So, what can cause the pressure to drop downstream of a screw pump? The pump could be worn out. But, that would not be reversible. It is much more likely that the system resistance to flow is reduced. What can cause that? A relief valve (PSV or full-flow) could be diverting flow. But, why would that change when the compressor is running or not running? The pressure controls (assuming a spill-back, pressure control valve) could open to divert flow. But, why would that change? The resistance of oil flow through the coolers could change. Unlikely. The resistance to flow through the filters could change. That one seems believable. We have one system where the pump capacity is barely enough and a relatively small change in oil temperature changes the system pressure because of pressure drop through the filters. When the compressor is not running, the oil temperature drops down to the setting of the oil heater (let's guess 90 °F). When the compressor runs, the temperature increases to the setting of the temperature controller (let's guess 120 °F). Less resistance through the filters and the oil supply pressure to the bearings goes up, not down. That is not it. The last option that comes to mind is a drop in resistance to oil flow to the bearings. I am not sure what would cause that. Perhaps something with the flow to the overhead lube oil rundown tank (if you have one). We really need a drawing of the oil system configuration to guess any more.
Johnny Pellin
