Find flow given: diameter, length, pressure

[sphynx_000]

I am trying to find the flow through a pipe between a pump and an outlet

I know
-gauge pressure just after the pump
-pipe diameter
-pipe length
-elevation change

Any idea how to go about this?

 

[Q_Goest]

Check this thread:
https://www.physicsforums.com/showthread.php?t=211590

Open up the attached document "Pipe-Flo Pro.PDF" (post #4) for pipe flow analysis.

Or just post your known information and I'll shove it into a program.
- Pipe ID
- Pipe length
- Upstream pressure
- Fluid type
- Fluid temp at upstream pressure location
- Number of elbows or bends
- Elevation change
- Valves (need Cv)
- Either flow or downstream pressure. Can't do both.

 

[sphynx_000]

hummm,

I don't know flow or downstream pressure,

 

[minger]

between a pump and an outlet

Are you sure you don't know the downstream/exit prsesure?

 

[Bob S]

You can work backwards using this:
http://www.pipeflowcalculations.com/flowrate/index.htm

 

[FredGarvin]

What kind of pump is it? What fluid? If it's a centrif then you are in a pinch. You would need to determine the total head of the system and then use that to compare to the pumps pump curve to find out what its output flow would be.

If it is a positive displacement pump then it's pretty straight forward.

 

[sphynx_000]

water, displacement pump...

I think i have it about figured out, I will post some results soon

 

[sphynx_000]

Ok, now I am stumped again.

Could someone confirm this for me?
- my p1= gauge pressure after pump, and p2=0 at outlet?
- Also my v1=v2, same diameter pipe, incompressible fluid, water

I have used the Darcy–Weisbach method to calculate the friction head loss. Now I need to use Bernoulli equation to calculate the elevation head loss right?

 

[FredGarvin]

Your assumptions are correct for the pressures and velocities.

If it is a PD pump, you have your flow. Conservation of mass means that what the pump is putting out is your flow. Since it is water and treated as incompressible, your volumetric flow rate is also constant. The only reason at this point in calculating head loss is to find out expected pressures somewhere along your pipeline.

What EXACTLY is it that you want to calculate?

 

[sphynx_000]

maybe I am making this way to complicated.


All I readlly need to do is find the flow.,

 

[minger]

As Fred was saying, if you know the pump and have the curves, you should be able to tell your flow by the head its producing.

 

[sphynx_000]

These are my only knowns


-pressures
-elevations
-bends
-diameter
-length

 

[Q_Goest]

As mentioned, it would help to know if it is a positive displacement (PD) pump (ie: such as a reciprocating piston or gear pump) or is this a centrifugal or similar style pump? For PD pumps, the flow rate is fixed regardless of pressure. For centrifugal pumps, the flow rate is a function of dP across the pump. That dP needs to be determined in order to find pump flow for a centrifugal. To determine dP, you will need to determine pressure drop through the pipe so it can be matched to the 'pump curve' for the pump you have.

 

[FredGarvin]

Now I see exactly what you are looking at. This is referred to as a TYPE 2 problem.

First set up an equation using the energy equation (the Bernoulli equation with the added loss terms) to come up with an equation that is a function of f and V.

You'll need to guess at the friction coefficient and then calculate V and Reynolds number. Use the Reynolds number you just calculated to look up the friction coefficient in a Moody chart. If the initial guess and the looked up values are close, you are done. If they are not close, make a new estimate for f and repeat.

 

[sphynx_000]

Ok, I think I finially starting to learn enough to solve this problem.

One more general question.


With bernoulli's equation like...

[p/y+V²/2g+z]₁ - [p/y+V²/2g+z]₂ = h_L

The h_L is the head loss due to what?

I am confused if this is due to friction, elevation, or both?

 

[Q_Goest]

Ok, I think I finially starting to learn enough to solve this problem.

One more general question.


With bernoulli's equation like...

[p/y+V²/2g+z]₁ - [p/y+V²/2g+z]₂ = h_L

The h_L is the head loss due to what?

I am confused if this is due to friction, elevation, or both?

Head loss is a generic term for a drop in a fluid's pressure. Your equation above is correct, but to calculate the total head loss (total pressure drop) in a pipe, the Bernoulli equation is insufficient. Bernoulli's assumes there is no frictional losses, so the fluid per Bernoulli's would behave like a perpetual motion machine, never loosing total pressure. To fix this, the Darcy Weisbach equation is added into the Bernoulli equation as shown in the manual I posted here:
https://www.physicsforums.com/attachment.php?attachmentid=12408&d=1201620608

See page 14, equations 15 and 16.