I saw a couple of check valves in a type of hydraulic system that I consider myself to be very familiar with, and, believe it or not, I couldn't figure out why they were there! After trying (very hard) to "crack" the logic behind them, I had to ask the OEM tech that was there. To my credit - the man got just as surprised as I was and also confirmed that he hadn't seen this application of check valves before, but he was very kind to call his colleagues at the headquarters and inquire about the function of the check valves - and got an answer that explained everything.
Even though I am feeling ashamed that I didn't figure this one out on my own, I still want to share this "check valve scenario". It's definitely not common, but it's pretty cool!
So here it goes. The hydraulic system in question is a very big closed loop devised to drive hydraulic cylinders. When I say very big I mean three R15 two-inch hoses for the tail end and two for the rod end. The schematic is simplified (for obvious reasons) but the point is - I am used to seeing these loops built like the drawing on the left, and what I found was the drawing on the right, with two SAE 6000 - 2'' check valves sandwiched between the hose ends and the cylinder ports!
Before I give you the answer - take a moment to reflect on their purpose. Any theories?
OK, I'll tell you. That particular installation was unusually big - so big that the volume of the oil in the connecting hoses was comparable to the volume of the cylinder. If the hoses were connected in the classic (check-valve-less) manner, the oil inside them would not recycle properly, leading to all sorts of problems in the future, and so the insertion of a check valve in one of the parallel lines made sure that part of the respective cylinder chamber volume would renew on every cylinder cycle!
I find this "flushing" solution rather elegant.
The max rod flow is less than 1000 l/min in this system, which means that even with the check valve "enforcing" single hose operation during rod retraction, the flow velocity in the 2'' hose is still below 8 m/s, which is "still acceptable".