9 Assessing Probability of Failure Flashcards
9 Assessing Probability of Failure
9.1 Introduction to Probability Analysis
Highlights
- The POF analysis shall address all credible damage mechanisms to which the equipment being reviewed is or can be susceptible
- It shall address the situation where equipment is or can be susceptible to multiple damage mechanisms (e.g. thinning and creep)
- The analysis should be credible, repeatable, and documented
- Damage mechanisms are not the only causes of loss of containment
9 Assessing Probability of Failure
9.1 Introduction to Probability Analysis
Other causes of loss of containment
a) seismic activity,
b) weather extremes,
c) overpressure due to pressure-relief device failure,
d) operator error,
e) inadvertent substitution of materials of construction,
f) design error,
g) sabotage.
9 Assessing Probability of Failure
9.2 Units of Measure in the POF Analysis
Highlights
- POF is typically expressed in terms of frequency
- Frequency is expressed as a number of events occurring during a specific timeframe
- For probability analysis, the timeframe is typically expressed as a fixed interval (e.g. one year or one run lenght) and the frequency is expressed as events per interval (e.g. 0.0002 failures per year or per one run lengh)
- For a qualitative analysis, the POF may be categorized (e.g. high, medium, and low, or one through five). In this case, it is appropriate to associate an event frequency with each probability category
9 Assessing Probability of Failure
9.2 Units of Measure in the POF Analysis
Two examples of association of event frequency with each probability category
Pending
9 Assessing Probability of Failure
- 3 Types of Probability Analysis
- 3.1 General
Highlights
- These approaches have been categorized as “qualitative” or “quantitative.”
- The methodology used for the assessment should be structured so that a sensitivity analysis or other approach may be used to assure that realistic, though conservative, probability values are obtained .
9 Assessing Probability of Failure
- 3 Types of Probability Analysis
- 3.2 Qualitative POF Analysis
Highlights
- A qualitative method involves identification of the units, systems, or equipment, the materials of construction, and the corrosive components of the processes.
- Based on knowledge of the operating history, future inspection and maintenance plans, and possible materials deterioration, POF can be assessed separately (assigning a category) for each unit, system, equipment grouping, or individual equipment item.
- Engineering judgment is the basis for this assessment
- The categories may be described with words (such as high, medium, or low) or may have numerical
descriptors (such as 0.1 to 0.01 times per year)
9 Assessing Probability of Failure
- 3 Types of Probability Analysis
- 3.3 Quantitative POF Analysis
Highlights
- One example is to take a probabilistic approach
where specific failure data or expert solicitations are used to calculate a POF. - Another approach is used when inaccurate or insufficient failure data exists on the specific item of interest. In this case, general industry, company, or manufacturer failure data is used.
- This general failure data is used to generate an adjusted failure frequency that is applied to
equipment for a specific application - Qualified persons should make these modifications on a case-by-case basis
9 Assessing Probability of Failure
- 4 Determination of POF
- 4.1 General
Two main considerations for determining POF Regardless of whether a more qualitative or a quantitative analysis is used
a) Damage mechanisms and rates of the equipment item’s material of construction, resulting from its operating environment (internal and external).
b) Effectiveness of the inspection program to identify and monitor the damage mechanisms so that the equipment can be repaired or replaced prior to failure.
9 Assessing Probability of Failure
- 4 Determination of POF
- 4.1 General
Steps for analyzing the effect of in-service deterioration and inspection on the POF
a) Identify active and credible damage mechanisms during the time period being considered (considering normal and upset conditions).
b) Determine the deterioration susceptibility and rate. For example, a fatigue crack is driven by cyclic stress; corrosion damage is driven by the temperature, etc. A damage accumulation rule may be available to mathematically model this process (see API 579-1/ASME FFS-1).
c) Using a consistent approach, evaluate the effectiveness of the past inspection, maintenance, and process monitoring program and a proposed future inspection, maintenance, and process monitoring program.
d) Determine the probability that with the current condition, continued deterioration at the predicted/expected rate will exceed the damage tolerance of the equipment and result in a failure or failures.
9 Assessing Probability of Failure
- 4 Determination of POF
- 4.2 Determine the Deterioration Susceptibility and Rate
Highlights
- Combinations of process conditions and existing materials of construction for each equipment item shall be evaluated to identify active and credible damage mechanisms.
- One method is to group components that have the same material of construction and are exposed to the same internal and external environment. Inspection results from one item in the group can be related to the other equipment in the group.
- For many damage mechanisms, the rate of damage progression is generally understood and can be estimated for process plant equipment.
- Deterioration rate can be expressed in terms of corrosion rate for thinning or susceptibility for mechanisms where the deterioration rate is unknown or immeasurable (such as stress corrosion cracking).
- Susceptibility is often designated as high, medium, or low based on the environmental conditions and material of construction combination. Fabrication variables and repair history are also important
9 Assessing Probability of Failure
- 4 Determination of POF
- 4.2 Determine the Deterioration Susceptibility and Rate
Sources of deterioration rate information
a) published data and unpublished company data,
b) laboratory testing,
c) in situ testing and in-service monitoring,
d) experience with similar equipment,
e) previous inspection data.
The best information will come from operating experiences
9 Assessing Probability of Failure
- 4 Determination of POF
- 4.2 Determine the Deterioration Susceptibility and Rate
Damage rates
- Damage rates will often vary as the mechanism progresses
- In some cases, the mechanism is self-limiting and damage will nearly arrest. In other cases, damage will occur in a slow, stable manner until it reaches a point where failure occurs
- In some cases, damage by one mechanism may progress to a point at which a different mechanism takes over to control the rate of further damage (e.g. pitting that gives rise to stress corrosion cracking)
9 Assessing Probability of Failure
- 4 Determination of POF
- 4.2 Determine the Deterioration Susceptibility and Rate
Parameters for considering in the determination of damage rates
a) fluid stream composition, including electrolytes and ions in solution;
b) temperature, humidity, and corrosiveness of the atmosphere or soil;
c) process temperature;
d) flow velocity;
e) dissolved oxygen content;
f) fluid phase (liquid, vapor, or gas);
g) solution pH;
h) flow stream contaminants;
i) process operating phase (operation, shutdown, wash, etc.);
j) mechanical properties of the metal (hardness, cold work, grain size, etc.);
k) metallurgical properties and corrosion resistance of the alloy;
l) weld properties: heat treatment, hardness, residual stresses, sensitization, inclusions, etc.;
m) component geometry (crevices, local turbulence, etc.);
n) coating and lining condition (no holiday);
o) relative size of anodic and cathodic regions;
p) solubility of corrosion products;
q) use of corrosion inhibitors (type, quantity, and distribution);
r) process control and variations from normal (excursions, frequency of unplanned shutdowns, regenerations, etc.);
s) existence and quality of IOWs.
9 Assessing Probability of Failure
- 4 Determination of POF
- 4.3 Determine Failure Mode
Highlights
- POF analysis is used to evaluate the failure mode (e.g. small hole, crack, rupture) and the probability that each failure mode will occur
- Failure mode primarily affects the magnitude of the consequences
9 Assessing Probability of Failure
- 4 Determination of POF
- 4.3 Determine Failure Mode
Typicall failure modes according to mechanism modes
a) pitting generally leads to small-hole-sized leaks;
b) stress corrosion cracking can develop into small, through wall cracks or, in some cases, rupture;
c) metallurgical deterioration and mechanical damage can lead to failure modes that vary from small holes to ruptures;
d) general thinning from corrosion often leads to larger leaks or rupture;
e) localized corrosion can lead to small- to medium-sized leaks and up to ruptures, depending upon the location and size of the localized corrosion.
