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PART D : Assessment of inspection findings
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© 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
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© 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
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© 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
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© 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
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© 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”
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© 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”
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© PMTSB 2006
Assessment of inspection findings
a b c d e f
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2
3
4
5
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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
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© PMTSB 2006
Assessment of inspection findings
a b c d e f
1
2
3
4
5
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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
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© 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
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© PMTSB 2006
Assessment of inspection findings
Localized corrosion on heads…..let’s try this problem
80%10% 10%
A
B
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© 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
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© 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”
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© 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
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© 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
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© 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
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© 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?