Pressure drop in bends when pumping viscous fluids 2

[rasmusagren]

Hi!
I have little prior knowledge of reology, but I have now come across a problem where I need to calculate the pressure drop in a pipe bend. The fluid is tomato paste (approximately power-law) so the flow will always be laminar.
Is it correct to assume that the pressure drop in a bend is proportonal to the pressure drop in an equvalent straight pipe of the same length? Is there a way I can calculate the proportion factor from angle, pipe diameter and bend diameter?
I do not ask for one-time resistance coefficients, but for a more general solution.

Your help will be most appreciated.

Regards
Rasmus Ågren

 

[Ashereng]

Is it correct to assume that the pressure drop in a bend is proportonal to the pressure drop in an equvalent straight pipe of the same length?

No.

Pressure loss will depend on radius of bends, pipe ID, roughness, etc.

If you need to calculated pressure loss, I would suggest using one of the piping software.

 

[WebWizz]

Dear Rasmus,

Most methods use equivalent length equations, but in fact these are incorrect for laminar flow. The method that I like is the '2k method' published in an 1981 article:
The two-K method predicts head losses in pipe fittings
W.B. Hooper
Chemical Engineering, Aug 1981
Pages 96 to 100

Just as in a straight pipe the friction in a bend increases at low Reynolds numbers. At high Reynolds the curve goes horizontal. The formula's are used in a web based calculation routine on

http://www.engineeringpage.com/engineering/pressure_drop.html

This only takes care of the laminar flow, it doesn't do anything with the rheology - but it is certainly a lot more accurate than the usual fixed k method.

Regards, Wim

 

[katmar]

I agree with Wim on using the 2-K method. As a second-best alternative, the "equivalent length" method is better than the fixed K method.

A good reference that describes piping calculations in food processing is Prof JF Steffe's book, which is available for download at

http://www.egr.msu.edu/~steffe/

See pages 128-136


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[rasmusagren]

Thank you all for your help! I think that I will go with the 2-K method.

Regards
Rasmus Ågren

 

[Ashereng]

rasmus,

My apology. I misread your OP.

Yes, a bend is usually treated by an "equivalent length".

I was thinking you said somthing along the lines of a 8" bend being modelled by an 8" length of straight pipe.

Sorry about that.