Fitness for Service
Pulp & Paper / Wastewater

Corrosion Probe has been helping customers in diverse industries determine what to do about service-related flaws in their equipment: operate the equipment as is, run and monitor, repair, alter or replace. Often the flaws are discovered during a scheduled equipment shutdown, so timely decision making is important for avoiding extended, costly shutdowns of equipment.

A fitness-for-service (FFS) assessment is a multi-disciplinary engineering analysis to determine if a piece of equipment is fit for continued, safe service. In addition to helping maintain safe and reliable operation, such assessments are used to increase inspection intervals, minimize shutdowns, determine if maximum allowable operating temperatures or pressures may be increased, or to de-rate a flawed vessel. FFS is also a useful tool for shutdown planning, developing inspection plans, extending the life of the equipment, and developing repair plans. API 579-1/ASME FFS-1 is the guiding document for performing FFS assessments.

Here are some examples of recent FFS assessments we have performed:

Waterside Cracking (Stress-Assisted Corrosion [SAC]) in Recovery Boiler Floor Tubes - Pulp & Paper Mill

A few cracked tubes previously had been removed from the boiler and examined to determine depths of the ID-initiated cracks. The approach involved considering a single crack of infinite axial length. Part 9 of API 579-1/ASME FFS-1 was used to determine FFS of such a crack at varying depths. The result was that the infinitely long crack did not fail the assessment until the depth was 80% of the tube wall thickness. The tubes that had been examined previously exhibited cracks extending a maximum of 17% of the tube thickness. This information reassured the mill that they had time to budget for, and plan a future repair.

Localized Thinning of a Wet Air Oxidation Reactor - Wastewater Treatment Plant

Inspection of the reactor revealed localized corrosion and stress corrosion cracking in the stainless steel weld overlay covering the horizontal vessel welds (where clad plates joined at vessel welds). This resulted in an initial recommendation to apply a high-nickel weld overlay to the entire interior of the reactor. CPI's subsequent FFS assessment revealed that rounded cavities measuring 7, 9, and 11 inches in diameter could reach a depth of 1.60, 1.45, and 1.35 inches, respectively (2-inch nominal wall thickness). This assessment provided the plant staff with a realistic idea of the severity of the damage, and prevented a panic reaction to overlay the entire vessel with an exotic alloy.

Localized Thinning of an Evaporator Train Surface Condenser - Pulp & Paper Mill

The shell of the tube-and-shell heat exchanger section of this vessel had suffered localized thinning and perforations due to internal erosion-corrosion from a tube leak. Permanent repair would have required extensive shell replacement and an extended, un-planned shutdown. CPI performed a FFS assessment based on the limiting design criterion of full vacuum pressure, and formulated a temporary repair plan. The FFS assessment allowed evaluation of "what-if" scenarios, including addition of stiffener rings and installation of external lap-patches over the thinned areas. The ultimate repair plan involved installing lap-welded patch plates over the thinned and perforated areas. This temporary repair was relatively fast and allowed mill staff time to plan a more permanent repair that could be executed during a planned shutdown.

Under-Stud Cavity Damage on a Recovery Boiler Wall Tube - Pulp & Paper Mill

Studded wall tubes in the recovery boiler exhibited corrosion underneath many of the studs, which appeared as "divots" in the external surface of the tubes. Mill staff were concerned, based on a previous FFS assessment by another engineer, that the boiler would suffer a leak prior to the planned repair a little over a year away. However, that assessment had been performed using calculations for general rather than local thinning. CPI utilized the less-conservative methods for locally thinned areas outlined in API 579-1/ASME FFS-1, and found that the remaining wall thickness at a 0.5-inch diameter divot could be as low as 0.045 inch and still meet the required maximum allowable working pressure. None of the field ultrasonic thickness measurements or lab analyses performed on tube samples revealed divot area remaining thickness of less than 0.084". The results of this assessment reassured mill staff that they could continue to operate safely for another year.


Service Areas

  • Corrosion and Materials Engineering
    • Corrosion Control/Protection Design
    • Corrosion Analysis and Material Selection Consulting
    • Failure Analysis - Mode and Root Cause Analysis
    • Cathodic Protection System Design and Evaluation
  • Structural and Mechanical Rehabilitation Services
    • Structural Design Engineering
    • Mechanical Design Engineering
    • Condition Assessment Evaluations
    • Structural Evaluation and Analysis
    • Shop and Field QC Inspection and Support
  • Mechanical Integrity and Inspection Design Services
    • Pressure Vessel and Tank Program Development and Implementation
    • Systematic Inspection Design (SID)
    • Risk-Based Inspection
    • Integrity Evaluation and Analysis
    • Fitness-for-Service Evaluations
    • Welding Consulting and Inspection
  • Coatings and Linings Engineering
    • Consulting
    • Failure Analysis
    • Project Management
    • Quality Control and Inspections
  • Project and Quality Management Services
    • Cost Estimating Services for all Construction Disciplines
    • Project Management Services, Including Planning, Scheduling, and Project Controls
    • Quality Control Inspection and Testing Services
    • Quality Assurance Program Development for New and Rehabilitation Construction
    • Quality Assurance Program Auditing and Oversight Services
  • Training Services
    • Alloy Selection for Corrosive Environments
    • Solving Corrosion and Materials Problems for Plant Engineers
    • Welding Essentials for Plant Engineers
    • Brick and Tile-Lined Equipment Inspection and Repair
    • FRP Essentials for Pulp and Paper Mills
    • Concrete Restoration and Protection
    • Quality Assurance for Capital Projects

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