# Find flow given: diameter, length, pressure

1. Jun 11, 2009

### 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

2. Jun 11, 2009

### Q_Goest

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.

3. Jun 11, 2009

### sphynx_000

hummm,

I dont know flow or downstream pressure,

4. Jun 11, 2009

### minger

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

5. Jun 11, 2009

### Bob S

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

Last edited by a moderator: May 4, 2017
6. Jun 11, 2009

### 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.

7. Jun 12, 2009

### sphynx_000

water, displacement pump...

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

8. Jun 12, 2009

### 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 Darcyâ€“Weisbach method to calculate the friction head loss. Now I need to use Bernoulli equation to calculate the elevation head loss right?

9. Jun 12, 2009

### 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?

10. Jun 12, 2009

### sphynx_000

maybe I am making this way to complicated.

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

11. Jun 12, 2009

### 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.

12. Jun 12, 2009

### sphynx_000

These are my only knowns

-pressures
-elevations
-bends
-diameter
-length

13. Jun 12, 2009

### 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.

14. Jun 12, 2009

### 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.

15. Jun 12, 2009

### 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 = hL

The hL is the head loss due to what?

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

Last edited: Jun 12, 2009
16. Jun 12, 2009

### Q_Goest

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 in to 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.