PRV is a very simple and cheap kit and is readily available in market why do you want to design one?

The relief pressure and Vac. settings are dependent on your silo / hopper design strengths. Generally for big silos the pressure side is set around 50 mbar and a vac is 10 -25- mbar. Small hoppers relief can be set at higher as smaller vessels have higher buckling strengths. Only thing to take into account is the size of the PRV and it should be designed at the surge air flow and not peak conveying airflow. Since you are using a blower there will be very little or no surge but it is still recommended to have an over sized PRV. ■