Abstract: A hydraulic system (10) includes an electronically-controlled counter-pressure valve (60) that enables backpressure in a return line (36) of the system to be varied by a control unit (64). The system allows active control of the pressure in the return line to produce different return pressures for different situations. A higher return line pressure may be set to improve make-up or recirculating flow through an anti-cavitation valve (50). This may improve controllability of functions that benefit from backpressure, such as lowering loads (12). The control unit that controls the counter-pressure valve may take into account any of a wide variety of possible inputs when setting the counter-pressure valve.

Abstract: An extrusion resistant ring that is radially expandable and contactable to accommodate the relative movement of two joined bodies relative to each other. The extrusion resistant ring in combination with an elastomeric diaphragm provides a high pressure seal for down-hole oil drilling apparatus.