Quote Originally Posted by sterl View Post
The TS-Pilot Receiver: otherwise HPR...most likely has an internal weir at +/- 50% and once it clears the weir, a float valve at the "high" vessel delivers liquid to the medium temp vessel: That about right?

Any chance this little condition you got follow a compressor shutting itself off?

When a compressor shuts down on a T-Sypon arrangement: The liquid in the pilot receiver (reservoir) drops quickly...That because on standby the liquid in the Thermosyphon Return Line rises all the way to the weir inside that vessel...Whereas while there was oil flow that vertical would be near-dry.

So a compressor shutting down causes you float to close...Your suction pressure was low already or the machine would have stayed on: if it shut down on low load...SO

Float valve closed. No liquid to MT vessel. MT vessel pressure drops further. Liquid in level column froths and Hansen probe sees it as quick rise in liquid level. With no flash gas to vessel compressor simply pull vessel down to lower pressure. Liquid in vessel starts to cool and that generates "submerged vapor" like what happens when you snap the cap off a bottle of warm beer.

Pump pumps froth. No differential pressure. No flow through cooling line or bypass...Pump stops on Diff Pr Control.

5-seconds later: Stability resumes and pump will start and hold....

This can also happen when a condenser pump kicks in while the outdoors is cool...the thermosyphon vessel looses its continuity of liquid cause this new condenser sucks back on the liquid delivery and the float closes...And the whole thing follows through as described. I expect your intermediate vessel sits high and the inlet piping to the pumps is not quite clean; but can't tell that from here.
As far as I remember nobody mention that this issue happen when compressor turn off. Even when it off, it takes time to cool oil in the oil cooler. So it will take time for liquid to fill up the return line.
I believe in sudden change of the load. It can happen if several evaporators off or make up solenoid close.