End cut for maximum flow rate in pipe

Assuming the length of the pipe is much greater (say L>= 5D) than the diameter, I don't think the angle of cut will make much difference, other than to maybe shape the flow of the water.

Hello Guest,

The best flow can be achieved with the square cut. Additionally, a pipe end flush with the dam wall (pond side) will give a better flow than if the pipe end extends beyond it (called a Borda entrance).

Hope this helps.

Mike

Can you elaborate on the Borda entrance, because my pipe extends approx. 4 ft beyond the dam. Also my pipe is 150 ft long and when completely submerged in the pond the flow at the end is less than 1/3 of the diameter of the pipe is this normal?

Hi Guest,

This is what I think the layout is at present. If it is different, let me know.

Also, I don't know what you mean by:

"the flow at the end is less than 1/3 of the diameter of the pipe"

It seems like you are comparing a flow rate to a diameter - can't do that! Please clarify.

Mike

Thanks Mike,

The diagram is correct. What I meant was, as per your diagram, If I have a 16 in. diameter opening collecting water how come I do not have 16inches or close coming out of the pipe. Is it because as the velocity increases the volume decreases?

Bull

Hi Bull,

You mean the inlet pipe diameter is greater than the outlet diameter? You said that the inlet diameter is 16". What is the diameter of the pipe at the outlet?

Mike

OK, I think I understand now. Disregard my last post. The flow profile - that is, the cross-sectional area of the stream at the discharge is only about 1/3 of the cross sectional area of the pipe.

I would say that it's mainly due to the fact that it slopes down. And yes, the average velocity at the discharge is higher than the average velocity at the inlet, but the flow rates must be the same.

In your situation, I think that entrance effects are negligible - it's the length and surface profile of the inside of the pipe where most of your losses are coming from.

Mike

The technical Literatur provides a lot of rules and calculation methods for flow of liquids. Try out www.pipeflowcalculations.com . For water it should be possible to earn sufficient results by using the theoretical formulas.

One important rule is the rule of continuation. It means: along a flow pass the flow rate is the same for all positions and sections of the flow pass. In fact the flow rate is Q = A w (cross area multiplied by mean flow velocity) you can calculate the mean flow velocity (w) for any point of the flow pass (it will change in proportion to change in cross section area).

The other important rule is the famous Bernoulli Principle, which is based on a description of energy contents of a flow (sum of energy in a flow = constant). On hands of the Bernoulli principle you can calculate the flow conditions on hands of geometrical conditions like pipe size and lenght, gravity height, pipe ends design, pipes internal surface quality (determining the pipes flow resistance). ==> see mentioned link for details. In case the pipes entry point is not totally flooded special additional rules need to be envoked.

For the posted problem the only driving energy for a flow is the gravity height between outlet and entry point. This potential energy creates a flow against resitances. The flow resistances typically are proportional to the square of flow velocity ( ζ = f(w²)). Especially the entry and outlet sections of flow passes (a pipe) have specifical flow resistances - depending on their design. For instance:

A pipe simply cut at outlet point has a flow resistance factor = 1. If the outlet is made like a booster (max. 15° angle), the resistance factor reduces to 0.05. For entry points the smallest resistance can be earned if the entry point is direct in the wall of the wall (sharp edged: resistance factor = 0.5), and not reaching out of the wall (increases resistance factor to about 1.0) into the open water. If you would make the entry point shaped like a trumpet end (greatly rounded with r=1/2 pipe diameter) the resitance factor goes down to 0.05.

Any valve, bend diameter change of pipe will create resistances. Having problems with the flow capacity of a pipe all sources for resistances shall be considered and minimised.

Assuming the entry point is flooded, but outlet stream is filling the pipe outlet diameter just 1/3 is an indication for significant flow resistances. If the entry point is not permanent flooded (air sucked in; water is curling around the entry point) the flow is disturbed (= additional resistances). Due to the used pipe slope, the pipe is quite long in comparision to the gravity height. The flow resistance along the pipe might be the true bottle-neck for the flow restrictions. This can be calculated as well.

Albert

Completely agree with the previous answer.

The only reason for cutting the pipe at an angle would be to help avoid turbulance in a high discharge system i.e. where the straight cut end of the pipe finishes close to the bottom of the drainage reservoir and sludge is disturbed.

The shape of any end cut will not matter, as far as increasing flow rate is involved.

Length of outflow pipe, if very long, will slightly reduce flow rate; so reduce it to the barest minimum.

Further increase in flow rate will be possible for a given length of pipe, only by replacing it with a large diameter pipe; the larger the better.

Shrikant Dhekne

"Further increase in flow rate will be possible for a given length of pipe, only by replacing it with a large diameter pipe; the larger the better. "

I beg to differ. Changing diameter will only change velocity of flow - not volumetric flow rate. Flow at one point in the system is always equal to flow rate in another point unless pipe branches in two. Think about it, if no fluid is leaving the system, flow has to remain the same, but velocity can change (decreases as pipe diameter increases)

Regards,

g

Assuming the source water stays constant you can increase volumetric flowrate by increasing pipesize, to an extent. That's not the point however, the point is he's not getting too little, he's getting just about right for the size and DP from the source, assuming no mass restriction at the inlet. And yes you can get more flow on smoother surfaces, but at what cost benifit for this application? I think he just expects more of the discharge diameter to be full and can't figure out why it isn't.
Just look at any gravity fed pipe, they all lay at 1/3.

Do not forget smooth out the pipe after cutting it as the burrs will hamper flow.

When the pipe is completely submerged the flow at the end of the pipe is less than 1/3 of diameter of the pipe is this normal for 150 ft pipe?

Do you mean that the pipe is 1/3 full of water at the discharge end?

Yes this is what i meant.

Sounds like you have a restriction in the pipe to me.Have you checked for this?Is this a new installation or an old one? Maybe moss or debri is clogging the pipe.Are you in an area affected by Zebra Mussels?I would think you would have a full pipe at the exit end unless something is restricting it.

Is a sink hole forming at the inlet to the pipe on the surface above it?If so, the inducted air will reduce the volume of the outgoing water.Is their a baffle or grate at the entrance to the pipe? If so, check for clogging.Should have full pipe otherwise.

The friction in a almost full tunnel is lower than the friction in a submerged tunnel. (should be true for pipes as well)

At the gradient of your pipe the flow in the pipe will always open up at the end.

If you restrict the flow at the entrance to achieve a opening at the top you might get a bigger flow.

Find a book on tunnel design.

You would only have a full pipe if the differential head from entrance to exit is sufficient to overcome friction losses in the pipe itself. What I suspect you are seeing is the total amount of flow the pipe is capable of producing at the differential avilable, and 1/3 would seem about right. If you are looking for more for some reason, you need a bigger pipe.