# Dust off your fluid dynamics knowledge

Ok, so here’s my issue:

I live out in the country and am on a well. I recently ran 1,200 ft of 1" pipe from my holding tank to my front field for a new garden I planted. approx 1,000 ft is level but the last 200’ go up hill.

There is a 1" line from the holding tank which branches to about 5’ of 1/2" pipe which then exits the building and then it goes to back to 1" pipe for 1,200 ft.

My pressure is not as high as I would like. I think one problem is the 1" -> 1/2" -> 1" change in pipe diameter. Another is the fact that the pump is pushing a 1" water column 1,200 ft long.

I’m wondering if I re-plumb it to remove the 1/2" pipe will my problems be solved, or do I need a bigger pump?

If my ailing memory of physics serves (and it probably doesn’t), going from the 1" to 1/2" pipe causes an increase in fluid speed and a decrease in fluid pressure (speed & pressure being inversely proportional); while going from the 1/2" to 1" reverses that, which should balance out…I think. So I’m guessing what you need is a bigger pump.

I was going to say ask your wife if you need a bigger pipe or more pump… but… ba da bump.

Correct. The fatter pipe after the constricted pipe is pulling on the water in the constricted area. The water volume per second in that constricted area is consistent with the rest of the pipe.

The 1/2" pipe is not your problem. I suppose you could seek more confirmation on this, but I’m very certain that ubongwah and myself are correct. There might be some idealization going on here though, and what irks me about applying Bernoulli’s Principle to this is that if you slightly constrict a regular garden hose, you do indeed get less flow.

edit: It seems some of that water will change from potential energy to kinetic energy, so maybe there is a net loss in pressure due to this not being an ideal situation. That’s how carburetors work.

edit2: Nevermind, the pressure in the direction of the flow is greater. After the pipe fills up with water there should be no change in flow rate at all.

i.e. get a bigger pump

Yeah, get a bigger pump. Or a bigger pipe. The transition to different bores will have a negative effect because of the eddies that’ll suck up energy at the transition, as will any other kind of ‘lateral’ change… but it’s not as great as the problem of moving a buttload of water a long way. You’ve basically got 1000 ft of hosepipe… plus a hill. Imagine blowing on the end if you want to see how your pump feels…

A bigger pipe will just move the same amount of water at a lower velocity. His pressure coming out of the showerhead should be the same.

My parents’ house is on a well, has to pump a distance of about 100 feet, and the water pressure is really weak if you try to shower while the washer is on.