API PUBL 2558:1993 (R2001)

The American Petroleum Institute (API) contracted with Cermak Peterka Petersen, Inc. (CPP) to conduct a wind tunnel study to determine the local wind velocities, wind directions, and roof pressures on External Floating Roof Tanks (EFRT). The results of this study are to be used to improve evaporative loss calculations for roof fittings on EFRT. The third edition of API Publication 2517 (1989) is the first to include roof fittings as a potential source for evaporative loss. Previous publications have limited the scope to include rim seal and stock clingage losses, assuming roof fitting losses to be negligible in comparison. The loss factors used in the evaporative equations were derived from experimental data which correlated stock evaporation to the wind speed over the roof fitting. The current procedure, as described in Section 2.2 of API 2517, specifies that an average wind speed at the tank site or from the nearest local weather station should be used as the wind speed over the roof fitting in the evaporative loss equations. This average approach wind speed may differ Substantially from the actual wind speed over the fittings, from which the loss factors were derived. For lower roof levels, in particular, the inaccuracy in wind velocity may cause overestimates of the evaporative loss potential for the roof fittings. The primary objective of this study is to develop a relationship between the airport wind speed and the wind speed at roof fittings. In addition to wind speed, the relative wind direction of air flow over the EFRT roof top will be analyzed as an aid to evaluating evaporation losses for fittings which exhibit some level of directional sensitivity. For documentation purposes and for use in future evaluations of evaporative loss, roof top pressures were also measured across the tank. The following sections provide background into the current evaporative loss methodology and air flow characteristics, a description of the experimental program carried out during the study, results of the study, a revised method for calculating evaporation loss from roof fittings, and conclusion and recommendations.