These glorious images present main parts of a completely rebuilt Parker PAVC 100 open circuitpump, that worked for a couple of minutes,
before becoming the scrap. This is a
perfect example of how a new pump can become old in a very-very short
time due to, let's say, "confident incompetence" of technical personnel
involved in hydraulic equipment servicing and re-commission.
This is a CLASSIC case of bad start-up procedure. It
can not get more classic than this! Obviously, the person responsible
for putting the machine back in service didn't have a clue of how that
particular circuit was functioning, and assumed that, as long as this
type of pump had no drain line and the suction line was internally
connected to the casing, thus ensuring all the internal lubrication,
there was no need to take any special precautions before starting the
engine... Boy, was he wrong!!!
What was expected? The pump would, probably, make a couple
of "dry" turns, then would suck in the oil, expel the air to the
circuit, and there you'd have it - the system functioning,
everybody applauding, glasses of champaign being distributed by
beautiful ladies wearing only.... Well, let us just say it didn't went
as expected and look into it to see why.
First of all, this pump indeed has NO case drain,
and the suction line IS connected to the case. The pump is designed so
that it sucks oil though the openings in the cylinder block (pic.8),
and the suction line oil inside the casing is providing all the
necessary lubrication. All the leakage oil goes to casing and is sucked
back again by the rotary group. Pretty standard arrangement,
eliminating the need for a case drain. So what went wrong?
Well, two things weren't taken into consideration - suction line configuration and the pump's control.
Suction line - even though the pump's inlet was
below the tank oil level, the suction line was designed in a way that
formation of an air pocket was possible. After the oil tank had been
drained, cleaned and re-filled, a significant amount of air was trapped
inside the suction line piping.
Control - the pump's control was a closed center
load sensing control, which, by definition, meant the pump would go to
almost zero displacement as soon as the stand-by pressure would have
been reached in the pump's outlet. And, to top it up, the servo system
of this model is designed in a way, that both pressure induced and
mechanical forces cause the swashplate to go to minimum displacement
(take a look at pic.8,
you'll note that the centerline of the swasplate is slightly offsef
from centerline of the rotary group), which meant that right from the
moment the shaft started turning, there was a mechanical force trying
to destroke the pump.
What really happened? The combination of the two
factors described above mixed with the inexperienced yet self confident
technical personnel. When the engine was turned on, and the pump
started turning (immediately at nominal speed), it sucked the air
pocket from the suction line, and as soon as the air cushion pressure
at the pump's outlet (remember that it is connected to a closed center
distributor!), combined with the mechanical rotary group induced
forces, was enough to destroke the pump, the swashplate went to minimum
displacement, the pump stopped suction, very little oil had entered the
pump at that point, and the pump was turning completely dry up until the piston shoes started literally to melt down!!!At
that point someone probably noticed the unusual noise (and the smell)
the pump was making and stopped the machine, but it was already too
late...
Had it been a different control, combined with an
open center distributor, or a truly flooded inlet, the pump would've
been fine. In that case, the air in the suction line should have been
drained before starting the pump (there is even a bleeding plug in the
inlet port!), and the distributor should've been activated right after
the start up (some movements should've been made), to give the LS
signal for the pump to go full stroke and drain the remaining air. The
technician, performing the start up, shoud've had prior experience with
these pumps and be aware of what to expect and what "normal" and
"suspicious" pump sound is.
The saddest thing is the fact that the pump had been supplied with a set of detailed start-up procedures in writing, huge WARNING signs all over it and everything...
So, next time you re-commission your hydraulics,
make sure you know the particularity of the circuit, and your personnel
know what they are doing. Most of the times re-commission is not
difficult, but it is something more than just mounting an overhauled
component and pushing the "start" button.
Despite all the warning signs, caution notices,
attention signs, red flag signs, and other warnings, both written and
oral, I still see "oups!" start-ups every now and then... I am guessing
it will never stop, but I am pretty sure the InsaneHydraulics readers
won't be the ones responsible for that.
