# Pipe flow calculation

Help me out. A level 200mm diameter pipe has one end 300mm under water. It then goes through a wall into a space. What amount of water will flow through it per second? Playing around with some on line calculators I get about 340 litres a second but that seems a bit high.

It depends on the length of the pipe.

What do you mean “a level pipe”? Is this a horizontal pipe? (I was picturing a vertical pipe with one end submersed 300mm under water.) Also, what is the dP across the pipe? In other words, (a) is this gravity flow, or is there a pump? (b) what pressure is at the other end - only the head from 300mm of water, I presume, or is this “level” pipe coming off a tank under pressure?

A pipe can pass an infinite amount of fluid depending on the dP driving it.

• Jinx

OK, here’s how I do it. I don’t have my Cameron’s Handbook of Hydraulics with me, but I’ve got some Excel spreadsheets with some data. You’ve got 300 mm static head as the driving force, and the pipe diameter is 200 mm. I’m assuming that that’s the inside diameter, which is pretty close to the ID of a schedule 40 8" pipe. Schedule 40 8" pipe has an ID of 202.7 mm. I don’t know the pipe material, but I’ll assume it’s steel.

The flow will increase until the friction loss equals the hydraulic head available. In an 8" pipe the losses will not build up to 300 mm until the flow equals 4583 gpm, with is around 290 liters per second. At that flow the friction loss is 30.008 feet per 100 feet of pipe.

So, my assumptions were: The only driving force was the static head, the pipe was 8" schedule 40 steel pipe, the pipe was horizontal, the fluid was water at 60 deg F, the static head does not change as the water flows, in other words, it is the instantaneous flow. For other than that, you need calculus.

The flow seems high, but do a reality check. If you have a swimming pool, and you poke a one meter long 200 mm diameter pipe through the side almost one foot below the surface, how much water do you think will come out?

Bill, I was asleep…I guess he was trying to describe a horizontal pipe coming off an atmospheric tank filled to 300mm of water. :smack: :smack: :smack:

I work with atypical configurations and applications, so the obvious escapes me…

• Jinx

OK, good. Thanks guys. I used an online calculator but by the time I had worked out that the flow was up to what it seemed was correct, the water was doing 11 m/s. The speed of it seemed too high. Anyway, nice to know I was on the right track.

Jinx, not a pipe off an atmospheric tank filled to 300mm. In fact, more like a pipe through a dam wall 300mm below the water line.

Without knowing the specifics of the online claculators, I think there are conditions here that may differ than what they were intended for.

I’m thinking about the dimensions of the system. You didn’t specify, but if it’s assumed that the centerline of the 200mm pipe is at a depth of 300mm, then the top of the pipe is under a static head of 200mm/water, and the bottom of the pipe is at 400mm/water. That a pretty large variation of head, something that the calculators may not be equipped to handle. So the flow is probably going to vary quite a bit with depth at the entrance of the pipe. At some point downstream, the flow will become uniform. The problem is the pipe is only 5 diameters long, and I doubt that the flow will become uniform in such a short length given the large variation of head at the entrance of the pipe. Again, the calculator might assume uniform flow.

The model the online caculator is designed for might not match your conditions. I would guess that it assumes a uniform head at the entrance and uniform flow along the pipe length. Maybe **Bill Door ** would know if the differences are enough to warrant a deeper investigation (though I guess it also depends on what the application is).

Given that the Reynolds number was around 1.8 X 10^6 I figured it was turbulant flow and used the centerline to calculate the flow. I also neglected the entrace and exit effects, figuring that with a 200 mm diameter and 8.96 m/s flow they would be negligible compared to the friction.

Hey, it’s close enough for engineering, I never claimed to be a scientist.

I wondered about this, too. In any event, I really just wanted to get into the ballpark, which no doubt is where Bill has got me.