Fitness for Service

A fitness for service (FFS) evaluation is a process for assessing the structural integrity and safety of components that may have experienced damage and deterioration during their service life.

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The goal of an FFS is to determine whether a component can continue to operate safely and with adequate performance, or if it needs repair, replacement, or modification. For power and process plant components, one usually uses the methodology set out in the ASME FFS-1/API-579 recommended practice guideline.

The boiler or HRSG is exposed to a variety of degradation mechanisms during operation and shutdown. Regular inspection and good maintenance practice can minimise damage and degradation, but not eliminate it. When damage is identified, what are your options? For pressure parts in critical service, where contract penalties apply, it may be desirable to delay repairs until the next planned outage or shutdown.

Tetra can provide a full range of services in the field of Fitness for Service assessments according to world recognised standards:

• API 579-1 / ASME FFS-1
• BS7910
• Fitnet
• B31.G
• DNV-RP-F101
• R5 / R6

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The complexity and scope of an FFS vary according to the particular problem under investigation. The first step in any FFS evaluation is to assess the extent and nature of the damage or degradation that the equipment or structure has undergone, such as creep, cracking, corrosion or mechanical deformation. Based on this initial evaluation, the ASME/API approach then specifies three levels of analysis. The level 1 assessment is the simplest and takes the least time. The analyst relies on experience coupled with use of some basic calculations and screening curves to make a judgment on the fitness for service. Typically this approach is most conservative, similar to design code calculations.

If the level 1 results do not provide sufficient confidence or do not meet the required criteria for acceptance, then a level 2 assessment is performed. This requires more complex analysis and calculations, such as finite element analysis (FEA). The objective is to reduce the amount of "conservatism" in the analysis and thereby see if the component can be placed back into service. Finally, if the result of the level 2 analysis still shows that a negative result, a level 3 can be attempted. More elaborate analysis and data collection from inspection or laboratory tests are used to further lower the remaining conservatism in the level 2 results and thereby potentially show an acceptable result for the component. In all cases the experience of the analysts is critical. Tetra Engineering has performed numerous FFS studies on a wide variety of components in the boiler and steam generation cycle. This provides the necessary assurance that we will deliver a suitably informed decision to clients on whether their component can continue to operate.

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