Fitness
for Service (FFS) Evaluations
Fitness-for-Service
(FFS) assessments are quantitative engineering evaluations
that demonstrate the structural integrity of a flawed or damaged
component. The results of a FFS Assessment can be used to:
-
Evaluate the integrity of damaged components
- Make
run/repair/replace decisions that maximize equipment availability
and improve performance
- Reduce
unnecessary repairs and avoid unplanned shutdowns
- Accurately
predict equipment and process behavior
The
ASME and API design codes and standards for pressurized equipment
provide rules for the design, fabrication, inspection, and
testing of new pressure vessels, piping systems, and storage
tanks, however, these codes do not address the fact that equipment
degrades while in service.
Fitness-for-Service
(FFS) assessments are quantitative engineering evaluations
that demonstrate the structural integrity of a flawed or damaged
component.
The
Equity Engineering Group has performed many FFS assessments
for a wide range of clients. Our engineers are the prime developers
of the landmark API Recommended Practice for Fitness-for-Service,
API-579. David Osage is internationally known as an industry
expert and leader in the development and use of FFS technology.
He heads numerous Code committee and industry activities to
develop new assessment technologies. The architect and principal
author of API-579, Osage also developed many of the assessment
methodologies and supporting technical information. Gerrit
Buchheim, a recognized expert in materials engineering and
identification of damage mechanisms, has extensive experience
using FFS technology in conjunction with Risk Based Inspection
(RBI). Buchheim's contributions to API-579 include the incorporation
of damage mechanisms in assessment methodologies, NDE for
flaw identification and sizing, and remediation techniques.
The
Equity Engineering Group's involvement in API-579 and the
planned ASME Post Construction FFS standard places us on the
leading edge of FFS technology. Our FFS and RBI expertise
allows us to help clients make informed run/repair/replace
decisions to ensure safe operation. We develop equipment inspection
and maintenance strategies that maximize equipment availability
and improve sustainable maintenance performance. The Equity
Engineering Group's use of FFS technology has saved our clients
significant sums in unnecessary repairs and shutdowns-some
representing a return on investment greater than 200 to 1.
We
provide the full range of capabilities required to perform
comprehensive FFS assessments, including damage mechanism
identification, materials testing, NDE and monitoring, and
analytical capabilities for detailed mechanical integrity
assessments and development of repair recommendations. Our
unique blend of analytical skills and owner-user experience
allows us to provide practical, cost-effective solutions to
critical equipment problems. We specialize in providing the
appropriate analysis to solve problems no one else can tackle.
The Equity Engineering Group utilizes nonlinear FEA to conduct
advanced mechanical integrity assessments to predict equipment
and process behavior. Use of conventional tools can lead to
conservative assessments, which may result in unnecessary
repairs, replacement, or non-optimum equipment operation.
Some
examples of how we used API-579 Level 3 Analysis advanced
techniques to evaluate FFS include:
Training
Courses - Fitness For Service
We
offer comprehensive FFS training courses on the background
and application of API-579. Our standard 3 day course is geared
toward plant inspectors and engineers, and covers Level 1
and 2 methods. We offer a 1.5 day course for plant inspectors
that emphasizes Level 1 methods and NDE for all assessments.
We also conduct a 1 day workshops on Level 3 methods with
example problems and software tool demonstrations.
VCESage
Software and Training
VCESage
software is a powerful suite of program modules that consolidates
a group of mechanical engineering assessment capabilities.
VCESage contains the latest FFS assessment capabilities in
API-579 and provides accurate, easy-to-use design/analysis
technologies that can be used to analyze existing and/or new
equipment for structural adequacy, code compliance, rerating
and remaining life. Training in the use of VCESage to model
FFS decisions is also available.
Contact Mary Buchheim
if you would like a demo copy of VCESage software.
Case
Study 1 - Ring Joint Cracking
The
Equity Engineering Group conducted an assessment for hydrogen
processing vessel with cracks in the ring groove radius region
of a ring joint style flange. Results of an elastic-plastic
numerical fracture mechanics assessment showed that the driving
force for crack propagation was high for a localized region
near the ring groove radius, but decreased significantly for
deeper cracks. Also, the highest stresses occurred during
the bolt-up operation. The material's properties indicated
that it had good resistance to hydrogen-assisted crack growth
during downtime and to crack advance during service. Our recommendation
that the unit be started up with no repairs to the flange
cracks resulted in a large cost savings and shutdown minimization.
Case
Study 2 - Out-of-Roundness Analysis
A
new pressure vessel in a petrochemical plant was dropped during
field erection, resulting in shell distortion exceeding Code
limits. To evaluate the shell's integrity, The Equity Engineering
Group constructed a finite element model based on field measurements.
Field data points were read directly into the solid modeling
program. Results of fatigue assessment and evaluation of the
collapse strength under internal pressure loading indicated
that the shell was acceptable for future operation at the
original design conditions. Based on our FFS evaluation, the
plant avoided costly repairs as well as unplanned operational
interruption.
Case
Study 3 - Pitting Analysis
A
refinery sludge tank had numerous 1-inch diameter pits in
the lower course of the tank. The refinery asked The Equity
Engineering Group to evaluate the pitting and develop fill
restriction, permissible pit-size and/or spacing limits, or,
if needed, repair details.
Nonlinear
FEA determined that no fill restriction was required with
the current size and density of pits. We developed pit size
and spacing limits the plant could use to determine when repair
would be necessary.
Case
Study 4 - Shell With Distortions Under External Pressure
A
major fire in an oil refinery resulted in excessive distortion
of a vacuum tower vessel's shell. Using field measurements
of the shell profile, we used nonlinear FEA to compare the
external pressure capacity to that of an undamaged shell.
Our analysis indicated that the integrity of the vessel was
not compromised. The Equity Engineering Group's assessment
eliminated the need for costly repairs and extension of the
unplanned shutdown, resulting in savings of more than $500,000.
Case
Study 5 - Jacketed Vessel Crack
We
performed Finite element analysis, piping flexibility analysis
and fitness-for-service assessments of a jacketed polymer
mixing tank that had crack-like flaws and severe distortion
in the jacket, vessel and the main rectangular flange. We
determined that the flaws were fatigue damage and buckling
resulting primarily from thermal stresses cycling over the
10 to 15 years of operation. We designed modifications to
the vessel's side walls, flanges and base plates that resulted
in stress reductions that will improve the fatigue life of
the repaired tank by more than a factor of 3.0.
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