Insane Hydraulics

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Measuring Low Flow Rate With a Needle Valve

Last weekend, I had to go back to the H1B motor hydraulic proportional displacement control with its mysterious built-in control leakage - the issue is still "under investigation" and, hopefully, the client will soon send us one of these motors for a closer inspection and I will be able to run a series of tests on the control (and, of course, conduct a proper back-engineering session and report on the results), but today I want to talk about the flow-gauging trick that I had to "deploy" to evaluate the tiniest flow rate in the pilot line without having proper flow-measuring gear.

Every tech involved in troubleshooting hydraulic systems knows that intermittent failures are the worst to deal with - the machine works fine for a while, and then, suddenly, it does not, but when the tech arrives at the scene - it's all good again. The malfunction I had to deal with last Sunday was exactly that - the core-drilling rig "acted out" a couple of times on the surface, and after we tweaked the displacement control threshold setting - it was tested tens of times and never failed. And then, when it got to the underground drilling site - the pilot pressure dropped again, causing the loss of drilling speed. It stayed like that for a while, and - boom - it was OK again all of a sudden! I was, actually, very lucky to have caught the exact moment on video - I haven't asked the client for permission to publish it, which is why I am not disclosing any details yet, but I will definitely do that, and hopefully, I will soon be able to show you exactly what I am talking about.

But for now, all you need to know is that I had to somehow evaluate the tiny flow rate in a pilot line, and I didn't have the right gear for that. I am talking about a flow rate of less than one liter per minute. The pilot line of the hydraulic motor was supplied through a 0.6-mm orifice from a 40-bar source, so for a pressure drop of 10-20 bar I would be looking at 0.5-0.7 liters per minute at most, and I wanted to see if I could detect any changes in the flow rate - which would be an indicator that oil consumption of the H1B displacement piloting piston varies dynamically - but, once again, I will detail on that in a separate post, for now let us concentrate solely on the flow rate measurement technique.

Ideally one wold need a small volumetric flow meter for that (for example - oval gear), but since I didn't have one - I had to use a different solution, and here it is:

Obviously, when you know the characteristics of an orifice, you can evaluate the flow rate by reading the pressure differential across it. I didn't have a calibrated orifice, and I had no access to the 0.6-mm orifice hidden in the control panel of the rig, but I had the next best thing - a "repeatable" orifice, so to speak, i.e. a needle valve, which I could insert in the line, set to a certain position (positions), log the pressure drop, and then recreate the test in our shop putting the valve at exactly the same setting (by counting the number of complete turns from the closed position) - and figure out what magnitude of flow-rate I was dealing with.

For now, all I can tell you is that the tests confirmed an oscillation in the consumption of the pilot flow between 0.35 and 0.8 liters per minute, and I don't know why it was happening, but I sure as hell hope that I'll figure this one out. And in case you wonder about the case pressure of the motor - I read it as well, and it was at steady zero bar at all times, with the drain line running straight to the tank.

Now I want to submit to your attention the empirical reference numbers for the needle valve that I used in my tests - the VRFU 90º 1/4 from OM (Oleodinamica Marchesini) - these valves are cheap, but the build quality is not bad at all and the adjustment is quite precise.

Here you can see me running the flow test of the valve on our test bench. I let the oil (mineral, Vg46) heat up to about 35-40 Cº before logging the flow rates, and you can also see that I used two "normal" digital pressure gauges along with my wireless pressure sensors - I did this because the app can calculate the delta P between two readings in real-time, which is super convenient, and I used the "occasion" to double-check if the sensors were still working properly. I was determining the flow rate with my oval gear volume counter - I would run the valve for a minute (two to three minutes for lower flow figures) and then see how much oil went through the counter. The tests were performed from minus 2 to minus 6 turns, and I used a second needle valve to control the flow - I would close it and then open slowly aiming for the target pressure drop. I logged the flow rate for drops between 2 and 20 bar in 2-bar increments. Using a relief valve instead of the restrictor would probably be better - but I already had that needle valve as a convenient tool prepared for the bench fast couplings - and I opted for using it to save some time.

Here are the results in a table, the leftmost column corresponds to the numbers of complete turns from the closed position, and the topmost row indicates the delta P figures (from 2 to 20 bar):

2 4 6 8 10 12 14 16 18 20
-2 0.04 0.06 0.08 0.12 0.15 0.19 0.22 0.27 0.42 0.54
-3 0.09 0.21 0.37 0.57 0.80 1.05 1.25 1.49 1.75 1.95
-4 0.42 0.86 1.40 1.97 2.44 2.88 3.28 3.76 4.16 4.53
-5 1.50 2.24 3.19 4.11 4.68 5.28 5.84 6.35 6.88 7.30
-6 4.83 7.27 9.03 10.67

The same results in a graph form:

So, as you can see - having these numbers allows me to use this needle valve as a primitive flow meter capable of estimating very small flow rates. This technique has obvious drawbacks, and the use cases for this measurement are limited, but the technique itself is totally valid. Actually, it just occurred to me that I could use this needle valve rick to evaluate flow in a dynamic load sensing line - something that I've been thinking about for a long time.

Anyhow, if you run a similar test on a needle valve of your choosing - you will get yourself a high-pressure and tiny flow rate capable flow meter basically for free!

P.S.:

When I got back to the shop on the next day, it occurred to me that engraving the average flow increment values on the side of the valve would make it even better, so that's what I did: