Problem with pressure in pipeline

[Discreet544]

Hi Folks;

As we all knew, and I used to experimentally observe it, as the temperature of water increases, pressure drop through pipeline (kPa/m) decreases. Now, I am getting the opposite, while the results are so consistent and reproducible !!! As the water temperature increases, pressure drop increases too !!! I have calibrated the pressure transducers, and those look fine. Any guess ? Has anybody seen such a situation ?

Also, as we all knew again, with decreasing velocity, friction factor increases (remember Moody diagram). Now for my case, I am getting decreasing trend for temperature range of 20 to 30, while the trend is increasing for above 30 !!!

I am running water in a 2" closed-loop pipeline of 46 m length, with a magnetic flow meter and digital pressure transducers.

Thanks;

 

[TD2K]

What are you holding constant when you consider/evaluate the pressure drop at two different temperatures?

If I take two cases at constant mass flow rate (or constant standard barrels per time) and I raise the temperature, the density will decrease slightly and the viscosity will increase, both those changes would directionally raise the pressure drop.

 

[MortenA]

I think that the reason for dP dropping when T goes up is that the viscosity of water drop as temperature increases. This causes the Renoylds no. to drop and thus the f to drop (not so much the viscosity). As a matter of fact, since the density decreases (and the dP also does) a centrifugal pump will then be able to deliver som more volume flow. I dont know if theres a pressure gradient?

Your system sound like a test/lab bench?

Best regards

Morten

 

[Discreet544]

@TD2K : I compare the pressure at certain velocities. By the way, both the density and viscosity decrease by increasing temperature of water.
@ Morten: Yes, this is a 46m length closed-loop pipeline.

 

[TD2K]

Duh, where was my brain, yes, the viscosity would go down as the temperature goes up.