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PART D : Assessment of inspection findings

© PMTSB 2006

Assessment of inspection findings

Brief overview of API 510

• Basic reasons for inspection:

1. Determine physical condition of vessel

2. Determine type, rate, and causes of deterioration

3. Determine if vessel remains fit for continued service for its intended function

© PMTSB 2006

Assessment of inspection findings

Let’s try some problems to assess the following;

• Uniform corrosion on shell

• Localized corrosion on shell

• Corrosion on heads

• Scattered pits

© PMTSB 2006

Assessment of inspection findings

Given the following table from API 510

Remaining

Life

Internal inspection

interval

Remaining

Life

External inspection

interval

>20 years 10 years >10 years 5 years

4 – 20 years ½ life 4 – 10 years ½ life

2 – 4 years 2 years 2 – 4 years 2 years

0 – 2 years Full life 0 – 2 years Full life

© PMTSB 2006

Assessment of inspection findings

Uniform corrosion…..let’s try this problem

• In April 2002, thickness was 10mm

• In April 2005, thickness reduced to 9.7mm

• The minimum required thickness for vessel is 8.2mm

• Determine the following;

1. Remaining corrosion allowance

2. Corrosion rate

3. Remaining life

4. Internal inspection interval & next inspection date

5. External inspection interval & next inspection date

© PMTSB 2006

Assessment of inspection findings

Localized corrosion…..let’s try this problem

• RULES: Least thickness is averaged over a longitudinal length of;

Smaller of ½ vessel diameter or 20”, for diameter up to 60”

Smaller of ⅓ vessel diameter or 40”, for diameter above 60”

© PMTSB 2006

Assessment of inspection findings

a b c d e f

1

2

3

4

5

6

Localized corrosion…..let’s try this problem

• Given the following;

Vessel diameter is 96” (2.44mm)

Required thickness is 7.0mm

Distance from 1 – 6 is 36”

Distance from a – f is 30”

© PMTSB 2006

Assessment of inspection findings

a b c d e f

1

2

3

4

5

6

Localized corrosion…..let’s try this problem

• In April 2002;

Point Thickness

(mm)

Point Thickness

(mm)

1 9.8 a 9.8

2 9.6 b 9.6

3 9 c 9

4 8.7 d 8.7

5 9 e 9

6 9.1 f 9.1

© PMTSB 2006

Assessment of inspection findings

a b c d e f

1

2

3

4

5

6

Localized corrosion…..let’s try this problem

• In April 2005;

Point Thickness

(mm)

Point Thickness

(mm)

1 9.1 a 8.9

2 9 b 8.7

3 8.6 c 8.5

4 8.4 d 8.4

5 8.8 e 8.5

6 8.9 f 8.6

© PMTSB 2006

Assessment of inspection findings

a b c d e f

1

2

3

4

5

6

Localized corrosion…..let’s try this problem

• Determine the following;

1. Remaining corrosion allowance

2. Corrosion rate

3. Remaining life

4. Internal inspection interval & next inspection date

5. External inspection interval & next inspection date

© PMTSB 2006

Assessment of inspection findings

Localized corrosion on heads…..let’s try this problem

80%10% 10%

A

B

© PMTSB 2006

Assessment of inspection findings

Localized corrosion on heads…..let’s try this problem

• Remember the following;

If corrosion happens at knuckle area (Area A), use appropriate head formula

If corrosion happens at central area (Area B), use spherical head formula AND radius R shall be equal to K1D, where D is the shell diameter and K1 is from table 5-1 of API 510 D/2h

(h = depth of head)K1

3.0 1.36

2.4 1.08

2.0 0.90

1.6 0.73

1.2 0.57

© PMTSB 2006

Assessment of inspection findings

Localized corrosion on heads…..let’s try this problem

• Given the following basic data;

Head type : 2-1 elliptical head

P = 200 psi

E = 1.0

S = 20,000 psi

D = 80”

Original thickness = 0.66”

© PMTSB 2006

Assessment of inspection findings

Localized corrosion on heads…..let’s try this problem

• Problem : After 5 years in operation corrosion was found on Area A with a minimum thickness of 0.5”

• Determine the following:

1. Corrosion rate

2. Corrosion allowance

3. Remaining life

4. Internal inspection interval

5. External inspection interval

80%10% 10%

AB

© PMTSB 2006

Assessment of inspection findings

Localized corrosion on heads…..let’s try this problem

• Problem : After 5 years in operation corrosion was found on Area B with a minimum thickness of 0.46”

• Determine the following:

1. Corrosion rate

2. Corrosion allowance

3. Remaining life

4. Internal inspection interval

5. External inspection interval

80%10% 10%

AB

© PMTSB 2006

Assessment of inspection findings

Scattered pits…..the rule

• The maximum depth of pit shall not be more than ½ the required thickness of the shell

© PMTSB 2006

Assessment of inspection findings

Scattered pits…..let’s try this problem

• Given the following;

P = 200 psi

E = 1.0

S = 20,000 psi

D = 96”

Original thickness = 0.6”

Problem : What is the maximum pit depth allowed?