This tread is outside of my area of expertise so please excuse my ignorance but I would like to learn more about commercial refrigeration.
Could one of you please provide a simple explanation of what your talking about. I get that you're discussing head pressure control during low ambiant but I'm not familiar with the valves being discussed (A8,A9,ORD,OPR). I'm assuming it's a valving system similar to a headmaster condensor pressure control that effectively reduces the size of the condensor therefor increasing head pressure.
Also, the term pumps has been mentioned several times in the same context of refrigerant. Are you referring to the compressor or some other pump? And what's a drop-leg and how does it relate to the head pressure control? And what's SCT? Is it sub-cooling temperature?
Again, forgive my ignorance, but I would really like to understand this better.