METHODS AND SYSTEMS FOR THE NEGATIVE PRESSURE TESTING OF UNDERGROUND STORAGE TANKS CONTAINING HIGHLY VAPOROUS HYDROCARBON LIQUIDS

摘要

A method and system for detecting a leakin a high vapor-evolving hydrocarbon liquidunderground storage tank filled incompletely withthe liquid, wherein there is a projecting fill pipeconnected with the ullage which is filled withhydrocarbon vapor and air. A vacuum source isconnected to the fill pipe for withdrawing theatmosphere of the ullage and imposing a prescribednegative pressure. While some of the drawn offvapor and much of the air is exhausted toatmosphere, a significant portion of the ullagecontent is recirculated to build the relativeproportion of hydrocarbon vapor to air in theullage. The ullage is then isolated from the vacuumsource and atmosphere, and allowed to stabilize overa predetermined time period with a reduced rate ofhydrocarbon vapor release into the ullage. Therecirculation and stabilization are then repeated toachieve further saturation of the ullage withgasoline vapor and vacuum is applied to the ullageto reach a predetermined threshold negative pressurein the ullage. Over a predetermined time periodsuccessive negative pressure readings are taken todetermine if there is a rate of decay whichindicates the existence of a leak in the tank.