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    When a hydraulic "repairman" comes to the critical point in his career, where he begins to consider the use of a flow meter as something useful and necessary rather than a back-breaking burden, he automatically evolves from a "grease monkey" to a "technician". It is from that point on that he starts to carry that turbine and the needle valve everywhere he goes, and pump tests make more and more sense.

    In my early hydraulics years, every time I had to "apply" the stupid turbine, as I used to call it, I was imagining all the hard work the thrilling operation would give and all the oil my poor self would have to soak. With practice came experience, and now I know for a fact, that "avoiding" the use of a flow meter when troubleshooting complex hydraulic systems is like avoiding the use of an x-ray machine when treating a broken leg. I am not saying it's an easy or pleasant procedure, I'm saying that it should not be considered as something "extra".

    Flowmeters are widely used to test hydraulic pumps "in the field", as the pump is usually the suspect number one when loss of pressure/force/speed is detected. The best way to take it out of the troubleshooting equation, is through flow-testing under load, typically with the help of a needle valve to induce pressure. In most cases the time you spend to install the test gear is less than the time you would waste wondering around the machine and guessing what might be wrong without being sure the main pump is OK.

      Since the pressure is induced by flow restriction, ALL of the input energy is transformed into heat during such tests. This oil super-heating can be, generally, overlooked when you test an open loop pump, especially in systems with large oil tanks, as normally the time necessary to test the pump and make all the necessary adjustments is not enough to raise the oil temperature to unsafe levels. But what about closed loops?

     When you need to "field-test" a closed loop pump you, obviously, need to create a closed loop. In most circuits the loop flushing system will be placed inside the motor you've just disconnected, which means that when you start restricting the loop flow with the needle valve, it will be, basically the same very reduced amount of oil (minus the leakage, of course) going over and over around the loop, and which you will be super-heating with your needle valve.

     Just as an example, imagine that you have 100 lpm (around  25 gallons per minute) flow in the loop, and the pump test pressure is, say,  450 bars. The power transformed into heat will be 450*100/600= 75 KW. Let us say you've used around 4 meters of a one inch hose to create the loop. Along with the flow-meter, the whole loop will hold just around five liters of oil. Can you imagine how fast the temperature will climb when you are heating five liters of oil with a 75 KW heater? (Didn't count the heat taken away with the leakage flow, but the point remains the same - the loop over-heats fast!).

    I personally have witnessed water boil on test fittings after only a minute long tests. Although it is possible to test closed circuit pumps in this simplified manner, it is far from being perfect, as you are forced to make tests and adjustments in a "lightning fast" fasion, and also have to wait for some time in between the "test bursts" for the loop to cool down, unless, of course, your goal is to deliberately bust the pump. The risk of severe oil-overheating with all of the consequences will always be present.

   This is just an example of such a test I made a few years ago. It went well, but I had to wait for fifteen minutes in between the tests to cool the hoses down. Give me a call if you read this, James (James is the rig's owner I lost contact with).

     The best course of action for closed loop pump field testing is to use a calibrated pressure compensated flow controller, connected after the needle valve, that diverts controlled part of the loop oil to tank (schematics). There are two main advantages in this layout. First one, of course, is the fact that the oil will not overheat so fast, allowing you to make more extensive tests and adjustments. The second advantage is less obvious.

    Think a little about a closed loop function - as you know, the charge pump must provide enough flow to compensate for the internal leakage of both the pump and the motor. When you are testing your closed loop pump with a simple loop, you take the motor leakage out of the equation, which, in  cases of marginal efficiencies, can make all the difference between stamping the pump as "good" or "bad". Using a calibrated flow control valve to vent controlled part of the loop oil to tank is the perfect way to simulate the motor's internal leakage, thus making the test more reliable.

   Note that it is important to use a calibrated flow controller, to have an idea about the flow you are "stealing" from the loop. The one I use the most has almost linear characteristics, giving approximately 3 liters per minute per turn, 30 liters per minute maximum flow. All I have to do is count the turns and multiply them by three lpms.

    This technique also allows you to evaluate roughly, when needed, the maximum amount of leakage the given pump can take, as you can easily detect the point when the charge pressure drops. Or, if you know the charge pump flow, you can estimate the pump's internal leakage, again by noticing the exact "stolen flow" rate at the moment when charge pressure drops, and subtracting it from the charge pump flow. If you don't know the charge pump flow, you can estimate it by noticing the "leak" flow when the charge pressure drops, with the main pump at zero displacement. If you know the rpms, you can estimate the charge pump size, the list goes on... This is why using a flow regulator is better than using a flushing relief valve.

    This is a picture of a closed loop pump field test I made just a few days ago. It was a large and old A4V250, working at 350 bars and a thousand and something rpms, I still used my old 30 liter regulator, but I had to open it all the way. You can see from the picture that the loop hoses I used were quite short, yet I was able to test and adjust the pump (both pressure limiters and the cut off) without stopping, and the temperature inside the loop never rose over 70 C!

    If you understand how it works, this technique is very easy to apply and doesn't require much investment, providing more complete test results and facilitating the adjustments.

P.S. Please do not laugh at the flow meter from the pictures, despite the looks and age it is still quite accurate and can tell lots and lots of field stories...
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