How did Industry Standard Pipe Flow Velocities Develop?

[WINTER]

I would like to know how the Industry Accepted Flow Velocities in hydraulic line have been established. The accepted values are as follows:
- 2 to 4 ft/s: suction lines up to 500 psi
- 10 to 15 ft/s: pressure lines from 500-3000 psi
- 15 to 20 ft/s: pressure lines over 3000 psi
These values are comonly used in designing hydraulic systems. I realize the the reynolds number governs when flow changes from laminar to turbulent. I understand that the pressure drop for turbulent flow is greater then for laminar. But i am not certain of the mathematic significance of the pressure or how these accepted values have been established.
Thanks for your response
Jeff

 

[BobPE]

the pressure drop for turbulent flow is less than laminar i think you meant to say. the standards for industry are a lot of rules of thumb. There is merit to them. For instance, high velovities at reduced pressures causes cavitation at minor losses like valves. High suction velocities can cause significant reduction in NPSHa or turbulence to pump inlets...It's best to do the math than have to defend a rule of thumb to someone...



BobPE

 

[KenA]

Winter,
A lot comes down to economics. Higher velocity allows smaller pipes, but pressure drop per length of pipe increases approx proportional to velocity squared. So pumps and compressors require higher discharge pressures as the line size reduces. Rules of thumb came about as a rough economic trade off between pipe size and pressure drop. There are of course many other considerations, such as erosion at high velocities.
K

 

[hacksaw]

Good question.

It is not clear why such high velocities are permissible.

Velocities above 10 ft/s are prone to cavitation and a potential risk to modulating valves and flex hoses.

Not all hydraulic systems are small. In one case a 3" line (3000 psig, 150 gpm) was being planned.

http://www.nrel.gov/docs/gen/fy01/NN0041.pdf

Clearly it is a question that deserves closer scrutiny and more discussion.

The failure rates of hydraulic components are pretty high

http://www.mvr.usace.army.mil/failuredata/merged.asp

and the potential consequences are severe.