One of the first basic components you learn when you enter the world of
hydraulics is the RELIEF VALVE. Why? Because most of the times it is
the only component that prevents all that other hydraulic stuff from
exploding into your face. You learn that its function is important, its
correct adjustment is essential, and that fiddling with it is very
dangerous.
Even those who are only remotely connected to hydraulics,
have a general idea of what a pressure relief valve is and that its
main purpose is to limit system pressure. This is good, because the
valves are important, and this is bad, because those fearless fellows,
who know no other hydraulic components, but aren't fiddle-shy, will
tamper with the relief valves every time their machinery has a
malfunction, before handling it over to your work-shop.
It would be OK, if they left the adjustments as
before, but in most cases, the poor valves end up adjusted to
ridiculously abnormal values (or even completely blocked), which can
seriously jeopardize or render useless any further repairs.
This is much more common than you might think, and
that's why it is very important to verify correct adjustment and
function of pressure relief valves whenever you test a hydraulic
component equipped with one. Unfortunately, even experienced hydraulic
technicians sometimes forget or intentionally omit this simple step.
An example, something I saw just this morning - a
simple test of a simple power-pack, which had gotten a brand new size 1
gear pump. The power-pack was dead simple - electric motor, pump, tank,
pressure gauge, manual distributor and a relief valve - that's
it. The technician testing the rig had many years of experience, which
was promising a quick and painless test...
It was the unusual noise the pump made when
the power-pack was turned on that caught my attention. As I glanced
across the shop, I could see the analogue pressure gauge from a 10
meter distance. I wasn't able to see the reading, but I could see the
pointer at the end of the scale, and I knew it was a 400 bar gauge. A
thought crossed my mind - 400 bars - gear pump - holy crap! The rig was
immediately shut down, and, luckily, the pump withstood the offensive
treatment with pride. It did give the technician something to think
about, though...
I have seen this situation (with variations, of
course) way too many times - a hydraulic machine "looses pressure", and
the "troubleshooting circus" begins - with the first thing to "check" and
fiddle being the pressure limiter, of course. A mechanic (who believes
a common myth that all hydraulic valves are made on a "turn clockwise =
increase" basis) turns the adjusting screw one turn in - nothing
changes, another - nothing again, yet another - nothing! (this troubleshooting technique is
described here)
Then says something like - Well, my work here is done, give the rig to
them hydraulics guys, they'll know what to do about it!... Sounds
familiar, doesn't it?
Another similar attitude I get to see way more
often than I'd like is the adjustment of pressure relief valves
incorporated in closed loop hydraulic pumps (I am referring to the
models that have those valves adjustable, of course). Normally, such
pumps have two separate pressure limiting systems - one limiting
pressure by destroking the pump (like Hydromatik cut-off valve in
A4VGs), and one limiting pressure through cross-port relief. As a
rule, the first pressure limiting system serves to actually limit the
system pressure, and the second one is to cut through spikes and limit
pressure when the first one wasn't fast enough. This is why the first
system setting is some 30-50 bars lower than the second one, and is the
main reason the relief valves "skip" testing, as, unfortunately, often mechanics
assume that, as long as their setting is higher than the pressure
cut-off system setting, it is "presumably OK".
Skipping the adjustment of these valves does save
time and trouble (sometimes quite a lot of trouble, due to the
condition and the position of the adjusting screws, and, oh yes, the
temperature) but is a VERY BAD HABBIT due to the exact same reason I
described before - it is probable
that before sending the malfunctioning pump to your shop someone had
fiddled with the pressure relief valves during the "troubleshooting
venture", most likely leaving them blocked!
Besides simply checking a relief valve's setting, it is also very important to check it's adjustability
by lowering and increasing its adjustment value and checking its
response and repeatability. It may help you discover a malfunctioning
valve you wouldn't find otherwise, and will give you an idea of the
valve's bars-per-turn ratio, which is a handy future reference.
Whenever you come across a pressure relief valve in
a malfunctioning component, always check both its setting and
adjustability. It might take you an extra minute but is a good
practice, which in the long run will pay you back. Whenever you are
about to re-commission a machine that had gotten a new or overhauled
pump, make it a habit of yours to at least identify the main pressure
relief to see if it had been tampered with, although the safest
practice would still be loosening the adjusting screw and starting the
machine or power-pack with a low limiter setting.
Most importantly, no matter what hydraulic machine,
rig, pump or motor you are testing, do NOT ask yourself whether
to test or not to test this or that part of it - THINK OF A WAY to test
it! Practice shows that parts labeled "presumably OK" are often the
ones to cause the malfunction in the first place, so when I am asked a
question "to test or not to test?" I always respond "claro que
TEST IT!"
Beware of fiddlers, tamperers, tinkerers
and meddlers - they are out there and are longing for adjustments...
P.S. There are closed loop pumps that use
multifunction vales (like Sauer Danfoss series 90), which perform both
of the two functions - pressure relief (cross port) and pressure
limiter (destroking), and have a fixed non-adjustable off-set pressure
value between the two, which in case of the Series 90 rounds 30 bars,
for example. Such valves have only one adjustment for both functions.
