Insane Hydraulics

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Pressure Drop of an Elbow Fitting

I often deal with efficiency issues of all sorts of industrial hydraulic installations, and from my experience I can say:

first - I am yet to find a system that can't be improved (efficiency-wise),

second - for about 50% of the systems that I come across you can safely put the word "drastically" before the word "improve" in the sentence above, and

third - nobody cares about it as long as the equipment does what it is supposed to do.

That's the state of affairs with industrial hydraulics these days, folks. I believe that in the future, when all hydraulic systems are battery powered (which will only happen long after everything else is battery powered) the low efficiency of traditional designs will inevitably have to be addressed, but that's in the future. Like in a very distant one. For now - we've got what we've got.

Now, low efficiency caused by component or design choices is hard (if not impossible) to correct without a major investment, but there are always simple things that can be improved. One of the first such things to look at would be the routing, more specifically - the 90-degree fittings, which when "over-applied", can make a hydraulic system less efficient by introducing unnecessary pressure drop in constant flow applications (like permanently running hydraulic motors and stuff). I am talking about these bad boys right here:

We all know that they are kind of bad, and the fewer we install the healthier a line is, but let me ask you a question: Do you know how bad of a pressure drop do such elbow-fittings create? Do you have a ballpark figure even? And don't give me the usual "it depends", all right?

To be honest - I don't know for sure either. But you know what I can do? I can try and get that ballpark figure through good old hands-on testing. That is exactly what I did, and in this post, I am happy to present the results of my "amateur scientific experiments".

The idea was to measure the pressure drop of standard 90º machined elbow fittings (up to 4 fittings connected in series) in three "flow-bearing" conditions:

Our test bench goes up to 95 l/min, which means that a standard 90º -12 (1.1/16") JIC elbow, with a hole of 15mm would be the "good" fitting (9 m/s), the -10 with its hole of 12mm would be the "slightly undersized" (14 m/s), and the -8 with its hole of 10 mm would fall under the category of "grossly undersized", pushing the speed of the fluid up to 20 m/s at 95 l/min. Just what we need! I also threw in the 3/4 BSP sweep elbows (ø14 mm) for good measure - to see if the sweep elbows behave that much better. You can use this nice interactive diagram to get flow speeds fast when you need to, by the way.

Here are the "test supplies":

And here's the test setup:

As you can see, I am using my wireless pressure and flow measuring system - it makes measuring and displaying a delta-P along with the flow reading so much easier. The bench runs with VG46 mineral oil, and I kept the oil temperature between 35ºC and 40ºC in all the tests:

Some pics of the testing process:

And finally, here are the results (pressures in bar):

30 l/min 60 l/min 95 l/min
ø14mm, Sweep: 0 0 (3.2 m/s) 0.2 (6.5 m/s) 0.4 (10.3 m/s)
ø14mm, Sweep: 1 0 0.3 0.8
ø14mm, Sweep: 2 0 0.4 1.1
ø14mm, Sweep: 3 0.1 0.6 1.4
ø14mm, Sweep: 4 0.1 0.8 1.7
ø15, JIC -12: 0 0 (2.8 m/s) 0.1 (5.7 m/s) 0.3 (9.0 m/s)
ø15, JIC -12: 1 0 0.2 0.8
ø15, JIC -12: 2 0 0.4 1.2
ø15, JIC -12: 3 0.1 0.6 1.6
ø15, JIC -12: 4 0.1 0.8 2.1
ø12, JIC -10: 0 0.1 (4.4 m/s) 0.5 (8.8 m/s) 1.3 (14 m/s)
ø12, JIC -10: 1 0.1 1.0 2.5
ø12, JIC -10: 2 0.2 1.5 3.8
ø12, JIC -10: 3 0.3 1.9 4.8
ø12, JIC -10: 4 0.4 2.4 5.7
ø10, JIC -8: 0 0.2 (6.4 m/s) 0.9 (12.7 m/s) 1.6 (20.2 m/s)
ø10, JIC -8: 1 0.5 2.1 5.0
ø10, JIC -8: 2 0.8 3.3 7.8
ø10, JIC -8: 3 1.1 4.4 10.3
ø10, JIC -8: 4 1.4 5.6 12.8

The most obvious conclusion: the sweep elbows, even with their smaller diameter, perform better than normal (machined) elbow fittings, however, the difference becomes apparent only above certain flow. (Don't forget that I tested the sweep elbows first, and therefore with colder oil).

The 2-bar pressure drop of the 4 "good" elbows may not seem like much, but you always need to look at things in perspective. For example - if a hydraulic motor line employs four elbows, most likely the second line will be exactly the same, so the pressure drop will double. 95 l/min at 2 bar is about 300W of lost power, which is a whole percent in a 30 kW system! Four elbows in each leg of a hydraulic motor will actually make it two percent! So, if the elbows were installed for "purely aesthetic purposes", this means that in essence, the "builder" robbed the system of two percent of its available power, and then added a 600W heater to the oil tank!

Measly two percent don't matter, say you? Would you like a two percent salary increase? See? There's your answer! And these things compound, you know. Plus - let's not forget that often the lines and fittings are slightly undersized due to economical reasons, which pushes the flow-bending velocities way beyond the relatively correct 10 m/s, with the pressure drop increasing exponentially, making things way worse - something that you can witness if you look at the pressure drop values of the -10 and -8 fittings in the table above.

It is interesting to note that the pressure drop of an elbow fitting seems to be more or less reliant on the fluid speed, especially if you look at how close the pressure drops of the -10 fittings at 95 l/min and -8 fittings at 60 l/min are (14 m/s vs 13 m/s).

So yes, taking excessive elbows from lines matters, and you should do it. Obviously, this only makes sense for lines with a permanent flow that is high enough for such a change to matter.

Now, I do realize that elbow fittings are a "necessary evil", so to speak, and sometimes you can't finish an installation without them because there simply aren't any other type of 90º fittings around. I totally get it. But, still, there are two things that you should always do:

a) if there's a possibility to install a 90º hose tail instead of an elbow fitting, go for the 90º hose tail, because it is most of the time a good sweep elbow, and

b) make sure that you size the elbow fittings (and the lines) so that the max flow velocity in them is below 10 m/s.

Do these two things, and you will be golden!

Also - if you ever decide to upgrade a line, you should always carefully measure the pressure drop before introducing any changes, and then take the same reading after the changes are made. You'll be surprised how much of a difference can be made by removing unnecessary elbows and/or "elevating" a line's dash size!