sizing tubing for a hydraulic system- line losses

[subsearobot]

In designing a fluid power system, Parker (catalog 4300) recommends specific line speeds for pressure, return and intake. Seems simple, which I like...

An SSP design handbook has a table which correlates pressure loss per foot of pipe and flow rate with tube size. What I see looking at this table is: as flow area increases for a given line speed, pressure losses per foot *greatly* decrease. Especially in the size range where I am concerned (low HP systems). I assume (I know...) this table comes from bernouli eqns.

I also notice in lines over about 1/2", 25 fps line speeds are in the turbulent regime, but line losses still tend to decrease with increasing flow area.

I tend to design to acceptable losses, as opposed to the parker method. I'm not quite sure what to make to the turbulent flow issue- my only thought here is that turbulent flow would tend to erode lines from the inside, but again I am making as a$$ of myself by assuming that erosion is very long term issue, which will out-survive the rest of my equipment.

so, how can Parker, the god of hydraulics, make such a simplification? am I missing something? what about turbulent flow?

please discuss and enlighten me!

thanks in advance!

 

[hydtools]

You can design however you like. Parker and others, see Gates nomograph which is typical, use fluid power recommended fluid velocities to prevent turbulent flow and prevent excessive pressure loss(wasted energy).

http://www.gates.com/downloads/down...hicChart.pdf&folder=brochure&location_id=3047

Energy or power and pressure loss varies with the square of the velocity, that's why small changes in area can result in large changes in wasted energy and pressure loss.

Ted

 

[hydtools]

Correction: energy loss varies with the velocity. Velocity varies with the square of the conductor diameter. A small change in diameter results in a large change in velocity.
Sorry.

Ted

 

[subsearobot]

I realize I can design however I like! ;-) I am just asking if I am missing something. I am somewhat of a generalist, though I really enjoy my hydraulic projects, and want to be thorough.

as an example, (and I probably need to run some calculations to verify the validity of the SSP table, from whence this data came):

at 25fps line speed:
1/4" x .049 tube, delta P = ~24psi/ft
3/8" x .049 tube, delta P = ~7psi/ft
1/2" x .049 tube, dP = ~6.6psi/ft, TURBULENT FLOW
...
1.0" x.049 tube, dP=~2psi/ft, turbulent flow

so what I'm seeing here is: designing to 25fps does not in itself yield a universally low pressure loss (by an order of magnitude), 25fps does indeed yield turbulent flow at larger line sizes and turbulent flow does not yield as much losses as I would expect.

So this leads me to perhaps my real question: why a suggested fluid velocity at all? or am I still missing something?

 

[MikeHalloran]

At 25 fps, actually a little lower, acoustic flow noise becomes objectionable in otherwise quiet environments. If that's not an issue in your application, go ahead and dissipate some energy to save weight or space.



Mike Halloran
Pembroke Pines, FL, USA

 

[hydromech]

1) Parker are not gods of fluid power, they have grown by acquisition of other established fluid power solutions providers. They like to think they own the fluid power business...but they don't.

2) The turbulent flow charts are derived from mainly Moody's data and Reynolds numbers.

3) There is nothing wrong with turbulent flow if you can handle the power loss.

4) Turbulent flow can help is some cases. More responsive system. Less dirt in the system as it is harder for debris to settle.

5) Fluid Velocity details are guidelines, not rules.

 

[kcj]

Many of the 'rules of thumb' were established so that designers and salespeople and mechanics could size lines and hoses without understanding reynolds numbers, laminar and turbulence, etc etc.
And realistically, they ROT were usually quite 'good enough.'

They are also faster and simpler to use without calculating line losses, which I have found to be sort of a shot in the dark at times depending on assumptions with viscosity and temperature.
Always seems a bit odd to me to calculate line loss but then have a sharp 90 elbow that isn't accounted for.

I use the velocity calcs as a guide, because they are quick and easy, tempering it with a judgment of when a line is 'long' and when the number of fittings becomes significiant.

Suction or case drain lines are a different matter and I look quite closely at those, although usually judgement and velocity and specific gravity only, not calculations.

+1 on Parker comments. They are good in their niches, but not gods. They have good products but are big because of PacMan gobbling up every smaller company in sight. The website absolutely sucks for organization and to find a product. Even my local Parker rep uses the paper catalogs many times a day.

 

[subsearobot]

thanks everyone, enlightening points all around.