centrifugal pump need to keep a minimum recycle flow when the outlet is blocked or too low flow rate. it is to protect pump from vibration damage and motor damage. the restriction orifice is like a throttle valve, it produce a pressure drop. for large pumps we usually have a recyle control valve instead of a restriction orifice. I have seen a lot of recycle valve design but never see restriction orifice for recycle line without control valve (on/off). anybody a better view ?