From what I gather the Process Safety Time is the duration of time from surpassing an Operating Limit until you reach the Safety Limit and a failure occurs?
If it is specifically a spread sheet for tank levels which you're looking for, if you google "vessel volume.xls" and take the first link to Cheresources page, there is an excellent spreadsheet Montemayor posted years ago. I thought he had posted it somewhere within this forum also, but I couldn't locate it.