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SMR Outlet Pigtail Cracking & Creep Rupture

Bulk hydrogen production in a petroleum refinery by steam–methane reforming (SMR) utilizes high temperatures, moderate pressures and catalyst – filled tubes which generate high tube temperatures of up to 920 °C [1,690 °F ].

Creep and pressure / temperature cycling imposes severe loading on the internal and outlet tubing & piping.   Although API 530 provides high temperature design guidance, API TR 942-A indicates many operators continue to experience unexplained repetitive incidents of creep rupture of outlet pigtails and cracking of welded joints in outlet manifolds.   Operators are regularly reporting failures at intervals of 30,000 hours (3 years) when service target life is 100,000 hours (11 years).

Root Cause Failure Evaluation

Design of SMR heaters is carried out by use of industry practice documents and computer aidied engineering (CAE) software to perform tube / pigtail / header thermal expansion stress and reaction load analysis.   But, detailed RCFA indicates that these design tools are being mis-applied in many instances for the conditions of the operating environments.   These tools need to be properly understood, calibrated and interpreted to assess the high temperature creep damage mechanism.


Solution Assessment

Many failures have occurred with alloy 800HT piping & fittings, which have been underdesigned because of rote use and misapplication of high temperature design practices and CAE tools.   One alternative is to apply the design tools correctly or use materials more forgiving of design deficiencies. Proprietary HP micro-alloyed materials may provide a solution; requesting MTR's which include creep rupture data is strongly recommended in this case. The Table shows the analysis of outlet pigtails which had failed after 7 years of operation; routine calculations show acceptable design with the Robinson damage accumulation factor ΣD ≤ 1.0, while correctly derived & applied calculations predict failure in conformance to creep rupture failure theory, i.e., ΣD > 1.0.

 

SMR Outlet Collector Cracking


Creep Rupture Failure



John Aumuller, P. Eng., Ph. D.