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The Alberta Boilers
Safety Association (ABSA) expresses its opinion about
fitness for service assessments and the use of AP RP
579
4/13/04
The following article is reprinted
from ABSA THE PRESSURE NEWS, Volume 7, Issue 3, September
2002. http://www.albertaboilers.com/newsletter/
FITNESS FOR SERVICE ASSESSMENTS
Fitness for service (FFS) assessment is
increasingly being used as a means to assess if damaged
equipment may continue operation for some desired length
of time. Some typical FFS assessments include reviews on
pressure equipment which has experienced crack-like flaws,
fire damage, pitting corrosion, general or local metal
loss, blisters and laminations, weld misalignments, shell
distortion (out of roundness) and creep damage.
API 579 is a well known published recommended
practice since the issuance of which, industry gained a
useful tool to analyze, evaluate and monitor damaged pressure
equipment. API 579 provides a comprehensive consensus of
industry-recommended practices for making assessments for
quantitative engineering evaluations and for demonstrating
the structural integrity of in-service pressure equipment
containing flaws. We would like to note here that while
API 579 is a useful reference publication, it should be
treated strictly as such and in all situations, operating
pressure equipment must comply with the Safety Codes Act
and regulations and the adopted codes and standards.
For all FFS analyses, similar to other
engineering approaches such as provided for under Para
U2(g) of Section VIII Div. 1 of ASME Code, care must be
exercised to ensure that basic engineering principles are
adhered to and the intent of the reference publication
is understood and followed. Specifically with reference
to FFS analyses, we would like to highlight some concerns:
A significant concern is the use of a
plain-strain critical stress intensity or fracture toughness,
KIc,whose value has not been reconciled with
the environment under consideration. (e.g., API 579 requires
material toughness measured in the environment under consideration)
The problem quite often, is further magnified when such
values are used for cracks in welded joints. The use of
a KIc whose value has not been determined in
the correct environment, requires extreme caution and must
be based upon some reputable studies. In these circumstances,
the KIc values presented as being conservative
are not valid unless substantiated and justified.Industry’s
problem in finding authoritative data for their application
needs to be acknowledged; however, given the fact that
the equipment is damaged and may have serious defects,
the industry cannot be absolved of the responsibility of
using reliable assessment methods.
Other concerns:
- Particularly where cracks are
being considered, the methods used to determine the
state of stress in a component should include capabilities
to compute stress distribution based on loading conditions
and structural configuration.
- Defects must be accurately located
and sized. This requires the use of appropriate NDE techniques
by competent examiners.
- Crack growth analysis in stress corrosion
cracking, HAC and fatigue regimes require 'increment
of crack growth for a given cycle, mm/cycle’ or
'increment of crack growth per unit time, mm/time’ assessment.
Unless authoritative data is available that had been
arrived at from tests done in environmentally similar
conditions, actual tests would be required to determine
material data for crack growth calculations. A cracked
vessel in a stress corrosion cracking environment, that
is subjected to cyclic loading as well, complicates the
analysis even further.
- Capabilities of the equipment, suitability
of the NDE techniques and confirmation of competency
of the technicians can not always be assessed from the
reports of in-service monitoring.
Selection of knowledgeable, reputable
consultants who are experienced in conducting FFS assessments
for pressure equipment is absolutely essential.
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