# Water pressure from a natural water source

Discussion in 'Pumps and Tanks Well Forum & Blog. Water is life.' started by Matt62, Jul 23, 2013.

1. ### Matt62New Member

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At my fishing camp where there is no electricity, I get water from gravity fed water source. The source is maybe 200 feet above the top of the cottage. Local people say that to increase pressure, it's better use a larger pipe near the source and the reduce the diameter of the pipe 2 times on your way to the camp. Let's say 2" the 1Â½" and then 1". Is this true ?

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The weight of the water makes pressure, so more water above the tap means more pressure at the tap, and decreasing the volume of water above the tap means less water making pressure at the tap. Think of small and large holes (one each) side-by-side near the bottom of a tapered bucket. The smaller hole will squirt farther because it is small, and not because the bucket is tapered...and then the pressure drops as the bucket drains, of course. So, run a large line all the way down to get all the pressure and flow gravity can push your way, then let that pressure appear at your tap's smaller opening.

Last edited: Jul 23, 2013

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You can get the same pressure through a 1/4" line, you just won't get any volume. Friction loss is all about the volume and the speed at which it flows through the line. You get the exact same volume going into the pipe as you get coming out the other end, so reducing the pipe increases friction loss.

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Are you meaning to say 200' of 1/4" line coming down the slope would have the same pressure (but certainly not the flow) as 200' of 2" line, or are you saying a 1/4" line at the end of a 2" line would show the same pressure?

Just trying to sort that out in my own mind since a basketball and cannonball rolling down the hill would not hit the wall with the same force.

6. ### johnjh2o1Plumbing Contractor for 49 years

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The pressure would be the same no matter what size line it was. The volume would change but not the pressure.

John

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Got it. A square inch is a square inch, and any portion of it would still show the same PSI.

Many thanks!

Last edited: Jul 24, 2013

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Ja, the size of the ocean doesn't change the pressure at the bottom.

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10. ### valvemanCary AustinStaff Member

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When there is no water being used, the pressure at the bottom of a Â¼” pipe will be the same as with 2” pipe. But as the flow increases (water being used), the friction loss in smaller pipe reduces the flow and pressure at the bottom. The larger the pipe, the more pressure you will have at the bottom when water is flowing.

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I think what the locals are theorizing is that PSI is a factor when calculating friction loss. They want to gradually increase the pipe diameter as they get closer to the head. The pressure in the pipe increases by .433 PSI per foot of vertical drop, so at the one third transition from 2" to 1-1/2", not counting friction loss, you would have about 29 PSI.

I could be off base though on what the locals are theorizing. Perhaps it is simply about the pressure rating of pipe in relation to cost. 2" pipe needs a thicker wall to sustain the same pressure rating, so would be more expensive to use on the last third where pressure will be 80-90 PSI.

12. ### valvemanCary AustinStaff Member

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Some people think reducing the pipe size will increase the pressure. One guy explained to me he thought the water would be going faster in smaller pipe and that would increase the pressure.

He had a 1” pipe running a long distance under a road, and the pressure at the other end was not good. He wanted me to replace the 1” pipe with Â½” pipe so he would have good pressure at the end of the line. I tried to explain that larger pipe is needed to get more pressure at the end of the line. His thinking was the water would be going even slower in the 2” pipe than the 1” pipe, so the pressure would not be as good.

I couldn’t talk any sense into the man, so I declined the job. The next contractor came out and put in the Â½” pipe. Needless to say the pressure was terrible and he then had to go back and install 2” pipe to decrease the friction losses and increase the pressure. That contractor got paid twice. I didn’t even get a call to tell me I had been right the first time.

Sometimes is doesn’t pay to be right. Now that customer is too embarrassed to call me anymore, so I lost a customer for life for just being right.

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You should have told him you would install 2", then run a temporary length of 1/2" through it to try what he wanted, then only charge him for the 1/2" if he was right and the 1/2" line actually worked!

14. ### valvemanCary AustinStaff Member

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Wasn’t going for it. Thought I just wanted to put in 2” because I made more money. Basically ran me off for suggesting such a stupid idea. I think that is why he is embarrassed to call me back.

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That kind of thing (area x psi) is what had messed me up at the beginning here. Where there would be 80-90 PSI at the bottom of the hill, you could still easily use your thumb against the pressure at the end of a 1/4" tube where the palm of your hand would not be sufficient for stopping and holding back the flow of a 2" pipe. So while the PSI of each is the same, the larger has more "total pressure" or force just like the rolling cannon ball (more mass to stop) as compared to a basketball.

Last edited: Jul 24, 2013