Why does the reccomended maximum fluid flow velocity in pipes vary with pressure?

[HydroSi]

Hello,
I've been reviewing lots of technical literature relating to reccomended maximum fluid flow velocity in pipes. On numerous occasions I have come accross increasing maximum fluid flow velocities with an increasing operating pressure.
e.g. upto 100 bar, maximum flow velocity = 5 m/s. 250 bar to 350 bar, maximum flow velocity = 10 m/s. etc.

I would like to fully understand the reasoning for this, any help would be greatly appreciated.
I understand that as the pressure of the fluid increases the density and viscosity increase...Is it all related to this?
HydroSi

 

[btrueblood]

At higher pressures, the possibility of cavitation, or gas bubbles forming in an area where reduced pressure can exist due to velocity changes (these are not the same phenomena, but both cause noise, and cavitation can cause damage) is reduced.

 

[Compositepro]

Optimal velocity is mainly a balance between initial capital cost (cost of pipe and installation) versus operating expense (energy used due to pressure drop). High pressure pipe costs more so using a higher velocity can be justified in order to not have to go to the next larger pipe size.

 

[zdas04]

What is the "fluid" you are talking about. The answer is different for gases than for liquids than for non-Newtonian fluids.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[RobertHasty]

Also, what about "differential flow"?
For example, if the pump flow is X and we send this flow to piston side according to guidelines HydroSi mentioned, what about then when the flow X goes to rod side? In that case, flow that goes from piston side can be up to 2X (depending of area ratio)?

 

[HydroSi]

Thanks for the answers so far. The fluid would be a standard hydraulic mineral oil, grade 32 or 46.

 

[kcj]

Also, pressure drop is a function of velocity, but the pressure drop and energy losses for a given velocity don't affect all circuits in the same way.
Pressure drop compared to total available pressure is a higher precentage of the original pressure.

5 psi drop in a suction line would be unacceptable.
5 psi drop in a 25 psi return line is a significant issue.
5 psi drop in a 3000 or 5000 psi hydrostatic line is hardly measureable.

 

[Dougt115]

The analogy I was taught was.

Friction is a function of velocity. The faster you go the more friction force. Same with fluids. The faster you pump a fluid through a pipe the more resistance until they reach an equilibrium. Increase the pressure again and the velocity will seek a new equilibrium.

Pipe size, texture and geometry will all affect the friction.