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RBI 581
Oleh:
Dr. Ir. Haryadi
Development of Inspection Programs to Reduce Risk
JURUSAN TEKNIK MESINPOLITEKNIK NEGERI BANDUNG
2014
Introduction
Development of Inspection Programs:
that address the types of damage that inspection should
detect, and the appropriateinspection techniques to
detect the damage.
Reducing Risk Through Inspection:
discusses the application of Risk-Based Inspection tools to
reduce risk and optimize inspection programs.
Approach To Inspection Planning
Contains
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1. INTRODUCTION1. INTRODUCTION1. INTRODUCTION1. INTRODUCTION
The probability of failure due to progressive damage is a
function of four factors:
Damage mechanism and resulting type of damage
(cracking, thinning, etc.).
Rate of damage progression.
Probability of detecting damage and predicting future
damage states with inspection technique(s).
Tolerance of the equipment to the type of damage.
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2. 2. 2. 2. DEVELOPMENT OF INSPECTION DEVELOPMENT OF INSPECTION DEVELOPMENT OF INSPECTION DEVELOPMENT OF INSPECTION
PROGRAMSPROGRAMSPROGRAMSPROGRAMS
An inspection program is developed by systematically
identifying:
a. What type of damage to look for.
b. Where to look for it.
c. How to look for the damage (what inspection
technique).
d. When (or how often) to look.
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Design and construction data:
a. Equipment type (heat, mass, or momentum transfer) and function (shell and tube exchanger, trayed distillation column, centrifugal pump, etc.).
b. Material of construction.
c. Heat treatment.
d. Thickness.
Process data, including changes:
a. Temperature.
b. Pressure.
c. Chemical service, including trace components (such as chlorides, CNs,ammonium salts, etc.).
d. Flow rate.
Equipment history:
a. Previous inspection data.
b. Failure analysis.
c. Maintenance activity.
d. Replacement information.
Data
Inspection techniques are selected based on their ability to find the
damage type; however, the mechanism that caused the damage
can affect the inspection technique selection.
Table 9-7 qualitatively lists the effectiveness of inspection
techniques for each damage type listed in Table 9-2.
A range of effectiveness is given for some damage type/inspection
technique combinations based on comments from various sources,
including the API Subcommittee on Inspection.
Selection of the inspection technique will depend on not only the
effectiveness of the method, but on equipment availability and
whether or not an internal inspection can be made.
2.2 How To Look For Damage (Inspection2.2 How To Look For Damage (Inspection2.2 How To Look For Damage (Inspection2.2 How To Look For Damage (Inspection
Technique)Technique)Technique)Technique)
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Damage types are the physical characteristics of damage that
can be detected by an inspection technique.
Damage mechanisms are the corrosion or mechanical actions
that produce the damage.
Table 9-1 describes damage types and their characteristics.
Tables 9-2 through 9-6 list damage mechanisms by broad
categories.
The types of damage that can be associated with them are
also listed.
These lists of damage mechanisms were developed by several
API members of the Fitness for Service Program.
2.1 What Type of Damage To Look For and2.1 What Type of Damage To Look For and2.1 What Type of Damage To Look For and2.1 What Type of Damage To Look For and
Where To LookWhere To LookWhere To LookWhere To Look
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3. REDUCING RISK THROUGH 3. REDUCING RISK THROUGH 3. REDUCING RISK THROUGH 3. REDUCING RISK THROUGH
INSPECTIONINSPECTIONINSPECTIONINSPECTION
In this case study, a usually effective inspection was
performed after six years.
For the following analysis, it is assumed that the inspection
revealed an actual corrosion rate of 5 mpy vs. the predicted
rate of 10 mpy.
Figure 9-3 shows the damage subfactor table from the
technical module for general corrosion. The thick line on the
table shows the path traced by an inspection plan (this is
discussed further in the next section). Using Table 9-19, the
following steps show how the damage subfactor is calculated
for the risk assessment.
3.1 Measuring Risk Associated With Existing3.1 Measuring Risk Associated With Existing3.1 Measuring Risk Associated With Existing3.1 Measuring Risk Associated With Existing
Inspection SystemInspection SystemInspection SystemInspection System
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Step 1: Calculate the ratio ar/t.
This is the equipment age (or time in current service) (a) times the corrosion rate (r), in in./yr, divided by the original thickness (or thickness at time equipment went into current service) (t).
Example:
5 mpy (0.005 in./yr), 6 years old, original thickness 0.375 in. ar/t = 6 x 0.005/0.375 = 0.08.
Step 2: Determine the overdesign factor.
This is a correction factor selected from the table in Table 9-12 that will be applied to the damage subfactor. The correction is necessary because the subfactors from the table arebased on a vessel that has a corrosion allowance of 25% of the wall thickness, while the vessel in this example has a corrosion allowance of 50% of the wall thickness. Vessels with a greater corrosion allowance should have a lower damage subfactor, while those with less corrosion allowance should have a higher damage subfactor.
Example:
Original thickness = 0.375 in.,
Corrosion allowance = 0.1875.
tactual / (tactual Corrosion Allowance) = 0.375 / 0.1875 = 2.0.
The overdesign factor selected from the table, is 0.5; that is, the damage subfactors are to be multiplied by one-half (for subfactors greater than 1).
Step 3: Refer to Figure 9-3 to find the damage subfactor for this vessel.
At one inspection (of any effectiveness) and ar/t = 0.08, the damage subfactor is 1.
Step 4: Multiply results of Step 3 by results of Step 2.
APPROACH TO INSPECTION PLANNINGAPPROACH TO INSPECTION PLANNINGAPPROACH TO INSPECTION PLANNINGAPPROACH TO INSPECTION PLANNING
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Implicit in the ar/t lookup tables is a remaining life.
When the damage factor rises to 10 or higher with 4 or more highly effective inspections, then the equipment is at or near the end of its life. In other words, there have been enough inspections to have relative certainty about the corrosion rate, and additional inspections no longer improve the damage factor.
The inspection planning method solves for the number of years at which this point occurs (roughly ar/t = 0.4, with corrections for pressure and corrosion allowance).
If this value is one year or less, a diagnostics module is called to provide a warning that based on the entered corrosion rate, age, and number of inspections, the equipment is already at or near its end of life. Careful data checking and/or confirmation of equipment condition are recommended.
If the remaining life is greater than one year, determine the number of inspections needed to achieve a high confidence in the corrosion rate over the remaining life of the equipment.
This is expressed as the number of inspections of whatever effectiveness has been performed in the past, assuming that this is the preferred inspection type for this plant.
The number of inspections can easily be converted to an equivalent number of inspections of a different effectiveness, based on the following relationships:
One highly effective is equivalent to two usually effective, is equivalent to four fairly effective.
If CUI is applicable in addition to internal thinning, the target damage factor is set to 5 for each mechanism so that the combined mechanisms will not lead to a damage factor greater than 10.
4.1 Method4.1 Method4.1 Method4.1 MethodThinning Mechanisms:Thinning Mechanisms:Thinning Mechanisms:Thinning Mechanisms:
Determine the current technical module subfactor. If this is less than 10, then use the SCC module escalation factor (years since last inspection) of 1.1 to determine the number of years until a TMSF of 10 will be reached.
As a default, perform a Fairly effective inspection at that time as a check on the SCC condition.
If the current TMSF is greater than 10, use the relationships in Table 9-15 to determine the inspection level required.
It is recommended that the inspection be performed within three years of the last inspection, or as soon as practical if more than three years has elapsed.
4.2 Method4.2 Method4.2 Method4.2 MethodStress Corrosion Cracking:Stress Corrosion Cracking:Stress Corrosion Cracking:Stress Corrosion Cracking:
Current SCC TMSF Inspection Level Recommended
10 < TMSF < = 100 Perform Fairly Effective Inspection
100 < TMSF < = 1000 Perform Usually Effective Inspection
1000 < TMSF Perform Highly Effective Inspection
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4.3 Method4.3 Method4.3 Method4.3 MethodFurnaces Inspection Planning:Furnaces Inspection Planning:Furnaces Inspection Planning:Furnaces Inspection Planning:
Part 1. Long Term Damage:
If the current TMSF is less than 10, increment ti (operating
hours) by 10,000 (~1 year) until a TMSF of 10 is reached.
The number of increments is the time to the next inspection,
Tinsp. Use Table 9-16 to determine inspection requirements:
If the current TMSF is greater than 10, use the following
relationships to determine the inspection level required:
It is recommended that the inspection be performed within
three years of the last inspection, or as soon as practical if
more than three years has elapsed.
Part 2. Short Term Damage:
For the short term damage TMSF, perform following action.
Furnace Inspection Intervals With a TMSF Less
Than TenTinsp Inspection Method Inspection Time
> = 20 years Fairly Effective
Usually Effective
Highly Effective
5 years
10 years
20 years
> = 10 years, < 20 Fairly EffectiveUsually Effective
Highly Effective
3 years
6 years
12 years
> = 5 years, < 10 Fairly EffectiveUsually Effective
Highly Effective
Not Allowed
3 years
6 years
< 5 years Fairly EffectiveUsually Effective
Highly Effective
Not Allowed
Not Allowed
T insp
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Furnace Inspection Intervals With a TMSF
Greater Than Ten
Current SCC TMSF Inspection Level Recommended
10 < TMSF < = 50 Perform Usually Effective Inspection
50 < TMSF < = 500 Perform Highly Effective Inspection
500 < TMSF Perform Highly Effective Inspection plus perform Remaining Life Evaluation
Actions Required for a Short-Term TMSF
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4.4 Method4.4 Method4.4 Method4.4 MethodHigh Temperature Hydrogen High Temperature Hydrogen High Temperature Hydrogen High Temperature Hydrogen
Attack:Attack:Attack:Attack:
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