reliable assessment of damages.pdf

7/28/2019 RELIABLE ASSESSMENT OF DAMAGES.pdf

1/26N. Al-Khirdaji, AZTech Sr. Consultant

4.3 Reliable Assessment of Damages

Corrosion rate calculations

Remaining life calculations

Inspection Data Quality - 1

Risk is increased when there is lack of, or uncertainty, .

Information on integrity of plant can be generatedfrom the design, operational experience andinspection records, and from sound knowledge of thedeterioration mechanisms and the rate at whichdeterioration will proceed.

nspect ons can t en e p anne at appropr ateintervals using inspection methods that are able todetect the type and level of deterioration anticipated inorder to allow an assessment of the current and futurefitness-for-service to be made.




Completeness and accuracy of inspection data are

inspected equipment or piping component/circuit.

Such data will be used as the basis for determination

of corrosion rates, remaining life, and in fitness-for-

service assessments.

- , ,

the quality and veracity of the information is tested

and validated.


Audits of plant inspection records frequently

en y ncons s en or ncorrec recor e

measurements.

Any such anomalies should be highlighted and

investigated immediately so that they can be

corrected or verified if roven correct .



Flaw Measurement Accuracy

The inspection methods selected must be capable of

Inspection techniques used for flaw measurement

may be different to inspection methods used for

the detection of flaws.

The selection of appropriate flaw measurement

applicable flaw acceptance standard.

TML (Thickness MeasurementLocation) Monitoring - 1 The minimum thickness at each TML can be located

.Electromagnetic techniques also can be used toidentify thin areas that may then be measured byultrasonics or radiography.

When accomplished with UT, scanning consists oftaking several thickness measurements at the TML

. The thinnest reading or an average of several

measurement readings taken within the area of a testpoint shall be recorded and used to calculatecorrosion rates, remaining life,



TML (Thickness Measurement

Location) Monitoring - 2 ,

should include measurements at each of thefour quadrants on pipe and fittings, withspecial attention to the inside and outsideradius of elbows and tees wherecorrosion/erosion could increase corrosionrates.

As a minimum, the thinnest reading and itslocation shall be recorded.

TML (Thickness MeasurementLocation) Monitoring - 3 TMLs should be established for areas with continuing

, ,

potential localized corrosion as well as for general,

uniform corrosion

TMLs should be marked on inspection drawings and

on the piping system to allow repetitive

.

procedure provides data for more accurate corrosion

rate determination



Thickness Measurement Methods

-

the remaining thickness is approaching the minimum

required thickness, additional thickness measuring

may be required. RT or UT are the preferred methods

in such cases. Eddy current devices also may be used.

When UT measurements are taken above 150F

(65C), instruments, couplants, and procedures should

be used that will result in accurate measurements atthe higher temperatures.


Inspectors should be aware of possible

ources o measurement naccurac es an

make every effort to eliminate their

occurrence.

As a general rule, each of the NDE techniques

will have ractical limits with res ect to

accuracy




ultrasonic measurements include the following:

a. Improper instrument calibration.

b. External coatings or scale.

c. Excessive surface roughness.

d. Excessive rocking of the probe (on the curved surface).

e. u sur ace ma er a aws, suc as am na ons.

f. Temperature effects [at temperatures above 150F (65C)].

g. Small flaw detector screens.

h. Thicknesses of less than 1/8 inch (3.2 mm) for typicaldigital thickness gauges.

Evaluating Inspection Data

(PII RUNCOM Software)

RUNCOM: Corrosion Growth detected between successive inspections



Remaining Life Determination

The remaining life of the piping system shall be

calculated from the following formula:

tactual = the actual thickness, in inches (mm), measured at the

Remaining life (years) =tactual - trequired

Corrosion rate

me o nspec on or a g ven oca on or componen

trequired = the required thickness, in inches (mm), at the samelocation or component as the tactual measurement computed by

the design formulas (e.g., pressure and structural) before

corrosion allowance and manufacturers tolerance are added


The lon -term LT corrosion rate of i in circuits

shall be calculated from the following formula:

The short-term ST corrosion rate of i in circuits

Corrosion Rate (LT) =tinitial - tactual

Time (years) between tinitial and tactual

shall be calculated from the following formula:

Corrosion Rate (ST) =tprevious - tactual

Time (years) between tprevious and tactual




tinitial = the thickness, in inches (mm), at the same location as

commencement of a new corrosion rate environment

tprevious = the thickness, in inches (mm), at the same location as

tactual measured during one or more previous inspections.

The preceding formulas may be applied in a

statistical approach to assess corrosion rates and

remaining life calculations for the piping system.

Care must be taken to ensure that the statisticaltreatment of data results reflects the actual condition

of the various pipe components.


Statistical analysis employing point measurements is

localized unpredictable corrosion mechanisms.

Long-term and short-term corrosion rates should be

compared to see which results in the shortest

remaining life as part of the data assessment.

,corrosion specialist, shall select the corrosion rate that

best reflects the current process.



Remaining Life Calculations - 3

The long term and short term corrosion rates should

remaining life calculations.

If there is a significant difference between the two

corrosion rates, further evaluations should be made in

an attempt to determine the cause.

the shortest calculated remaining life.

Corrosion Rate Estimation 1Newly Installed Piping Systems or Changes in Service

new piping systems or for systems whose service has

been changed.

One of the following methods must be used to

determine the probable corrosion rate.

similar material and in comparable service.

Estimate based on the owner-users experience or from

published data for similar material in comparable

service.



Corrosion Rate Estimation 2

Newly Installed Piping Systems or Changes in Service Make initial thickness measurements after no more

.

Corrosion coupons or probes may be useful to

help determine when thickness measurements

should be made.

Make additional thickness measurements as

.

Corrosion Rate Determination

Existing Piping Systems

Corrosion rates shall be calculated on either a

s or - erm or a ong- erm as s.

If calculations indicate that an inaccurate rate

of corrosion has been assumed, the rate to be

used for the next period shall be adjusted to

a ree with the actual rate found



Maintenance Schedule Based on Crack

Length vs. Fatigue Life CurvesMaintenance schedules can be developed from crack length vs.

Catastrophic failure

- .

Critical crack length acr is determined based on KIc and maximum

design stress

The time till repair is determined

considering an appropriate factor

acr

Repair needed

Inspection

of safety i.e., ar= a

cr/(FS).

Remaining loading cycles beforerepair are determined from ai and

ar

ar

ai

Loading cycles, N

MAWP Determination - 1

The MAWP of a piping system must be determined

code (i.e., ASME B31.3 in the case of process plant

piping systems).

The MAWP of the system is that of the weakest

component within the system. Thus, in addition to the

,

considered (e.g., flanges, valves, etc.).




Computations may be made for known materials

comply with the principles of the applicablecode:

a.Upper and/or lower temperature limits for specificmaterials.

. .

c.Inspection requirements.

d.Reinforcement of openings.

e.Any cyclical service requirements.


If the pipe material is unknown, the MAWP

. .,

weakest) material and lowest weld joint efficiency

that would be permitted by the code.

When the MAWP is recalculated, the wall

thickness used in these computations shall be the

actual thickness as determined b ins ection see

5.6 for definition) minus twice the estimated

corrosion loss before the date of the next inspection

Examples 1 and 2 illustrate calculations of MAWP

using the corrosion half-life concept.




Allowance shall be made for the other loadings

n accor ance w e app ca e co e.

The applicable code allowances for pressure

and temperature variations from the MAWP

are permitted provided all of the associated

code criteria are satisfied.

Evaluation of Inspection DataMAWP Calculations Using Corrosion Half-life ConceptExample 1:

Design Temperature: 400F (204oC)

Pipe Material A 106 Gr. B

Pipe Size: NPS 16

Allowable Stress: 20,000 psi (138 mPa) ( (from B31.3)

Lon itudinal Weld Efficienc : 1.0 A 106 Gr. B is

seamless pipe)

Thickness Measured During Inspection: 0.32 in. (8.1 mm)

Observed Corrosion Rate: 0.01 in./year (0.254 mm/yr)

Next Planned Inspection: 5 years



Evaluation of Inspection DataMAWP Calculations Using Corrosion Half-life Concept

Estimated thinning until next inspection = 5 x 0.01 = 0.05in.

(5 x 0.254 = 1.27mm)

MAWP = 2SEt/D (From ASME B31.3)

= 2 x 20,000 x 1 x (0.32 2 x 0.05) /16

=

Since the MAWP > 500 (system design pressure), thesystem may remain in service at the design pressure

without repairs, replacements, or rerating.

Evaluation of Inspection DataMAWP Calculations Using Corrosion Half-life ConceptExample 2:

Next lanned ins ection 7 rs.

Estimated corrosion loss by date

of next inspection = 7 X 0.01 = 0.07 in.

(7 x 0.254 = 1.78mm)

MAWP = 2SEt/D

= (2 x 20,000 x 1 x (0.32 2 x 0.07) /16)

= ps g a

Conclusion: Must reduce inspection interval or determine thatnormal operating pressure will not exceed this new MAWP

during the 7th year, or renew the piping before the 7 th year.



Illustrative Example

Remaining Life Vs. Corrosion Rates and

Benefits of Conducting Repairs



Evaluation of Inspection DataMAWP Calculations Using Corrosion Half-life Concept

Example 3: An NPS 16 piping system has been inoperation for 10 years and has been taken out of service forts rst t oroug nspect on.

Pipe service - Gas with 3.5% H2S

Minimum required thickness - 0.28 in.

Originally installed thickness - 0.375 in.

Thicknesses measured at five locations: 0.36, 0.32,0.33, 0.34, 0.32

Required: Based on the information provided,

calculate corrosion rate

what maximum thickness measurement interval shouldbe used for this system?

Evaluation of Inspection DataExample 3 - Solution

The pipe service places this system into Class I.,

than 5 years based only on the service. Now checkthe remaining life criterion.

CRMax = (0.375- 0.32)/10 = 5.5 x 10-3 in./yr.

Available corrosion allowance = (0.32 - 0.28)= 0.04 in.

Maximum Interval = 0.04/(2 x 5.5 x 10-3 )= 3.6 years < 5 years

Maximum thickness measurement intervalis 3.6 years.



Minimum Required Thickness

Determination The minimum required thickness of a piping

sys em .e., e re remen c ness mus edetermined considering all applicable designloads.

The design pressure of the system will normallygovern the minimum required thickness.

However, local loading conditions (e.g., wind orearthquake, valve weights, local thermal displacementstresses, etc.) might govern the minimum requiredthickness in particular situations.

Both general and localized corrosion must beconsidered.

Minimum Required ThicknessDetermination - 2

In cases where there are significant safety or,

is prudent to increase the minimum required thicknessabove the calculated value.

This additional allowance is meant to account forunanticipated or unknown loads, undiscovered metal loss,tolerance in the thickness measurements, and resistance tonormal abuse.

In all cases, the normal code design formulas andallowable stresses must be used.



Assessment of Inspection Findings - 1

Pressure containing components found to have

capability (pressure loads and other applicable loads,e.g., weight, wind, etc., per API RP 579) shall beevaluated for continued service.

Fitness-for-service techniques, such as thosedocumented in API RP 579, may be used for this

.

The fitness-for-service techniques used must beapplicable to the specific degradation observed.

Reporting and Records for Piping

System Inspection - 1

Any significant increase in corrosion rates

s a e repor e o e owner user or

appropriate action.

The owner/user shall maintain appropriate

permanent and progressive records of each

i in s stem covered b API 570.




System Inspection - 2These records shall contain pertinent data such as:

p p ng sys em serv ce;

classification;

identification numbers;

inspection intervals; and documents necessary to record the

name of the individual performing the testing, the date, the

types of testing, the results of thickness measurements and

other tests, inspections, repairs (temporary and permanent),

alterations, or rerating.


System Inspection - 3 Design information and piping drawings may be

.

events affecting piping system integrity also should

be included.

The date and results of required external inspections

shall be recorded. (See API RP 574 for guidance on

.



Weld Acceptance Criteria - 1

General

-

The surface of the welds should be sufficiently freefrom coarse ripples, grooves, overlaps, abrupt ridges,and valleys.

The surfaces of the finished weld should be suitableto permit proper interpretation of radiographic andother required nondestructive examinations.

Weld Acceptance Criteria - 2

ASME B31.1

Visual examination consists of observation of

whatever portions of a component or weld are

exposed to such observation, either before,

during or after manufacture, fabrication,

assembl or test.



Weld Acceptance Criteria 3

ASME B31.1

The following indications are unacceptable:

cracks in external surface;

undercut on surface which is greater than 1/32

in. deep;

ac o us on on sur ace;

incomplete penetration (applies only wheninside surface is readily accessible).

Reinforcement of

Girth and Longitudinal Butt Welds

Maximum Thickness of Reinforcement for Design Temperature

>750F 350-750F



ASME Section IX

Rounded Indication Charts

ASME Section IXRounded Indication Charts



Fillet Weld Size B31.3

The size of an unequal leg fillet weld is the leg length of the largest

inscribed isosceles right triangle (Theoretical Throat = 0.707 x Size)

Fillet Weld Size B31.3

The size of an unequal leg fillet weld is the leg length of the largest

inscribed isosceles right triangle (Theoretical Throat = 0.707 x Size)



Typical Details For Flange

Attachment Welds B31.3

Figures (1) And (2) Slip-On Flange

Minimum Welding Dimensions

SW Components B31.3

Socket Welding Components Other Than Flanges



Assessment of Inspection Findings

Pressure containing components found to have

capability (pressure loads and other applicable loads,e.g., weight, wind, etc.,) shall be evaluated forcontinued service.

Fitness-for-service techniques, such as thosedocumented in API 579, may be used for this

.

The fitness-for-service techniques used must beapplicable to the specific degradation observed.



Any significant increase in corrosion rates

s a e repor e o e owner user or

appropriate action.

The owner/user shall maintain appropriate

permanent and progressive records of each

i in s stem covered b API 570.


26/26


System Inspection - 2These records shall contain pertinent data such as:

p p ng sys em serv ce;

classification;

identification numbers;

inspection intervals; and documents necessary to record the

name of the individual performing the testing, the date, the

types of testing, the results of thickness measurements and

other tests, inspections, repairs (temporary and permanent),

alterations, or rerating.



Design information and piping drawings may be

.

events affecting piping system integrity also should

be included.

The date and results of required external inspections

shall be recorded. (See API RP 574 for guidance on

.

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