Hydraulic Hose theory

JTknives

JTknives

Blade Heat Treating www.jarodtodd.com
Knifemaker / Craftsman / Service Provider
Joined
Jun 11, 2006
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So I have been doing a rather large amount of research into hydraulic systems and how to properly design them. And after all this research I have come to find out it's not as easy as I thought. First we need to narrow down what our goals are. This has also change over time from more tons to faster ram speed. So that's the direction we will look. Top Priority is ram speed and in that category I would put cylinder responce rate. By that I mean how long does it take from when I cycle the spool for the cylinder to move.

One thing I did not take into account early on was the compressibility of the fluid and the volumetric expansion of the hose it's self. I have seen numbers tossed around that with those two factors added togather adds up to over 40% increese in fluid in the hose. I started researching this when I came across an article saying using hoses to large can cause cylinders to become sluggish. It all comes down to fluid FPS in the hoses. Industry standard is 15-20FPS fluid velocity in hoses. Anything drastically over that number can start to cause cavitation and shorten its life. Contrary to what I always believed hose diameter does not affect flow rate just pressure.

So let's compare a 1/2" hose to a 3/4" hose connected to a 7gpm pump. A 1/2" hose would generate 11.5FPS and a 3/4" hose is 5fps. Each hose also has a pressure drop rating. 1/2" is around 10psi per 10ft and 3/4" is under 1.5psi per 10ft. So considering we are not running a lot of hose it does not affect us much. But the kicker is volumetric expansion of these hoses. Even if both hoses have the same VE factor the larger hoes will hold more fluid to the order of 2.25 times. This means the pump has to pump 2.25 times more fluid to pressurise the hose to the same pressure as a 1/2" hose. You might not think this is a big deal but it can be. Let's take say a 5ft section of both hoses. The 1/2" holds 11.8 cubic inches of fluid and the 3/4" holds 26.5 cubic inches. If we say the VE combined with fluid compressibility is 40%. At pressure it gives us an increese of 4.72 cubic inches on the 1/2" and 10.6 on the 3/4" hose. If we calculate the motor rpm and the pump output it will give us the delay from zero pressure to full pressure. My motor is 1745rpm and the pump is .92 cubic inch per revolution. With a 1/2" hose the delay is 5.13 revolutions and the 3/4" is 11.52 revolutions. Considering each revolution takes .0343839sec we get a time delay of .176sec for the 1/2" and .396 sec for the 3/4" hose.

All these numbers are really just fluff numbers to show a point that selecting the right hose is important. I don't know if the 40% is accurate or not or if it's for only one type of hose. But what it does show is the difference between these two hoses and how over sizing your hoses could end up hurting the very performance your going after. One thing larger hoses have going for them is larger fittings. Fittings have a much larger pressure drop then hose, especially 90° fittings. But you can work around this and kinda have the best of both worlds. Use reducers to and from valve assembles, pumps and cylinders. This way all your fittings can be say 3/4" which have a MUCH lower pressure drop then a 1/2" fitting. This will keep the pressure up and the smaller hose will keep the flow velocity up. It's funny becaus using larger hoses is a technique used in systems where excessive hydraulic noise is an issue or the systems need to be softened up alittle. The other great thing is smaller hoses are rated to a higher max pressure and much stiffer in responce rate. The other big advantage not normally discussed is hose bend radius. Each type and size of hose has a minimum end radius. If you exceed this minimum you compermise it's strength and life. 3/4" hose is a 4.75" minimum and 1/2" hose is 3.5" minimum. The funny thing about all this is the experts say for persized systems use 10-20fps for fluid velocity. If the pressure is constant then stay up closer to the 10fps but if the pressure is intermittent like our presses are then to stay at the upper end of max at 20fps. This means for my pump 6.95gpm and 20fps I need a 3/8" hose. Now don't that just go completely against what we have allways been told. And what's also funny is I'm running a 10hp motor so I'm putting out higher volume then most others out there that are normally around 5HP. You don't want to do the math on those pumps and the hoses thy should be running lol.

So I dont know if I'm way off in left field or not. All I know is what months of reading has shown me. So I greatly welcome any conversation on this topic here. I would be highly interested in what specs your running and how you feal it performs for you. Thanks guys for listening to my rambling like allways.

P.S. here is the chart that shows the hose size you need for your given flow rate.
Nomograph1.jpg
 
One more quick thing. A lot of people might be scared away from the pressure drop in something like a 3/8" hose which with my pump 6.95gpm is around 30psi per 10ft. You can loose that much pressure with a single 90° fitting. These small ID hoses have very tight min bend radius at around 2.5" which makes it possible to not need alot of fittings that you normally see in most press builds.
 
Not much to add but good luck on your build & learning new stuff.
Thats just as, if not more valuable as the finished product.

As a Electrician Millwright, I done fair bit of hydraulic repairs & reconfiguring stuff but never really design anything.
 
I read this twice and have made what should have been an obvious discovery................... You, sir, are much much more intelligent than I....
 
FWIW, you can usually assume that fluid is INcompressible. Most of that volumetric increase comes from the hose stretching and increasing diameter, not the fluid compressing. Now, if you get gas trapped in the system, that’s a different story.

In pipe flow (or hose flow, as the case may be), you get a rapid boundary layer growth on the walls inside the “pipe” as soon as the flow begins to move. This boundary layer thickness is determined primarily by the fluid properties (largely by its viscosity), and also to a lesser extent by the flow rate/speed (this can all be non-dimensionalized into a value known as Reynolds number).

This boundary layer will generally grow asymptotically to a nearly fixed thickness, regardless of pipe diameter. Thus, the thickness of this boundary layer becomes extremely important when your goal is flow rate and/or velocity rather than pressure. Too small a pipe, and you’ll choke the flow or get cavitation if the velocity is too high. Once the diameter begins to get larger than twice the boundary layer thickness, you will start to see potential flow rate increase by almost a squared factor with diameter. The boundary layer is the same thickness in a 1/2” hose as it would be in a 36” pipe, given the same fluid properties and velocity.

In summary, you want the smallest diameter hose that will give you the required flow rate, because as the cross-section diameter increases, you have more potential for hose wall flex. Since the volume will increase with the square of hose diameter as it stretches, you get to a point of diminishing returns.

HTH.

ETA: Flow rate is also generally fixed/determined by the smallest diameter in the ENTIRE wetted system. This is usually a fitting (90° are generally the worst case) or a flow path inside of a hydraulic cylinder, so keep that in mind too, especially if considering any kind of diameter reducer. Use of those can completely compromise an otherwise good design if you’re not aware of the above info.
 
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i4Marc said:
Couldn't you use steel braided lines to resist VE in the hoses?
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Most hydraulic lines are steel braided, with the 2 wire being the most common. That is 2 separate steel wire braids wrapped around the hose in layers. These hoses are what I’m talking about, thy can have a rather large VE ratio.
 
JTknives said:
Most hydraulic lines are steel braided, with the 2 wire being the most common. That is 2 separate steel wire braids wrapped around the hose in layers. These hoses are what I’m talking about, thy can have a rather large VE ratio.
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JT , father of my wife produces/ mounting fittings/ on high-pressure hydraulic hoses 30 year .If you need same answer I can ask him . He knows this stuff ........
 
Spyder, I've never put any math to it but I've been very aware of that boundary layer thing. I have spent a good bit of time and money building giant wood stoves to heat my greenhouses. The commercial models I started with completely ignored this boundary layer and got crappy heat exchange. I never got scientific about it but I locked onto the idea that to get heat exchange the material ( hot gasses on one side and water on the other) must "scrape" the surfaces of the heat exchanger. We also use an older Barko loader and slasher saw to handle the firewood. Constantly changing hydraulic hoses on that old girl but if you can get a log in close enough she will handle 30,000 pounds.
Interesting discussion y'all. If you want it to work well, do not ignore the math.
 
I'm sure this is a stupid question, but what about steel pipe instead of hose? The front end loader on my tractor uses hose where it has to move as the loader moves, and pipe in all the straight runs between joints. I haven't studied this stuff, but it seams like that would mostly eliminate the tubing expansion? But I'm sure there's a reason to use hose, that I'm just not considering.
 
Jesse Smith said:
I'm sure this is a stupid question, but what about steel pipe instead of hose? The front end loader on my tractor uses hose where it has to move as the loader moves, and pipe in all the straight runs between joints. I haven't studied this stuff, but it seams like that would mostly eliminate the tubing expansion? But I'm sure there's a reason to use hose, that I'm just not considering.
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Only cost and ease of building. It's another skill set to plan and run rigid tube.
 
kuraki said:
Only cost and ease of building. It's another skill set to plan and run rigid tube.
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definitely a whole other set of tools. Benders, double flaring blocks, and a press, etc.

They are professionals, so you have to know what you are talking about when you order from them, they make hot water hoses complex if you call unprepared. www.hose-man.com
 
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