Short Term Overheating | Materials And Corrosion Control

Damage Description

·         Eventually results in bulging and failure by stress rupture.

·         Ruptures are characterized by open “fishmouth” failures and normally with thin lip fractures

·         In ferritic steels especially, bulging is typically associated with local microstructural changes that confirm high temperature exposure

·         It is a fairly common damage mechanism in utility boiler tubes and is sometimes associated with starvation (loss of steam/water on the tubeside).

·         Bulging and distortion can be significant at low stresses, as temperatures increase.

Affected Materials/Equipment

·         Flow starvation can cause short term overheating and even failure in any refining furnace but it is particularly critical in hydrogen or steam reformer furnaces

·         Overheating may also be caused by deposit built-up inside furnace tubes. Furnaces with coking tendencies such as crude, vacuum, heavy oil hydroprocessing are susceptible to this overheating.

·         Refractory lined equipment in the Fluid Catalytic Cracking, sulfur recovery and other units may suffer localized overheating due to refractory damage and/or excessive firing.

Control Methodology

·         Check that metal temperatures do not exceed design limits

·         Monitor for flame impingement using visual inspection via peep holes, or local overheating with infrared guns or thermography.

·         Install and maintain bed thermocouples in reactors and minimize the likelihood of hot spots through proper design and operation.

·         Fired heaters require proper burner management and fouling/deposit control to minimize hot spots and localized overheating.

·         Utilize burners which produce a more diffuse flame pattern.

·         Perform remaining life assessment per API STD 530 or
API RP 579 to determine the impact of having higher temperature than normal and take remedial action if the calculated life is shorter than expected.

Monitoring Techniques

·         Check for distortion and diametric growth during turnaround inspection.

·         Metallographic replication and hardness testing can be used to micro-structural categorization and detection for evidence of overheating

·         Refractory lined equipment can be monitored with heat indicating paint and periodic infrared thermography.  Inspect for refractory damage during shutdowns.

Inspection Frequency

·         Internal inspection and diametric dimensional checks at T&I

·         Metallographic replication and hardness testing after every process upset or temperature excursion or as advised by CSD Metallurgical Specialist

KPIs

·         Number of metallographic replication and hardness testing checks

·         Number of life assessments

·         Calculated life fraction consumed

·         Number of temperature excursions (over design limits) and durations

·         Number of chemical cleaning operations carried out (boilers)

·         Number of decoking operations carried out (process heaters,
e.g., crude, vacuum, visbreaking)

·         Steam/hydrocarbon ratio (hydrogen or steam reformers)

Reference Resources (Standards/GIs/BPs)

·         NALCO Guide to Boiler Failure Analysis

·         API STD 530

·         API RP 579, Section 10