Insane Hydraulics
Bold and audacious blog about fluid power
These glorious images show a completely rebuilt Parker PAVC 100 open circuit pump, that worked for a couple of minutes before turning into scrap. This is a perfect example of how a new pump can become old in a very-very short time due to "confident incompetence" of technical personnel involved in hydraulic equipment re-commission.
This is a classical case of a bad start-up procedure. It can not get more classical 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 the oil in, expel the air out into the circuit, and there you'd have it - the system functioning, everybody applauding, glasses of campaign being distributed by beautiful ladies... Well, let us just say it didn't went as expected and let's try to discover why.
First of all, this pump indeed doesn't have a case drain, and the suction line is connected to the case. The pump is designed so that it sucks oil through the openings in the cylinder block, and so the suction flow through the casing is providing all the necessary lubrication and cooling. The leakage 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 in that particular system - suction line configuration and the pump's control and design peculiarities.
Suction line - even though the pump's inlet was below the tank oil level, the suction line was designed in such 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 swash-plate to go to minimum displacement (take a look at pic.8, you'll note that the center-line of the swash-plate is slightly offset from center-line of the rotary group), which meant that right from the moment the shaft started turning, there was a mechanical force trying to de-stroke the pump.
What really happened here? When the pump started turning (at nominal rpm right from the start, by the way) 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 directional valve), combined with the mechanical rotary group induced forces, was enough to destroke the pump, the swash-plate went to minimum displacement - but since very little oil had entered the pump at that point, it turned completely dry up until the piston shoes melted down! At that point I believe someone did notice 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 valve, 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 for that), and the directional valve 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, should have been aware of what to expect and what "normal" and "suspicious" pump sound was.
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, emails and oral warnings I still see "oups!" pump start-ups every now and then up to this day. I am guessing it will never stop, but I am pretty sure the readers of this blog won't be the ones responsible for that.