of pipe and the flow rate. Water flowing in a pipe produces friction on the sides which causes pressure loss. The faster the water flows, the more the pressure loss.

So, if you had a real big diameter pipe, your pressure will be higher at the bottom (less friction loss) assuming the flowrate is the same. A small diameter pipe will cause more friction loss.

It seems like a smaller pipe leads to higher pressure because we see water spray out a small nozzle at the end of a hose faster than a big one. The pressure before the nozzle is the same (house water pressure) and the pressure out in the air is the same (air pressure) it is just the velocity of the water that is different.

So, if the top of the resevoir is only 100 feet above the use point, you will not have anywhere near 125 psi

good luck

Answer 2

I agree with the previous answer although there are a lot more variables once the water starts moving and is not stagnant.

If your friend wants to do this, he would need to get a booster pump. Also, if he's just wanting to water the land, he can split the pipe off with y joints or t joints. Each line of the pipe would have essentially the same pressure as the original design at the bottom but he would get more water for the irrigation.

Answer 3

Your problem is that the bladder tank has ruptured. The bladder tank is that big blue (usually) thing near where the water line enters the home. With the tank to regulate pressure, the well pump is cycling on and off as water is being used, resulting in the pressure being high when the pump in on and near the shutoff pressure for the pressure switch, and down to nothing once it shuts off until the pressure drops to the level where the switch calls for more water. Replace the bladder and then use an air compressor to pressurize the bladder to 35-40 lbs.

Answer 4

Amazingly, water pressure only depends on the height of the water. You can see this because:

pressure = force / area

force = area * height * density * gravity

so, pressure = height * density * gravity

So this means that the size of the pipe doesn't matter.

I calculated the pressure for your situation and it comes out to about 45 psi.

Happy sprinkling dude!

Answer 5

100 feet = 1200 inches of water above any square inch.
that's 1200 cubic inches, which weigh about 600 ounces, or 50 pounds

so it is around 50 psi and the system should stand it.

Answer 6

What is the distance between the 12" pipe and the first sprinkler head? Will there be a single large pipe that feeds all sprinkler heads?

o o o o <-- sprinkler heads
I I I I
_I________I_______I____I______
_____MAIN PIPE______________
I I I I
I I I I
o o o o

Answer 7

HP = true vertial height x .052 x 8.33 pound per gal. for water, so
125 psi /.052/8.33 =288.6 ft. true vertial height, you'll need a pump to get 125 psi