FREE-VALVE’s UPVC back pressure safe relief valve is installed in the line of positive pressure discharge of metering pump,its function is to prevent siphon phenomenon occur. Removing the protective cap on the adjusting screw on site, then rotating and adjusting screw , the required pressure will be got by adjusting pressure gauge in the pipeline. Back pressure will be set within the range of 0-0.6 MPa by adjusting screw .