Fitness for service in the process industries
There is a common dilemma for engineers in process industries when it comes to flaws in components: “Can I run it? Should I repair? Is it time to retire the component?”
Those questions are vital to any asset integrity management program. If an engineer cannot make accurate, honest assessments, it can mean extended downtime, a loss of productivity or unnecessary safety risks.
Fitness for service is a well-known concept for engineers. Fitness for service evaluations are performed on components that are in service, and they are used to determine structural integrity despite the presence of some flaw. Engineers then determine whether or not the flaw affects the asset’s performance, or if it poses a significant threat.
“The presence of a crack or corrosion in a component doesn’t meant it needs to be taken out of service.”
The presence of a crack or corrosion in a component doesn’t mean it needs to be taken out of service, necessarily. In many cases, equipment may continue to operate despite such flaws. Replacement or attempts to fix may be unnecessary. There are ways to determine whether a flaw warrants repair, replacement, or no action at all.
Fracture mechanics and limit load analysis
According to Inspectioneering, an asset integrity management journal for the oil, gas and process industries, cracks pose the most serious material threat. To determine the extent of the threats cracks pose to assets, engineers apply the principles of fracture mechanics. These mathematical principles quantify fracture toughness, stress and flaw size.
Though cracks may be the most serious threat to assets, there are other conditions that require evaluation such mechanical damage (dents) and the thinning of materials caused by corrosion and/or erosion. These conditions may affect an asset’s load bearing capacity. So in these cases, determining what flaws are acceptable is based on load limit capacity.
Determining acceptable flaws and fitness for service in these respects often follows two methods. One is the BS-7910 standard from the British Standards Institute, which is a guide to the acceptability of flaws in metallic structures. In his paper “Effect of Residual Stresses on Fracture Resistance of Arctic Structures,” author William Magnor wrote that BS-7910’s main focus is on the structural integrity of welds. However, the standard also addresses brittle and ductile fractures in metallic materials that are not welded.
The other standard is API-579, which was developed by the American Petroleum Institute. Inspectioneering noted that API-579 details FFS assessment techniques for pressurized equipment often used in the oil and gas industries.
“The document contains 11 sections on assessment procedures for preventing damage mechanisms such as brittle fracture, general metal loss and local metal loss, pitting corrosion, blisters and laminations, weld misalignment and shell distortion, crack-like flaws, creep damage, and fire damage,” Inspectioneering noted.
“The first sign of damage should not necessarily signal to operating engineers that it’s time to replace an asset.”
In his paper, “Living with Defects: Replace/Repair or Prove Fit-for-Service?“, author Ron Selva wrote that the first sign of damage should not necessarily signal to operating engineers that it’s time to replace an asset.
Selva wrote that a component can be deemed fit for service if “it can be demonstrated (with acceptable safety margin) […] that the conditions to cause failure is not reached within a predetermined time period, giving due regard to its DMs (damage mechanisms) induced integrity risk and the HSE (Health, Safety & Environment) and business consequences of failure.”
The application of FFS technology, taking into account the factors and conditions that can potentially cause damage, and use of non-destructive testing techniques are changing the way engineers decided whether to run, repair or retire their assets. Such evaluations can be used to determine tolerable corrosion damage, crack sizes and their growth rates, operating measures that will prolong integrity and remaining component life.
Selva wrote that determining FFS can involve using BS-7910, API-579, or a combination of the two and can be done irrespective of the equipment’s design codes.
Inspectioneering noted that despite the success of such techniques, FFS cannot determine the absolute line between safe and unsafe operation. The more uncertain the input parameters, the more uncertain the predicted outcome may be. However, if engineers take a more conservative approach with input, the result would be a more conservative prediction of remaining asset life.