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Fitness for Service per API 579 in CodeCalc

Fitness for Service (FFS) assessment using API Recommended Practice 579 is performed to assess the operation safety and reliability of the process plant equipment, such as pressure vessels, piping and tanks for some desired future period. The assessment procedure will provide an estimate of the remaining strength of the equipment in its current state in which may have been degraded while in-service from its original condition.

Most pressure vessel design codes such as ASME Section VIII Division-1, do not address local thin areas (LTAs), requiring minimum component wall thickness to be more than a calculated value. On the other hand, Fitness for Service standards such as API-579 credits the area (where thickness > tmin) around the damaged portion. So, in this regard the API-579 code is better suited for pressure equipment with flaws.

CodeCalc and PVElite include Level 1 and Level 2 metal loss assessments covering the following flaw types:

  • Section 4, General Metal Loss.
  • Section 5, Local Metal Loss.
  • Section 6, Pitting Corrosion.

Main features include:

  • Data can be specified by thickness reading or profile, just like you would measure the data.
  • Consideration of proximity of the flaw to a discontinuity.
  • External loads consideration.
  • Remaining life calculation using thickness or MAWP approach.
  • De-rating the component if the flaw does not meet the passing criteria.
  • Extensive material database.
  • Easy to read report with equations-substitutions.
  • Option to change units at any time to any user-defined units. 
  • Graph of the thickness profile is generated

Download and try the API-579 capability in the working demo of CodeCalc or PVElite. You can also download the FFS brochure in pdf format.

The analysis can be performed using the Shell and Head Module of CodeCalc (Component Analysis module of PVElite) as depicted in the figure below:

Screen shot showing selection of API-579 as Analysis Type.

Typical approaches for FFS are as follow:

  • Identifying the flaw type and damage mechanism.
  • Considering the applicability and limitations of the specific flaw type procedure
  • Reviewing data requirement and gathering the data.
  • Applying the assessment techniques and comparing the result to the acceptance criteria.
  • Estimating the remaining life for the inspection interval.
  • Applying remediation as appropriate.
  • Applying in-service monitoring as appropriate.
  • Documenting the results.

Section 4 covers FFS assessment procedures for components subject to general metal loss resulting from corrosion and/or erosion. Meanwhile Section 5 is for the analysis of local metal loss or Local Thin Areas (LTAs) in which include groove-like flaw or gouge. 

A screen shot showing the selection of flaw type and correspoding input.

Flaw type selection

Measurement Data

The Assessment of General Metal Loss described in Section 4 can be performed using either point thickness (random type readings) or profile thickness (grid type readings) measurement data.

The localized metal loss assessment as described in Section 5, requires that data is specified as thickness profile. The data matrix can be setup by providing the number of points in both circumferential and longitudinal directions. User can also enter the CTPs in each direction.

Several options of measurement data area available; profile, CTP’s, and point thickness.

When the data is entered as a profile, the program generates a graphical representation of it

Graphic showing thickness profile generated by the program.

API-579 Section 6 covers flaw assessment procedures for components that are subjected to pitting damage. These types of pitting defects can be evaluated,

  • Widespread Pitting

  • Localized Pitting

  • Widespread Pitting with a LTA region

  • Localized Pitting with a LTA region

Additionally, pits can be located on either the inside or the outside component surfaces

Pitting damage is described using pit-couples, each is composed of two pits that are separated by a solid ligament. For non-uniform pit flaw, additional pit-couple measurements are required.  

The program evaluates flaws per Level 1 and Level 2 criterions. Level 1 assessment includes simplified methods with conservative assumptions, while Level 2 assessment requires a more detailed analysis and produces more accurate results.

The acceptance criteria is in relation with the desired remaining life and the design  pressure.  If the component is not acceptable, then remaining life could be reduced or vessel may have to be de-rated or a more advanced Level 3 evaluation (typically using FEA) may be performed.

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