California Air Resources Board

CARB staff are studying the occurrence of overpressure in underground storage tanks (UST) at retail gas stations. Most gas stations are equipped with an In-Station-Diagnostic (ISD) system which monitors UST pressure and set alarms when the recorded pressures exceed a specified criteria. The ISD pressure monitoring is intended to limit emissions from fugitive leaks and vent line emissions that would compromise the control efficiency of the Phase II vapor recovery system. 

CARB staff is working to develop new alarm criteria for ISD containment overpressure assessments. An ongoing study has revealed that the number of over pressure alarms increase significantly in the winter months when compliance with CARB’s 7 psi Reid Vapor Pressure gasoline volatility limit is not required. CARB staff have also determined that a high percentage of winter season ISD pressure alarms are not related to vapor recovery system equipment defects.