Weir SPM Full Bore Unloading Valves for drilling applications provide over-pressure protection for mud pumps operating under a variety of pressure and flow conditions. The valves are compact, simple to operate and rely on the system's pressure to open when a preset pressure is exceeded and automatically snap shut when the pressure drops. The valves are externally adjustable, and unlike shear relief valves which use common nails to trigger the valve, Weir SPM's Full Bore Unloading Valve utilizes nitrogen or rig air (properly filtered) to activate opening and closing. This model is available in 3" and 4" sizes. Operating pressures range from 5,000 psi to 20,000 psi. Standard service and sour gas models are available.