Velocity Variation through Pipeline 3

[Discreet544]

Hi

Imagine there is a closed-loop pipeline which we are pumping pure water. Reasonably, we expect the pressure-drop to increase towards the end, due to friction and minor losses. My question, however, is about the velocity.

When you half the length of the loop, at the same pump power, velocity increases, as there will be less total friction and maybe less pump slippage. However, at the same pump power, should we expect pressure drop/length to be different through pipeline?

In my experiment, I am experiencing different kPa/m at various positions. As the D and f is the same everywhere through pipeline, it can happen only if the velocity is different from point to point. But, when the pump power is kept fixed, the fixed amount of water is expected to be pumped and the velocity should be the same everywhere. Am I right ? If not, and the velocity reduces as well through pipeline, how do you justify the continuity of fluid ?

Thanks;

 

[zdas04]

If this was a real problem, and I had to "solve" it, I'd look at:

- The inside of all the pipe
- Gauge uncertainty
- Pump wear (the numbers are REALLY small, it wouldn't take much bypass flow to result in the graphs shown, if velocity is actually lower because the pump isn't pumping as much then the dP would be low for a given "velocity").

There is no theoretical condition that would cause observed behaviour, it has to be physical.

David

 

[Discreet544]

Dear zdas04. Thanks for your comments. By the way, this ""school project"" is being conducted by Alberta Agriculture Ministry !!

The problem is not only with water. As I already stated, I am using water just to run and calibrate the system. The major problem is with slurry. Unfortunately, for that case, it is not like all the results are shifted down by 4 kPa/m. But the trends are totally mixed up and I am getting nothing but a real mess of meaningless graphs!

 

[btrueblood]

What David said. If you have (re)calibrated the pressure transducers (i.e. adjusted fit to a standard) in the interim, it would also explain the shift.

 

[MortenA]

Its a very long tread and i may have missed it but:

In general i agree with biginch et al: dP is constant when expressed as dp/m for a pipeline if the ID is kept constant and the line is level. A number of this may cause this to APPEAR untru:

- The obvious: Changes in elevation
- Slack flow cause by changes in elevation
- Changes to ID (e.g. for crossings)
- Deposits or growth!

I think that the last could be an overlooked issue for a lot of fresk water lines. It could be organic growth but also changes to e.g. temperatre that would promote a solid deposit (or even organic growth) to occure at some point but not initially.


Best regards

Morten

 

[BigInch]

Might as well add constant temperature.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso