pressure side of the piping between pump 560 or if present optional check valve 565 and drain valve 570 and storage vessel 590 if or when the pressure exceeds a predetermined upper working limit the safety shutoff switch 580 opens which by means of the electrical connector 582 disconnects power from the compressor pump 560 motor which deactivates the pump 560 preventing excessive pressure from building up inside the storage vessel 590 and related components although safety shutoff defeats use of the apparatus 500 in keeping containment system 300 pressures from exceeding the first pressure threshold limit it is expected that this is a rare and abnormal condition which will not materially affect long term averages of positive containment system 300 pressures since carb requirements are generally based on weekly or monthly long term averages no adverse consequences will likely occur since fuel vapor and air mixtures pose a flammability safety hazard all the electrical components including compressor pump 560 motor solenoid activated drain valve 570 pressure sensor 520 pressure switch 580 and associated electrical connectors 582 522 562 572 are designed as either intrinsically safe circuits and devices or are enclosed in explosion proof housings as appropriate to ensure safety the capacity of the pressure storage vessel 590 and the maximum working pressure capability of the vessel 590 and other components 560 565 580 570 of the vapor storage system 550 determine the maximum volume of vapor and air mixture which may be removed from the containment system 300 during any one over pressure under pressure cycle of the containment system 300 for instance if the vessel 590 capacity is 1 cu ft and maximum working pressure capability of the components is 150 psi about.