Water AM - Pipe Flow

[GTScott]

Someone called me with this one so I thought I would throw it out there. I have an answer but would like to see if anyone else attacks this differently.

Given an 8" pipe (material not specified) at 0 slope with 3 feet of head on the pipe, what is the max flow that you can get through the pipe?

-GT

 

[DVINNY]

What's the square root of zero?

If you use 0.001 for slope, then you can get an answer, and I'll bet it's close to the real situation.

 

[GTScott]

Treat it as pressure conduit.

 

[EL Nica PE]

d/4

 

[SCPE]

treat it as a tank disharge.

 

[DVINNY]

treat it as a tank disharge.

good call

 

[EL Nica PE]

ok. where is the solution?

 

[GTScott]

I worked it two ways...one as a submerged orifice and then once as pressure conduit. Which would be correct? The answers are fairly different. The local expert here says to treat it as pressure conduit with P1, V1, and P2 out...solve for V2 and then get Q. I did it as a submerged orifice with a coefficient of about 1.6.

 

[EL Nica PE]

GTSCOTT,

did you get right answer?

 

[EL Nica PE]

I don't have my notes but if Iremember right isn't D/4

the max flow for a circular pipe? I feel like I'm missing something thou?

 

[DVINNY]

D/4 is for Open Channel, I think we need to look at this as a pressure situation

 

[JasonT33]

whats the downstream condition? submerged?

 

[EL Nica PE]

DVINNY,

are you sure? for some reason keep thing of d/4 for pipes

 

[EL Nica PE]

ok, I will look at my notes tonight!!!

 

[GTScott]

I don't know which answer is right...but here is what I got.

Orifice = Q=13.45 cfs

Pressure Pipe = Q=4.85 cfs

I thought this was some crazy simple question when I posted it. I am glad to see that I am not the only one who looks at it in a few ways.

 

[EL Nica PE]

those answers are to different to treat the problem either way.

 

[GTScott]

those answers are to different to treat the problem either way.

Thank you, captain obvious

Pressure pipe seemed like a good method to me, but my only concern with that method is that if you assume that V1 is 0, following that logic V2 would be 0 also. Using the orifice, that does not necessarily mean that the pipe is flowing full. Any other ideas?

-GT

 

[JasonT33]

I believe this is a Bernoulli solution.... discharge from a tank is derived from the Bernoulli equation for frictionless flow

 

[EL Nica PE]

Ok, here is the answer.

V=Sqrt(2gH)

v=Sqrt(2xgxh)=13.89=14ft/s

Q=VA

A=piexD^2/4=0.3491

Q=13.89x0.3491=4.84 cfs

so, I agree with GTScott.

 

[civengPE]

I don't understand why you can't just use the energy equation.

Never mind. I see you did now!