The screw compressors are: Howden, Sullair, Mycom, and Dunham-Bush. Most of these are at least 20 years old, and all run very well.
I'm qute familiar with Howden, Mycom and Dunham-Bush. Sullair I do not know at all. Proper maintenance and normal working conditions can keep them alive for a long time

The one thing I do not like about the pumper drums is that you have to maintain a minimum gas pressure for pumping the liquid. Otherwise they are very simple to understand. You are exactly right about the NRV's.
This one what we have here is working with condensing pressure. High pressure liquid cooler and pumper drum are in paralel connection do deliver a cold liquid to coolers. First we have liquid from pumper drum until is empty. Simultaneously we close the high pressure gas line to pumper drum and open equalizing gas line from pumper drum to surge drum to start gravity liquid filling into pumper drum. In the same time SV at cold liquid line from economiser is open to deliver cold liquid until pumper drum is full again and ready to deliver liquid. This is some kind of sequential work with about 3-5 minutes sequences for almost 30 years until now.

This thread is getting very long. Perhaps we should start a new one??
For that reason I started this one

Yes, almost forget nothing about controls and valves: old one Phillips Refrigeration, Johnson Controls, Danfoss or other..