#3 I would guess the original poster is not in the UK. So while the principles of DSEAR may well be relevant but legally not applicable. As Peter says much more information is required to decide if a vapour trap is needed. #1 is asking about a process safety problem, with the information given you can't say 'a possible catastrophy' may occur. It might only be a small vent that doesn't operate very often with adequate ventilation etc with zonal classification so there is no electrical equipment and/or ignition sources present. Equally COMAH won't apply if #1 is overseas. There may be local similar legislation. Also, again from the information given in #1 you can't say if COMAH applies, even if in the UK. With out knowing the quantities of LPG on site, the site inventory may or may not exceed the COMAH threshold limits for COMAH to apply. COMAH is essentially about writing a safety case to justify why an installation is safe. COMAH is also about emergency planning in the event of a process safety incident. COMAH as such will say very little about specific technical LPG vapour trap requirements. I would not accept in #1 'an audit' stating/requiring a vapour trap to be installed. I would expect a recommendation for a vapour trap to originate from a process safety study, most likely a HAZOP study. It is usual to have a fully contained vapour capture/recovery system. In a surface installation any vapour release would probably go to a flare system, if a large installation like and oil refinery or offshore platform. . The original poster should find an appropriate engineering/technical standard on which to Base any further decisions on modifications to the plant under consideration. Possibly an API standard.