Higher demands on outlet steam conditions has led to increased use of modified low alloy materials (9-12% Cr), such as Grade 91. The superior high temperature material properties of these types of steels (strength, fatigue resistance, creep resistance, improved steam oxidation resistance) gives the designer advantages with respect to boiler reliability and thermal performance. The advantages of using this type of materials are many; however, the long-term integrity has been a concern to the industry ever since the first installations.
A number of failures have occurred in recent years, some catastrophic; type IV creep cracking in the Heat Affected Zone (HAZ) has been the dominant failure mechanism. Advantageous mechanical properties can only be assured if heat treatment is properly performed through the entire fabrication and construction cycle. Improper welding or heat treatment will result in a product not fit for its purpose, increasing the risk of premature failures. Tetra performs risk assessments encompassing all key aspects that can contribute to component failure:
Finite Element methods are used to analyse large piping and boiler components in order to accurately determine the geometrical effects on the nominal stresses present. The results are used in a creep analysis to determine damage up to date and the remaining life of the components. If the plant is planning to change the operating process conditions, a boiler simulation study can be used to predict the local temperature and pressure conditions in tubes and piping in advance. Key components to be checked are identified and integrated into an Inspection Plan, for use by plant staff and NDT personnel.For more information contact Peter Jackson at peter.jackson@tetra-eng.com (North/South America) or Andreas Fabricius at andreas.fabricius@tetra-eng.com (Europe, Middle East, Asia).
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During the startup of the triple-pressure HRSGs, plant staff noticed steam coming from the stack and a discrepancy between feedwater and steam flows. On shutting down the unit a T23 superheater tube was found with a complete circumferential failure. The failed tube was removed and the bore was plugged with some difficulty due to poor access and the uncommon material.
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Since 2007 the ASME B31.1 Power Piping Code has included Chapter VII, establishing mandatory requirements for Operation and Maintenance. The key elements of this program are discussed here, and require that Operators outline a detailed maintenance plan for their Covered Piping Systems (CPSs). The Code also requires that operators perform regular Condition Assessments of their CPS ensuring that relevant degradation mechanisms are being covered.
Since 2007, the ASME B31.1 Power Piping Code has included mandatory requirements on the Operation and Maintenance (O&M) of Covered Piping Systems (CPS), previously called High-Energy Piping. The main concern is that the safety of these systems is very dependent on good O&M, which justifies its inclusion in the Construction Code. There is room for flexibility depending on individual circumstances but the responsibility is squarely laid on the Operating Company.
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