I've corrected errors made in my earlier, middle of the night post here, and deleted the original.
Corrected version, hopefully fairly accurate.
I was meaning the difference between potential pressure, into a dead end, and actual pressure when flowing.
If the return was closed, then when the relief valve opened pressure in the return would be full pump pressure, (and the relief function wouldn't work). But, as the return is not closed, (ignoring any check valve, which will open at much less than pump pressure), then there will never be a high pressure in the return.
The pressure limiting valve relieves excess pressure to the suction side of the pump, not the return line , as I had written. The flow is small, as it's only has to be enough to relieve the excess, above 7.5 Bar. The need for the regulating valve to open at all, will depend on the pressure the pump produces, which, in turn, depends on engine speed. My point is that the flow from pressure side to suction via the regulating valve is, I think, going to be quite small, and probably intermittent, depending on engine speed.
With regards to an air leak, it only needs a tiny gap for air to be sucked in by the pump vacuum.
But such a small gap may not let fuel out. That's how the bypass/restrictor in the air separator works, passes, (thin), air, not, (thicker), fuel.
Seems to work very well too!
The relief valve only "vents" enough fuel into the return, to drop the system pressure to the safe level, so only a small volume of flow.
My second thoughts anyway.
Mac.
Yes that is what I mean. It is there on the right. A slight flat on valve seat. I sealed off the base of the valve and blew air through that gap.
I don’t think it is there though to separate air. I think it is to allow pressure to equalise throughout the return when not running. The valve sets the back pressure on the injectors when running.
Edit. SSP thinks it is an air bypass, I’m not convinced. Need to do some thinking.
View attachment 122657
I can see the tiny bypass path, (which I'd call a restrictor, and has to be very small to work), passing air, but not fuel.
The pressure limiting valve keeps the return line at 1 Bar, fair enough, prevents fuel drain back.
Not sure about the "force equilibrium" though, where is the solenoid needle valve referred to located?
Mac.