sizing tubing for a hydraulic system- line losses
sizing tubing for a hydraulic system- line losses
(OP)
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$$ of myself by assuming that erosion is very long term issue, which will out-survive the rest of my equipment.
so, how can Parker, the god of hydraulics, make such a simplification? am I missing something? what about turbulent flow?
please discuss and enlighten me!
thanks in advance!
An SSP design handbook has a table which correlates pressure loss per foot of pipe and flow rate with tube size. What I see looking at this table is: as flow area increases for a given line speed, pressure losses per foot *greatly* decrease. Especially in the size range where I am concerned (low HP systems). I assume (I know...) this table comes from bernouli eqns.
I also notice in lines over about 1/2", 25 fps line speeds are in the turbulent regime, but line losses still tend to decrease with increasing flow area.
I tend to design to acceptable losses, as opposed to the parker method. I'm not quite sure what to make to the turbulent flow issue- my only thought here is that turbulent flow would tend to erode lines from the inside, but again I am making as a$$ 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!





RE: sizing tubing for a hydraulic system- line losses
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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
RE: sizing tubing for a hydraulic system- line losses
Sorry.
Ted
RE: sizing tubing for a hydraulic system- line losses
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?
RE: sizing tubing for a hydraulic system- line losses
Mike Halloran
Pembroke Pines, FL, USA
RE: sizing tubing for a hydraulic system- line losses
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.
RE: sizing tubing for a hydraulic system- line losses
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.
RE: sizing tubing for a hydraulic system- line losses