On API-661 and Assessment Procedures of Header Boxes and Nozzles for Air-Cooled Heat Exchangers

This paper presents a series of studies on header boxes and the attached inlet or outlet nozzles for air-cooled heat exchangers and associated data from API-661. Unlike cylindrical or spherical vessels, the rectangular-shaped cross section of a header box does not have a simple thickness equation that allows for determining the minimum required wall thicknesses from pressure. Though pressure vessel codes and relevant API standards provide certain design tool or fabrication data, a definite and consistent assessment procedure with necessary details is still not available. In many occasions this often imposes difficulties and confusion when conducting inspections and integrity assessments on air-cooled heat exchangers. In this study, stress analysis methods for vessels with rectangular cross sections and the attached nozzles are reviewed and applied to assessments of air-cooled heat exchangers. Minimum nominal thicknesses from API-661 are examined and compared to analysis data for a variety of conditions to investigate the applicability and restrictions of the standard on inspection and integrity assessments of header boxes and attached nozzles. As a result, FFS assessment procedures in three different levels, which are consistent with API-579 general assessment methodology, are developed and presented for header boxes and nozzles. This study may serve as a practical addition to integrity assessments and inspection of air-cooled heat exchangers.