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    This short post is a mere reminder to all that hydraulic machines can have more than one hydraulic oil tank, and that not all double gear pumps are internally connected at the suction side.

    The first time I came across one of these was in the field, naturally. It was an agricultural tree planting trailer, equipped with sophisticated hydraulic leveling system that ensured that all trees were planted vertically regardless of the terrain, and also a combination of jacks to move the whole planting assembly up and sideways. The hydraulics was powered by a small double gear pump and the detailed schematics was available, showing the two pumps, two independent oil distributors and one oil tank.

    In the course of troubleshooitng process I required to test some priority valves I suspected were malfunctioning. During the test I connected a needle valve to the outlets to simulate loaded condition. The tank port of the needle valve had to be connected to the tank, of course, and the easiest and the fastest solution was to stick a hose into the filling cap. Don't laugh now, the expected flow rates were of around 2-5 liters per minute ( a gallon), so the oil shower risks were minimum, and, as the flow was so low, I actually had to SEE the oil stream to evaluate flow rate. 

    I turned the clutch on, and concentrated my attention on the pressure gauge as I was raising the pressure. When I looked up a couple of seconds later, I saw, to my GREAT SURPISE, the oil poring out quietly (sneaky bastard!) from the completely topped up oil cap! I rushed to the controls and disengaged the clutch to stop the circus, but all too late - the frame, valves, hoses and the ground underneath the tank got all covered with oil! And I HATE cleaning-up oil spills!

    When I calmed down and looked at the machine with the "seeing" eyes, I saw that it was equipped with another oil tank, and the double gear pump had a separating oil retainer in the intermediate plate. The two separate circuits were working with two separate oil tanks, and during my test I was using the wrong oil tank to dump the oil... I thanked out loud the good engineer who'd made the schematics, clearly showing ONE oil tank, and then slapped myself in the face (not literally) for not paying attention.

    This happens a lot, I mean people assuming that double gear pumps always work from the same oil tank. The reason for it is simple - most of the times, 99 out of 100 maybe, they do, and the standard, most common double pump assembly has suction sides of the pumps connected internally. But then there's that one case when they don't, and if you miss it and, say, supply a "normal" double pump instead of a "separated" one, or make a test like I did, you end up transferring hydraulic fluid form one tank to the other. Further consequences may vary form disastrous to hilarious...

    That case taught me to pay additional attention to all double pumps and systems using those pumps, and also to always check if the supplied schematics corresponds to the reality. So whenever you come across a double (triple) gear pump with the suspiciously "fat" intermediate plate, check if the system has more than one oil tank - consider this as another tip to keep you from unexpected hydraulic trouble.