We have 3 centrifugal chillers (water cooled) at a site in town. There is a problem with oil return/oil pressure and this has been happening off an on for a while. The service technician changed the filters etc last week, and left the valves on the oil line closed. We opened them up but the machine is still tripping out on low oil pressure. The sump is half full and empties when the oil pump runs. We backed off the chiller to try and get some oil back and I believe he added some to top up as well. I went over earlier and closed off the purge drum and tried to get the chiller running to get some oil back, but nothing would work. Our service people are going back to drain the oil from the evaporator and check the filters. Personally I think the compressor maybe the problem. condenser pressure was a little low, and oil at about 50kPa