Hydraulics is not my specialty, so please excuse me if I mis-use terminology, etc.
We have a design with a hydraulic cylinder that is required only for emergencies; it is very important for it to remain retracted during normal operation. The extension pressure line has a closed ball valve to prevent accidental operation. The retraction line remains pressurized (3000 psi) to keep the cylinder retracted, and a pilot operated check valve prevents rod end fluid from returning to tank unless there is extension pressure.
I am wondering about the reliability of this design, because it seems to me that fluid could leak past the piston seal from the rod end to the blind end. The difference in surface area (due to the presence of the rod) might result in the piston creeping, even though fluid cannot return to tank. There is no load on the rod in either direction.
However, here is the counter-argument. The piston was designed to hit a mechanical stop of machined steel in the blind end of the cylinder when fully retracted. I do not know the finish specification for the contacting areas, but am guessing it is pretty smooth but not mirror finish. This mechanical contact area is supposed to reduce the effective surface area of the piston face. Indeed, if it did so effectively, the rod side would have a greater surface area and the cylinder would remain solidly retracted regardless of leakage.
I am questioning this on a couple of points: a) a single piece of grit could completely eliminate the surface area reduction; and b) it seems implausible to me that the pressurized fluid could be excluded completely from the contact area.
Let's forget the grit issue for a moment. The main question is then, will the fluid that penetrates into the crack between piston and hard stop exert a force? I assume the scale of surface features even on a relatively smooth surface is going to be bigger than the hydraulic oil molecules, but perhaps surface tension is important in such small spaces.
If anyone can share their thoughts on this, I would be grateful.