Should I repair or replace my equipment? Can it remain in operation? A fitness-for-service approach.

Fitness for Service and Finite Elements Analysis

08 August 2017

Regardless of the quality of a Mechanical Integrity (MI) program, degradation or damage eventually occurs in virtually every facility. When it does, personnel need to understand what has caused the damage and whether the equipment can remain in operation, be repaired, or be replaced. Intertek Asset Integrity Management (AIM) provides the remaining-useful-life service that gives you the information you need to make the right decisions.

Nondestructive examination (NDE) of equipment is a standard periodic maintenance activity used to manage today's in-service assets. Generally, the NDE itself is quite straightforward; however, problems can arise when the NDE technician discovers a significant weld or base metal flaw indication. This leads to the following questions:

  • Should the flaw be repaired before placing the component back into service?
  • Can the repair be delayed until a future planned outage?
  • What Code or Standard should be used to evaluate the component?

Once a flaw is detected and sized, a Tolerable Flaw diagram can be used by the responsible party to determine the remaining allowable operating time.  This operating time could be zero months, in which case the flaw must be repaired before restart, or it could be as long as 5 years or more.

The result?  Immediate answers on the component's suitability-for-service, without having to wait for an after-the-fact engineering analysis to determine the seriousness of the flaw.

For pressurized components in power plants, chemical plants, or petrochemical plants the National Board Inspection Code is often the governing standard. This standard provides repair guideline and allows for Fitness-for-Service (FFS) approaches using ASME FFS-1/API 579.

  • The initial FFS Level 1 evaluation is intended for use at the plant inspection level. An increasing level of complexity is required for the analysis of defects or conditions that do not pass the previous level.
  • At the Level 2 evaluation, standard design calculations are made to take credit for any extra fabricated thickness, which can in turn be used as additional corrosion allowance.
  • A Level 3 evaluation is intended to be used by an engineering specialist in the appropriate field. At this level complex calculations are often employed, including finite element analysis and fracture mechanics.

Intertek's multidisciplinary team, with specialist knowledge in areas of fracture mechanics, fatigue, finite element analysis, materials, inspection and welding, has conducted many failure analyses in the industry, as well as hundreds of fitness-for-service analyses using the guidelines in ASME FFS-1/API 579. The information gathered in the failure analyses allows us to augment the information in ASME FFS-1/API 579 and permits us to provide the most comprehensive assessment of a component possible. Using the Fitness-for-Service approach can allow the owner of the equipment to safely extend the operability of the equipment while avoiding expensive and unnecessary repairs. 

To learn more visit www.intertek.com/aim or download one of our FFS technical papers:

Fitness-for-Service Evaluation of Off-Gas Scrubber

Fitness-for-Service Creep Life Evaluation

Mr. Cronin's expertise is in analysis of steady-state and transient stress, strain, and temperature fields using finite element and finite difference methods; fatigue and fracture analysis of large steam turbines, generator rotors, jet engine disks, pressure vessels, spacecraft structures, and launch vehicle platforms; risk analysis procedures for use in making run/repair/retire decisions; nondestructive examination using ultrasonic, magnetic particle, and radiographic methods; development of stress intensity factor solutions using finite element methods; and nonlinear stress and heat transfer analysis.