Lesson 11 Corrosion_new2 Corrosion Calculations

8/21/2019 Lesson 11 Corrosion_new2 Corrosion Calculations

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API 510 Preparatory ClassAPI 510 Preparatory Class

Lesson 11Lesson 11

Corrosion CalculationsCorrosion Calculations


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A 60 foot tower consisting of four (4) shell courses was

found to have varying corrosion rates in each course.

Minimum wall thickness readings were taken after 4 years

and 6 months of service. All original wall thicknesses

included a 1/! corrosion allowance.

"he to# course$s original thickness was .%1&'!. "he #resent

thickness is .%000!. "he second course downward had an

original thickness of .%'!.

uring the ins#ection it was found to have a minimum wall

thickness of .%4*!. "he third course was measured at .440!

its original thickness was .'00!. "he +ottom course had an

original thickness of .6&'! and measured to +e .'*'!.

,orrosion -am#le ro+lems


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etermine the metal loss for the to# course the corrosion

rate for the second course the corrosion allowance

remaining in the third course the retirement date for the

+ottom course.



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olution A2

"3 ,35-2

Metal loss euals the #revious thickness minus the #resent

thickness.

revious .%1&'!

resent 7.%000!

.0125" Metal Loss



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-,38 ,35-2

,orrosion rate euals metal loss #er given unit of time.

revious .%'09

resent 7 .%4*0! .0260"

Metal Loss : = .006"/ Per Year


yearsTime

Loss

5.4

"0260.


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";


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>3""3M ,35-

5emaining life euals the remaining corrosion allowance

divided +y the corrosion rate.

1. 5euired "hickness

3riginal "hickness .6&'!

3riginal ,. A. 7.1&'!Required Thickess .500'



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&.5emaining ,orrosion Allowance

Actual =all "hickness .'*'9

5euired Thickness 7.'00!

Re#aii$ %orr. &llo( .0)5'

%.,orrosion 5ate

3riginal "hickness .6&'!

resent "hickness 7.'*'9

Metal Loss .0!0"

Metal ?oss .0%09 : %orrosio Rate = .006"/Yr.

"ime 4.' @ears



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4. 5emaining ?ife

,orrosion Allowance.0*'9 : 14.& @rs. 5emaining ?ife

,orrosion 5ate .006!/@r.



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,orrosion

ust Milling Around


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,lass CuiD

ust Milling Around

1. ,onvert the following decimal fractions into Mils.

.1*'B : EEEEE Mils

. 0&%B : EEEEE Mils

.00*B : EEEEE Mils.

.0011B :EEEEEEMils.

&. ,alculate or #revious 5emaining ?ife #ro+lem using Mils.

,orrosion Allowance.0*'9 : 14.& @rs. 5emaining ?ife

,orrosion 5ate .006!/@r.


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olution

ust Milling Around

,onverting the following decimal fractions into Mils.

.1*'B : 1*' Mils

.0&% : &% Mils

.00* : * Mils.

.0011 : 1.1 Mils.

,orrosion Allowance *' Mils : 14.& @rs. 5em. ?ife ,orrosion 5ate 6.9 Mils/@r.


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"ime

A year has 1& #arts

"he net issue is how do you handle the odd months in

a year. ased on 4.416 years and a metal loss of '0

Mils we haveHMetal ?oss '0 Mils : %orrosio Rate = 11.! Mils/Yr.

"ime 4.416 @ears


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,lass CuiD

"ime

A vessel shell course was re#laced in 3cto+er of 1*0.

"he initial or +aseline thickness was .'0'B. "he most

recent actual thickness measured was .4'B in anuary

1**1. =hat is the corrosion rate +ased in Mils #er year

in anuary of 1**1.


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olution

.'0'(t7initial) 7 .4'(t7actual) : .0&0B : &0 Mils

3ct.1**0 I 3ct.1*0 : 10 years #lus 8ovem+er ecem+er

of 1**0 and anuary of 1**1 soH 10 yrs. % Mos.

10 7 %/1&th (%/1& : .&'0) : 10.&' years

"hereforeH

Metal ?oss &0 Mils : ,orrosion 5ate : 1.95 Mils/Yr .

"ime 10.&' @ears


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@es in the A< '10 there are the ?ong "erm and hort

"erm. ;ere is the ?ong "erm. Grom API 510 6. pa!e 6"#

"he long term (?.".) corrosion rate shall +e calculated from

the following formula2

Corrosion rate (L.T.) :

,an there +e two ,orrosion 5atesF

t(actual)andt(inital)between)time(years

t(actual)-t(inital)


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"he short term (S.T.) corrosion rate shall +e calculated from the

following formula2

Corrosion rate (S.T.) :


actualtandprevioustbetween(years)timeactualtprevioust -


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t initial : the thickness in inches(millimeters) at the same

location as t actual measured at the initial installation or at

the commencement of a new corrosion rate environment.

t previous : the thickness in inches(millimeters) at the same

location as t actual measured during a #revious ins#ection.

Long-term and short-term corrosion rates should *e

co#+ared to as #art of the data assessment. "he authoriDedins#ector in consultation with a corrosion s#ecialist shall

select the corrosion rate that +est reflects the current

#rocess.



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-am#le2 A vessel shell had a second set of ultrasonic thickness

measurements after 1 year of service the original +aseline

wall thickness was 0.'00B and the second set revealed that

the shell was now at 0.4*B. Give years later a third set of

wall readings were taken and the shell was measured to +e

0.4'*B.

=hat value should +e used in the 5emaining ?ife

calculationsF ,om#aring short term to long term corrosion

we find the following values. 8ormally the most aggressivewill +e used and in this case it will +e the ?ong "erm

,orrosion 5ate.



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L.T. $ $ 6.% Mils/ year

S.T. $ $ 6 Mils / year

"he remaining life of the vessel shall +e calculated from the

following formula2

5emaining life (years) : (6.4)


years60.4590.500 -

years5

0.459-0.489

rate corrosion

reuiredt-actualt


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t actual : the actual minimum thickness in inches determined

at the time of ins#ection for a given location or com#onent.

t required $ the reuired thickness in inches at the same

location or com#onent as the t actual measurement

com#uted +y the design formulas (e.g. #ressure and

structural) +efore corrosion allowance and manufacturerJsK.

-am#le2 etermine 5emaining life of the vessel shell

course in the eam#le a+ove. T reuired thickness of the

shell course is 0.%B (known as t minimum). ,om#are S.T. andL.T. corrosion rates as follows2

S.T. rate : 0.006B or 6 Mils a year

L.T. rate : 0.006B or 6. Mils a year



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"herefore we will use the most aggressive corrosion rate

found to +e the ?ong "erm 5ate.

5emaining life (years)

=hat would +e the maimum length of time +efore the net

ins#ectionF

A82 L the 5emaining ?ife or 10 years whichever is less.

There,ore- 1.)/2 = .)5 ears


Yrs Mils

Mils!9.!

8.6

2

"0068.0

"2.0

yeara0.0068"

0.##8"-0.459"


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=hat would +e the maimum length of time +efore the net

ins#ectionF Grom2 A< '10 pa!e 6"&

6. Internal An 'n"(trea) Inspection

"he #eriod +etween internal or on7stream ins#ections shallnot eceed one half the estimated remaining life of the

vessel +ased on corrosion rate or 10 years whichever is

less.


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(a) (the second a) Gor a corroded area of considera+le siDe

in which the Circumferential Stress Govern the leastthickness along the most critical element of the area may +e

averaged over a length not eceeding the following2

1. Gor vessels with the inside diameters less tha or equal to

60 iches one half the vessel diameter or &0 inches(hicheer is less.

&. Gor vessels with the inside diameters $reater tha 60

iches one third the vessel diameter or 40 inches (hicheer

is less.

-am#le '. age '7%

,orrosion


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Gor a corroded area of considera+le siDe in which the

Circumferential Stress Govern the least thickness alongthe most critical element of the area may +e averaged over

a length not eceeding the following2

1. Gor vessels with the inside diameters less tha or equal to

60 iches one half the vessel diameter or &0 inches(hicheer is less.

&. Gor vessels with the inside diameters $reater tha 60

iches one third the vessel diameter or 40 inches (hicheer

is less.

,orrosion and Minimum "hickness -valuation

5.*


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Whe the area cotais a o+ei$ the distance on either side

of the o#ening within which the thicknesses are averaged shall

ot eted *eod the li#its o, the rei,orce#et as de,ied i

the &M3 %ode.

,orrosion and Minimum "hickness

-valuation

ee Gig. 7% and refer to reinforcement calculations fordetermining the etra metal in the shell.

Gor ?imits2

d ( t t r ) or !(t " t

n ) ( t t

r )

4se the Lar$er

, i d Mi i "hi k


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Widel scattered +its may +e ignored as long as thefollowing are true2


-valuation

1. "he remaining thickness +elow the #it is !reater than

one7half the reuired thickness. (1/& t reuired)1.0B I 0.1&'B : 0.'B the reuired thickness

"herefore no #it eual to 0.'B/& : 0.4%'” or greaterN

Pit ! 0.02 7 0.!58 Accepta+le.


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2. The total area o, the +its does ot eceed square

iches (4' s. centimeters) within any ich (&0

centimeter) dia#eter circle.

=e will assume the #its to +e circles and use the formula i

5adius uared to determine the area of each #it.

it O 3ne %.141 (0.10/&)&

: 0.0'&

: 0.0&&6 s.in.

it O "wo %.141 (0.%%0/&) & : 0.16' & : 0.0'' s. in.

it O "hree %.141 (0.&'0/&) & : 0.1&' & : 0.04*0 s.

in.

it O Gour %.141 (0.%/&) & : 0.11' & : 0.1116 s. in.

Total- 0.26 sq. i.

less tha sq. iches Acce#ta+leN

,orrosion and Minimum "hickness -valuation

, i d Mi i "hi k


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%. "he su# o, their di#esios along a strai$ht lie (ithi

the circle does not eceed 2 iches

traight line for #its 17%74

O1. 0.10B

O%. 0.&'0BO4. 0.%B

Total 0.)8 7 2 iches Accepta+le

=hen the sur,ace at a (eld with a Point factor other tha 1.0

as (ell as the sur,aces re#ote ,ro# the (eld is corroded an ide+edet calculatio using the a##ro#riate weld Point

factor must +e made to determine if thickness at the weld or

remote from the weld governs the allowa+le working

#ressure.


-valuation


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Gor this calculation the sur,ace at a (eld icludes 1 ich on

either side of the weld (measured from the toe of the weld)

or t(ice the #ii#u# thickess o either side o, the (eld9

(hicheer is $reater .


-valuation

, i d Mi i "hi k


31/33

=hen measuring the corroded thickness of elli#soidal

and toris#herical heads the governing thickness may +e

as follows2

1. "he thickness of the kuckle re$io with the head ratingcalculated +y the a##ro#riate head formula.

&. "he thickness of the cetral +ortio o, the dished re$io in

which case the dished region may +e considered to +e as#herical segment whose allowa+le #ressure is

calculated +y the ,ode formula for s#herical shells.


-valuation


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%. Gor calculating the s#herical #ortion using the s#herical

formula the following a##lies to find the s#herical radius ?2

a. ? : the 3 of the Torispherical hea,

+. ? : the


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Lesson 11 Corrosion_new2 Corrosion Calculations

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