API Publ 323:1994 pdf download.An Engineering Evaluation of Volumetric Methods of Leak Detection in Aboveground Storage Tanks
There are two approaches to detecting leaks from an aboveground storage tank (AST) by means of the volumetric method. The first is the conventional approach in which measurements of the level and temperature of the product are made with a precision level sensor and a vertical array of closely spaced, precision temperature sensors. The second is a mass-measurement approach, which employs a differential-pressure sensor to measure the level changes. In a tank with verti- cal walls, a differential-pressure sensor inherently compensates for the level changes produced by the thermal expansion and contraction of the product betweeen the pressure port, which is located near the bottom of the tank, and the product surface. Because of the possibility of large horizontal gradients in the rate of change of temperature of the product in an AST (gradients which cannot be accurately measured with a single vertical array) the mass-measurement approach should, in theory, have a performance advantage over the conventional approach. As part of Phase III of the American Petroleum Institute’s (API’s) project to develop and evalu- ate the performance, in actual operational environments, of different technologies for detecting leaks in the floor of ASTS’, a controlled experiment was conducted in a 117-ft-diameter tank at Mobil’s refinery in Beaumont, Texas, during late May and early June 1992. The purpose of this experiment was to evaluate the performance of both approaches to volumetric testing. The tank contained a light fuel oil, and data were collected over a continuous 28-day period. Two vertical arrays of thermistors were placed at two locations inside the tank to determine the magnitude of the horizontal gradients in the rate of change of product temperature. Temperature measure- ments of the tank’s exterior shell were also made.
2 BACKGROUND
A methodology for testing ASTS for small leaks with a volumetric test method was developed in the Phase II field tests. Volumetric measurements were made in a 114-ft-diameter AST contain- ing a heavy naphtha. Two three-day data sets were collected, one in which the tank was filled to a level of 17 ft and the other to a level of 10 ft. Estimates of the magnitude of the important sources of ambient noise were made at each level. The results of the field tests indicated that compensation for thermally induced volume changes is essential for the detection of small leaks. Changes in the temperature of the product, in response to diurnal cycles, were found to be the largest source of volume fluctuation. These were difficult to measure with sufficient accuracy for effective compensation when only a single vertical array of thermistors was used. The reason for the insufficiency of the single array was that the rate of change of temperature differed at various locations along the horizontal axis of the tank. The volume changes induced by the ther- mal expansion and contraction of the wall were found to be much smaller, but because they were still large in comparison to a small leak, compensation was required if robust detection performance was to be achieved.
API Publ 323:1994 pdf download
PS:Thank you for your support!
