These compressors (2 high stage, 2 booster ) each have a "viltech" controller, so pressure, temperature and cycle timing are more accurately controlled, (no system PLC) and yes this does cycle off as I do not like to let them run for extended periods of time fully unloaded. Unloaded head temperature appears to build up in this mode. I have used a stethoscope on start up on each head and do not here anything unusual. It does not stay off for more than 2-4 minutes and has a drive for a soft start. controls for speed control have not been installed. Vilter has stated that they can go to as low as 50% speed (fully loaded) and maintain adequate oil pressure. Liquid trapping is possible, but I did not think it could happen in the 2-4 minutes of off time. The ammonia liquid for head/oil cooling is from the high temp recirculator and regulated down to feed each compressor. When you refer to a non-return valve, is that the same as a check valve? There is a high level safety on the recirculator as well.