Equipment: Separated head Closure of  Propane Dehydrator Heater (High pressure steam tube side, propane shell side) and  Butane Dehydrator Heater (High pressure steam tube side, butane shell side) The separated head closure uses independent shell-side and tube-side closures. This is ideal in cyclic and shock service, high temperature, and mid-pressure applications. Separate tubesheets: Handle high temperature differences and cycling more effectively than shell-and-tube exchangers.

Problem:a steam leak in the flange due to apparent steam cuts in the outlet flange groove was observed on the tubesheet gasket of propane dehydrator heater. This typical failure has been observed on numerous occasions during the past 12 year the unit has been in operation. This gasket failure / steam leaks has been a chronic issue as steam and HC have repetitively leaked. This heat exchanger has been designed by Brown-Fin-Tube as a hairpin type design

Analysis: The ASME B31.3 and ASME Sec-VII MA-2 calculations confirmed that the piping loads on the tubesheet nozzles and the flange design stresses were well within acceptable limits. Additionally, the WRC-510 calculations confirmed that the resulting bolt load during the transient when compared to minimum operating bolt load after gasket relaxation effect has been accounted for, establishes that that the gasket will continue to seal during the 1^st thermal cycle transient load has been applied.  The detailed FEA analysis simulated the loss of additional gasket seating stress due to the inherent design of ‘Eccentric steam-inlet’ line causing a non-uniform thermal gradient to be within acceptable range. However, upon completing a further thorough ASME PCC-1 analysis in accordance with the WRC-538, it established that the existing spiral wound gasket required a target assembly gasket seating stress of 17,824psi, consequentially requiring a bolt tensioning value of 61ksi to perform effectively. In general, during assembly, the bolt is pre-stress only upto to maximum of 45ksi. This has been attributed to the root cause of the chronic leak issue.Also it was established that the current flange design was limited to a bolt tensioning value of 52ksi, rendering the existing Flexitallic spiral-wound gasket unsuitable for the tubesheet flange joint design application. In conclusion, it has been established that the Flexitallic CHANGE gasket overcomes the limitation posed by the Spiral-wound gasket for application in the 10E-22 operating service.

Results Conclusion:TSU has completed As the existing spiral wound gasket was found unsuitable for the application, TSU has recommended to upgrade to a Flexitallic CHANGE® gasket. Implementation of this recommendation will eliminate the repetitive gasket failures and thus ensure reliable and safe operations