Abstract
High pressure threaded closures designed to ASME VIII Div.1 and Div. 2 are used extensively in heat exchangers in the hydroprocessing industry, where design conditions in excess of 100 bar and 450 °C may be required. When there is a requirement to open for inspection and maintenance, the breech-lock or screw-plug design offers a cost-effective alternative to TEMA channel closure types because it eliminates the large cover flange and bolting (often associated to welded diaphragms), allows for sealing adjustments without being dismantled and provides quicker opening and closing operations. For services designed to operate with a pressure differential between circuits that cannot be isolated (High-High type) an internal assembly must be designed such that the sealing of the tubesheet gasket is maintained under all possible operating conditions. The assembly must transfer the loads from the bolts to the tubesheet gasket but also remain elastic over a range of operating conditions.
The past work on this assembly type is reviewed and root causes of exchanger leaks reported in the literature are discussed. A finite element analysis study showed that typical designs for the internal load-carrying components undergo plastic deformation due to thermal expansion under certain operating conditions, leading to a loss of gasket seal. While sealing may be temporarily restored by acting on external bolts, for example by hot torquing, subsequent thermal cycles can result in further deformation such that the assembly may require unplanned maintenance or repair.
The results of the study were used to guide development of a new closure concept using a conical disc spring to ensure adequate gasket stress for sealing of the tubesheet while maintaining elastic behavior of components under load for all anticipated operating conditions, including upsets. Finite element models were used to compare the new approach to the existing practice and to design a demonstration unit, which has since been fabricated. The conical disk spring for the demonstration unit was tested, confirming the elastic behavior over the anticipated load range and enabling calibration of strain gauges. The complete prototype was tested with mechanical and thermal loads to validate analysis results and confirm ease of assembly and disassembly. The new closure design can reduce plant shutdown time by eliminating leaks and damage during operation of screw-plug exchangers.
