Acceptability of flaws in metal structures
Where inspections are carried out using Non-Destructive Testing (NDT) methods to determine equipment integrity, acceptance levels are required for any flaws that are found to determine if the equipment can continue to operate. Acceptance levels are sometimes provided in standards but are often not specific enough for the types of flaws that develop during operation or there are no acceptance levels available. In this situation and Engineering Critical Assessment (ECA) using fracture mechanics can be used to:
- Determine the size that the flaw needs to be, to start propagating;
- The critical flaw size that the equipment will fail at; and,
- How long the flaw will take to grow from its starting size to the size that will cause failure.
Types of Flaws
Types of flaws that can be assessed in metallic structures include:
- Welding defect such as; lack of fusion or penetration or undercut;
- Local thinning due to corrosion or erosion;
- Porosity; and,
- Shape imperfections such as misalignment or imperfect profile.
Fracture mechanics can be used to assess flaws on equipment that is subject to cyclic forces or high temperature creep that cause the flaw to grow. The calculations involve an iterative process, calculating the flaw growth for each stress cycle, which often results in millions of iterations before the results are produced. Typical applications for this type of assessment are:
- Rotational loading (e.g. shafts, mine winder drums, sheave wheels, bucket wheel reclaimers, conveyor shafts and drums, vehicle axles).
- Cyclic loading (e.g. structural beams, structural pins, bolts, vibrating screens).
- Cyclic pressures (e.g. pressure vessels, emptying and filling of tanks and bins, pipework).
- Cyclic temperatures (e.g. high temperature pipe work, boilers, furnaces, smelting equipment).
- Creep due to high temperature or stress corrosion cracking (e.g. Furnace components, smelting equipment).
The following graph shows the resulting flaw growth with time that is produced as part of a typical assessment.
Assessment of flaws using an Engineering Critical Assessment (ECA)
BS7910 – Guide to methods for assessing the acceptability of flaws in metallic structures, provides a method using the Engineering Critical Assessment method that assesses the flaw in more detail than would typically be done under the primary design standard or quality control standards. This could result in acceptance of a flaw or defect that might have been rejected by a more conservative and generic quality control criterion.
The assessment of the flaw can be applied to structures with existing flaws or to assess a new structure to determine its susceptibility to flaw growth in high stress areas. If a flaw is existing, calculations can be performed to estimate if the crack will continue to propagate and how long it will take to reach the point of failure. This information can be used to determine if the equipment can continue to operate safely in the short term and predict how soon the item needs to be repaired or replaced before it fails. The information can also be used to determine NDT frequencies required to monitor the flaw during the period between detection and repair or replacement. If a new structure is assessed, the result of the analysis can be used to refine the design before it is manufactured and determine optimal in-service maintenance and NDT inspection intervals.
The assessment of flaws using ECA methods can be beneficial in several ways:
- It can allow equipment to stay in service when it would otherwise need to be taken offline if assessed under normal acceptance criteria. This enables repair, remediation or replacement work to be planned, rather than needing to instigate an unplanned shutdown immediately to conduct repairs.
- NDT periods can be optimised based on the actual equipment operational criteria, rather than generic estimates based only on experience or standards.
- Fracture mechanics is specific in assessing the effect of the stress around the flaw, and thus potential issues can be highlighted that would not otherwise be picked up during a normal stress analysis or Finite Element Analysis (FEA) during the design processes.
Rapallo has experience in completing Engineering Critical Assessments using fracture mechanics in accordance with BS7910. If you want to benefit from this type of assessment, please contact us to discuss further.