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...
