I am looking for some advice on the sizing of hydraulic lines in a secondary controlled device I am designing. The device must accomodate 120 gpm, and the flow is divided up internally in the device (details omitted). I have been told that 1" dia is appropriate for 120 gpm. But as I divide the area down proportionally with the internal flows, the cross-sectional areas of these passageways becomes so small I am second-guessing my methodology.

My approach is this:
Considering steady-state operation, all flows must be at the same velocity.

For example, we start with 120 gpm flowing through 1" dia. 1" dia = .785398 sq in. If we now divide that flow into halves, we have (2) 60 gpm flows each flowing through passageways with cross-sectional areas of .392699 sq in (because .785398/2=.392699). As the flows are further divided, this same proportional relationship is held, and the fluid velocity is always constant no matter which sub-flow you are considering.

I have taken this approach to minimize the internal accelerations and decelerations of the flows in order to minimize energy losses and maximize efficiency. I have even minimized the number and degree of flow redirection. The minimization of internal energy losses is imperative to the success of this device.

Basically, my question is this:
Is there a rule of thumb for determining line sizes/cross-sectional areas? Please do not respond with a bunch of equations relating dia's and velocities. I already know all of them and am using them. My question is more about the validity of my approach of keeping all flows at the same velocity.

I ask because it occurs to me that my approach would be correct IF the fluid properties (viscosity, bulk modulus, air entrapment) were proportional to the flows. But this is not the case. In reality, all fluid properties remain constant and only the flows change (as they are subdivided, treated, and rejoined). This, I believe, results in a situation where, although the fluid velocities are equal, a smaller flow flowing through a proportionally smaller passageway contributes a disproportionally larger energy loss. Because of this, I believe my current approach is flawed.

Suggestions, anyone?

ELTON what end result are you looking for??

I always size lines for minimum pressure drop at maximum flow so Energy Waste is at a minimum. Larger lines may cost more up front but higher pressure to push oil through under sized lines and a Heat Exchanger to cool the oil cost for the life of the circuit.

I get the feeling you are trying to get equal flow by having equal pressure drop??????

If not please try to be a little more specific about what it is you are actually trying to accomplish with Flow Line Sizing without giing any secrets away.

Normally flow in Medium to high pressure lines should be 15-20 Ft./Second for nominalPressure Drop. I always use a Fluid Power Data Book sold by Womack Machine Supply in Dallas, TX 214-357-3871. They also have a web site for poroducts and eduvational material. The Data book is shown at the educational site.


http://emka.xs4all.nl/dP/index.htm

If equal flow is what you need you will only get it by ine sizing if final pressure NEVER FLUCTUATES and I mean NEVER and each line is the same length and has the indentical number and type of fittigs. Not impossible but usually not worth the effort since something always changes sooner or later and gives different flow volumes.

Synchronizing flow is best done by some hydraulic, mechanical or electrical means according to how accurate you need the split.