API 510 Course
Lesson Plan
(Calculations – Internal and External Inspection Intervals)
I.Corrosion Rates and Inspection Intervals
Able to calculate;
a. Metal Loss (including corrosion averaging) (API-510, Para 7.4)
b. Corrosion Rates (API-510, Para 7.1)
c. Remaining Corrosion Allowance (API-510, Para7.1)
d. Remaining Service Life (API-510, Para, 7.2)
e. Inspection Interval (API-510, Section 6)
NOTE: These calculations can be open and/or closed book exams.
II.Joint Efficiencies
Determine;
a. Weld Joint Categories (ASME Section VIII, UW-3);
b. Type of radiography (full, spot, or none) performed basis the nameplate markings (RT-1, RT-2, etc.); (UW 11)
c. Joint efficiency by reading Table UW-12;
d. Joint efficiency for seamless heads and vessels Sections per UW-12 (d); and
e. Joint efficiency for welded pipe and tubing per UW-12 (e).
NOTE: Determining joint efficiency may be part of the internal pressure problem since joint efficiency "E" is
used in the formulas for determining required thickness or Vessel Part MAWP.
API 510 −Calculations
Short Term Corrosion Rate
Long Term Corrosion Rate
tprevious − tactual
CRST = # of years between tprevious &tactual
Remaining Life
tactual
RL =
Section 7, par
7.1.1
tinitial − tactual
CRLT = # of years between tprevious & tactual
Internal Inspection Interval
− trequired
Interval = Lesser of ½ of RL or 10 years
Corrosion−Rate
External Inspection Interval
Required Thickness
Interval = Lesser of 5 years or Internal Interval
Minimum
Thickness
Internal or Onstream Interval
PR
t=
2[(SE) - (0.6P)]
- Lesser of 10 yrs or ½ Remaining life
tm =
t +c
- if remaining life is less than 4 yrs, full life up to 2
years
Corrosion-Rate Calculations
• Corrosion rate equations;
Short Term Corrosion Rate
tinitial − tactual_LAST
tprevious − tactual_LAST
CRST = # of yearsbetween t
previous
Long Term Corrosion Rate
& t actual _LAST
CRLT = # of years between tInitial &tactual_LAST
Variables for Thickness Calcs
•
tinitial
=
tactual
= actual (or LAST) thickness measured
tprevious
= thickness recorded during previous inspections.
thickness at initial installation or beginning of a new corrosion
rate environment.
trequired
= thickness required for internal pressure as calculated by design
formulas (before corrosion allowance and manufacturer tolerances
are added.
Years
=
Years between thickness readings
NOTE: LT and ST corrosion rates must be compared to see which rate results in shortest remaining life .
5
Uniform Corrosion
Uniform Corrosion
7
Pitting Corrosion
9
Pitting Corrosion
10
Corrosion-Rate Calculations
Practice Question # 1
Section 7, par
7.1.1
• A vessel has been in service for 8 years. The original shell thickness was 0.875” and Full RT was performed
during fabrication. The required thickness is 0.850”. Last measured thickness taken 3 years ago indicated a
remaining thickness of 0.865”. What is Short-Term corrosion-rate?
12
Corrosion-Rate Calculations
Section 7, par
7.1.1
Practice Question # 1
•
A vessel has been in service for 8 years. The original shell thickness was 0.875” and Full RT was performed during fabrication. The required
thickness is 0.850”. Last measured thickness taken 3 years ago indicated a remaining thickness of 0.865”. What is Short-Term corrosion-rate?
Calculate Short term corrosion rate
What is Known ?
tprevious −tlast
Corrosion−rate =
tprevious = 0.875
# of years between t previous
t & t last
=
0.875 −0.865
5
=
0.002 inch/yr
tlast = 0.865
How many years between readings?
1.
Previous reading is the ORIGINAL thickness (i.e.
ZERO years of service)
2. Last reading taken after 5 years of service
(8 – 3 = 5)
13
Corrosion-Rate Calculations
Practice Question # 2
•A vessel has been in service for 7 years and the current thickness of the shell is 1.125”.
Inspection records indicated an original thickness of 1.300”. What is corrosion-rate?
Corrosion-Rate Calculations
Practice Question # 3
A vessel was placed in service in 1980. Initial thickness measurement taken before the vessel was placed into
service indicated a shell thickness of 1.125 inch (28.57 mm). Thickness measurements taken in 1990 indicated
a shell thickness 1.115 inch (28.32 mm). The required thickness of the shell is 0.990 inch (25.15 mm). The
most recent thickness measurement for the shell was taken in 2000 and indicated a remaining thickness of
1.060 inch (26.94 mm). What is corrosion-rate?
Corrosion-Rate Calculations
Practice Question # 4
• Inspection records contain the following information for a particular condition
monitoring location;
Thickness
0.875 inch (22.22 mm)
0.863 inch (21.89 mm)
0.840 inch (21.59 mm)
Years of service
0 (New)
5
10 (Current)
• What is the corrosion-rate for this condition monitoring location?
Corrosion-Rate Calculations
Practice Question # 5
After 10 years of being in service, a vessel is inspected and the thinnest
shell section is found to be 1.250 inch (31.75 mm). If the original
thickness was 1.275 and the required thickness is 1.125 inch (28.57 mm),
what thickness of material is available for corrosion allowance?
Corrosion-Rate Calculations
Practice Question # 6
A vessel was placed in service in 1995. Initial thickness measurement taken before the vessel was placed
into service indicated a shell thickness of 0.825 inch (20.95 mm). Thickness measurements taken in 2004
indicated a shell thickness 0.820 inch (20.83 mm). The required thickness of the shell is 0.775 inch (19.70
mm). The most recent thickness measurement for the shell was taken in 2013 and indicated a remaining
thickness of 0.811 inch (20.60 mm). What is corrosion-rate?
Corrosion-rate Determination
• Determined by SHORT term or Long Term Calculations (API 510, par 7.1.1.2)
• Newly installed or Change in Service (API 510, par 7.1.2)
1. Calculated from data of vessels in similar service.
2. Estimated from Owner-User experience
3. Published Data
4. On-stream determination after 1000 hrs of service.
• May have different corrosion-rates for large vessels with multiple zones. (API
510, par 6.5.3)
Remaining Life Calculations
• Remaining Life equation;
Remaining Life
RL =
tactual
tactual_Last
Section 7, par
7.2.1
−
trequired
Corrosion−Rate
Variables for Remaining Life Calcs
= actual (or LAST) thickness measured
trequired =
thickness required for internal pressure as calculated by design
formulas (before corrosion allowance and manufacturer tolerances
are added)
NOTE: Corrosion rate is the rate (i.e. Short or Long term rate) which results in
shortest remaining life .
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Remaining Life Calculations
Section 7, par
7.2.1
Practice Question # 7
•
A vessel has been in service for 8 years. The original shell thickness was 0.625” and Full RT was performed during fabrication. The required
thickness is 0.575”. Current measured thickness indicates a remaining thickness of 0.600”. What is Remaining life?
What is Known ?
Looking at the formula for “Remaining Life”, what is UNKOWN?
Remaining Life
=
− trequired
tlast
Corrosion−rate
tprevious =
0.625
tlast =
0.600
Unknown?
Corrosion−rate =
?????
Since the “CORROSION−RATE is unknown, the 1st Step is to determine the Corrosion−rate.
26
Remaining Life Calculations
Practice Question # 7
•A vessel has been in service for 8 years. The original shell thickness was 0.625” and Full RT was performed during fabrication. The required thickness is
0.575”. Current measured thickness indicates a remaining thickness of 0.600”. What is Remaining life?
−
tlast
trequired
Corrosion−rate
=
Remaining Life
What is Known ?
Step # 1 – Calculate the CORROSION-RATE:
tprevious −
Corrosion-rate =
tlast
# of years between tprevious & tlast
=
0.625 −0.600
8
=
0.003
tprevious =
0.625
tlast =
0.600
# of years between =
Previous and Last
8 years
27
Remaining Life Calculations
Practice Question # 7
Step # 2 – Calculate the REMAINING LIFE:
tl ast
Remaining Life =
− trequired
Corrosion−rate
=
0.600 −0.575
0.003
=
8 yrs
What is Known ?
trequired =
0.575
tlast =
0.600
Corrosion−rate =
0.003
28
Remaining Life Calculations
Practice Question # 8
A vessel has a single corrosion-rate of 0.020” per year (0.508 mm/yr). The required thickness for
the shell and heads is 0.750 inch (19.05 mm). The actual thickness of the head is 0.875 inch and
shell is 0.900 inch. What is the remaining life of the vessel?
29
Remaining Life Calculations
Practice Question # 9
A vessel was placed in service in 1980. Initial thickness measurement taken before the vessel was
placed into service indicated a shell thickness of 1.125 inch (28.57 mm). Thickness measurements
taken in 1990 indicated a shell thickness 1.115 inch (28.32 mm). The required thickness of the shell
is 0.990 inch (25.15 mm). The most recent thickness measurement for the shell was taken in 2000
and indicated a remaining thickness of 1.060 inch (26.94 mm). What is remaining life?
Inspection Intervals and Next Inspection Date
• Internal Inspection Intervals (API 510 par 6.5.1.1)
• Internal or on-stream inspections and thickness inspections shall not exceed one half the
remaining life of the vessel or 10 years, whichever is less.
• Whenever the remaining life is less than four years, the inspection interval may be the full
remaining life up to a maximum of two years.
• External Inspection Intervals (API 510 par 6.4.1)
• Interval not exceed the lesser of 5 years or the internal/on-stream interval.
The interval is establish by inspector or engineer in accordance with the owner/user’s
QA System.
• CUI Inspection Intervals
• Should be part of the inspection plan, but no interval requirements mentioned in API- 510
• “SHALL” be considered for externally insulated vessels and those that are in “intermittent”
service or operates at temperature between;
• 10oF and 350oF for carbon steel and alloy steels
• 140oF and 400oF for austenitic stainless steels
• 280 oF and 350 oF for duplex stainless steel
Internal Inspection Interval
Section 6, par
6.5.1v
Practice Question # 10
A vessel has been in service for 8 years. The original shell thickness was 0.625” and Full RT
was performed during fabrication. The required thickness is 0.575”. Current measured
thickness indicates a remaining thickness of 0.600”. What is Internal Inspection Interval?
32
Internal Inspection Interval
Practice Question # 10
•A vessel has been in service for 8 years. The original shell thickness was 0.625” and Full RT was performed during fabrication. The required thickness is
0.575”. Current measured thickness indicates a remaining thickness of 0.600”. What is Internal Inspection Interval?
What is Known ?
Internal Inspection Internal = lesser of ½RL or 10 years
Remaining Life
Section 7, par
7.2.1
=
− trequired
tlast
Corrosion−rate
tprevious =
0.625
tlast =
0.600
Unknown?
Corrosion−rate =
?????
Since the “CORROSION−RATE is unknown, the 1st Step is to determine the Corrosion−rate.
33
Internal Inspection Interval
Practice Question # 10
•A vessel has been in service for 8 years. The original shell thickness was 0.625” and Full RT was performed during fabrication. The required thickness is
0.575”. Current measured thickness indicates a remaining thickness of 0.600”. What is Remaining life?
−
tlast
trequired
Corrosion−rate
=
Remaining Life
Any Unknowns?
Don’t know the Corrosion-rate
What is Known ?
Step # 1 – Calculate the CORROSION-RATE:
tprevious −
Corrosion-rate =
tlast
# of years between tprevious & tlast
=
=
0.625 −0.600
8
0.003
Section 7, par
7.1.1
tprevious =
0.625
tlast =
0.600
# of years between =
Previous and Last
8 years
34
Internal Inspection Interval
Practice Question # 10
Section 7, par
7.2.1
Step # 2 – Calculate the REMAINING LIFE:
tlast
Remaining Life =
− trequired
Corrosion−rate
=
0.600 −0.575
0.003
=
8 yrs
What is Known ?
trequired =
0.575
tlast =
0.600
Corrosion−rate =
0.003
35
Internal Inspection Interval
Practice Question # 10
Step # 3 – Calculate the Inspection Interval
Inspection interval = lesser of 1/2RL or 10 years
= lesser of ½ (8 years) or 10 years
= lesser 4 years or 10 years = 4 years
Internal Inspection Interval = 4 years
36
Internal Inspection Interval
Practice Question # 11
A vessel has a single corrosion-rate of 0.020” per year (0.508 mm/yr). The required thickness for
the shell and heads is 0.750 inch (19.05 mm). The actual thickness of the head is 0.875 inch and
shell is 0.900 inch. What is the next on-stream inspection interval?
37
External Inspection Interval
• Practice Question # 12
Section 6, par
6.4.1
A vessel with a required thickness of .375” that operates at 60 oF at 300 psig was placed in
service in July 1996. Thickness readings taken prior to placing this vessel in service indicated
a thickness of 0.725”.
Thickness data taken July 2006 indicated a thickness of .464”. What should the external
inspection interval be?
Next Inspection
API-510, Para 6-1 and 6-2
Next inspection date = Last inspection date +
interval
•A question requiring you to calculate “next inspection date” may be complex or simple.
1. Simple question would only require you to determine inspection interval and then next date.
2. Complex question would require you to calculate corrosion-rate, remaining life, inspection interval, then next
inspection date.
42
Next Inspection
• Practice for “Simple” calculation
API-510, Para 6-1 and 6-2
Practice Question # 13
• A vessel that has a remaining life calculated from the thickness data
taken March 2010. The remaining life of the vessel is 7 years. What is
“next External inspection date”?
43
Next Inspection
API-510, Para 6-1 and 6-2
Practice Question # 14
What is the Next Internal Inspection date with the following
information?
Remaining Life (yr)
March 2000
March 1995
16
.324
.356
44
Evaluation of Pitting
API-510, Para 7.4.3
Section 7, par
7.4.3
• Widely scattered pits can be ignored, if;
Rule # 1
Remaining Thickness below pit is greater than ½ the Required Thickness
Vessel Thickness
= 2.0”
Depth of Pit
= 1.06”
Corrosion Allowance = 0.250
Retirement Thickness = 1.75”
45
Evaluation of Pitting
API-510, Para 7.4.3
• Widely scattered pits can be ignored, if;
Section 7, par
7.4.3
Rule # 2
Area of the pitting below the corrosion
allowance has an area less than 7 in2 within
an 8” diameter circle.
46
Evaluation of Pitting
API-510, Para 7.4.3
• Widely scattered pits can be ignored, if;
Section 7, par
7.4.3
Rule # 3
Sum of the length of pits within any 8” line, must be less than 2”
47
Evaluation of Pitting
Practice Question # 15
Section 7, par
7.4.3
A recent inspection discovered isolated pitting in the vessel wall. Minimum required wall
thickness is 1.250”. Current thickness of the vessel is 1.625”. What is the minimum allowed
remaining thickness allowed at the deepest pit?
48
Evaluation of Pitting
Section 7, par
7.4.3
Practice Question # 15
• A recent inspection discovered isolated pitting in the vessel wall. Minimum required wall thickness is 1.250”. Current
thickness of the vessel is 1.625”. What is the minimum allowed remaining thickness allowed at the deepest pit?
“Minimum allowed remaining thickness below the pit is ½ the required thickness”, Therefore, the
minimum thickness allowed at the deepest pit is;
( ½ required thickness = 1.250”/2 = 0.625”)
Corrosion allowance
Required
Thickness
Remaining thickness below
pit
½ of Required
Thickness
49
Evaluation of Pitting
Practice Question # 16
Section 7, par
7.4.3
• During a recent internal inspection of a 36” ID vessel with 0.985” wall thickness, three isolated pits were discovered. The
pits are within a 8” diameter circle, with the sum of their dimensions along a straight line is 1.53”. Vessel has a required
thickness is 0.895” and corrosion allowance of 0.625”. Dimensions of the pits are as follows;
Pit 1 – 0.665” x 0.250” x 0.350” deep
Pit 2 - 0.720” x 0.300 x 0.420” deep
Pit 3 - 0.600” x 0.250” x 0.300” deep
Which of the following is true?
a.
b.
c.
d.
Pits can be ignored
Pits are unacceptable based on sum of the pit dimensions along a 8” straight line.
Pits are unacceptable due to total area of pitting within an 8” diameter circle.
Pits are unacceptable due to insufficient remaining thickness below the deepest pit.
Evaluation of Pitting
Practice Question # 17
Section 7, par
7.4.3
• During a recent internal inspection of a 36” ID vessel with 2.375” wall thickness, three isolated pits were discovered.
The pits are within a 8” diameter circle. Length of the pits dimensions along a straight line is 0.875”, 0.450” and
0.725”. Vessel has a required thickness is 1.55” and corrosion allowance of 0.250”. Can these pits be ignored? If not,
why not? Dimensions of the pits are as follows;
Pit 1 – 0.875” x 0.450” x 0.850” deep
Pit 2 - 0.450” x 0.600 x 0.420” deep
Pit 3 - 0.750” x 0.750” x 0.300” deep
Inspection Plans
Section 5, par
5.1
A. Inspection plan must be established for all pressure vessels and
pressure-relieving devices.
B. Inspection plan developed by inspector or engineer.
C. Corrosion-specialist must be consulted for inspection plan for
vessels operating above 750oF.
D. Inspection plan shall be evaluated based on present or possible
types of damage mechanisms.
E. Methods and extent of NDE shall be evaluated to assure they can
adequately identify the damage mechanism and severity of
damage.
59
Types of Inspection
Section 5
A. General
•Inspections should be conducted in accordance with the inspection plan
•Prior to performing an inspection, the inspector should be familiar with;
• Thorough understanding of the inspection plan
• Operating conditions since the last inspection (API 572 par 9.1)
• Applicable damage mechanisms
• Prior history
•New inspection intervals shall be established if operating temp increases, operating pressure increases
or process fluid changes. (API 510 par. 6.2.2)
61
Types of Inspection
B. Internal Inspection
•Interval is lesser of ½ remaining life or 10 years. If remaining life is LESS than 4 years, interval can be the full remaining
life up to max of 2 years. (API 510 par 6.5.1.1).
•SHALL be conducted by the inspector (API 510 par 5.5.2.1)
•Primary reason for internal inspection is to find damage that cannot be found by external CML’s (API 510 par 5.5.2.1)
•Internal inspection performed inside the vessel (API 510 par 5.5.2.1)
•Internals may need to be removed to facilitate the internal inspection. Likely will not need to remove 100% of the
internals. (API 510 par 5.5.2.2)
•Inspector should consult with Corrosion Specialist to determine if it is necessary to remove any linings and/or deposits (API
510 par 5.5.2.3)
•Vessels in non-continuous service, the interval is based on number of years of actual service, instead of calendar years,
provided the vessel when idled is separated from process stream & not exposed to corrosive streams.
62
Types of Inspection
API 510 par 5.5.1, page 5-4
C. On-stream Inspection
•Interval same as INTERNAL inspection.
•Should be conducted by either an inspector or examiner. (API 510 par 5.5.3.1)
•On-stream inspections performed by examiners shall be authorized/approved by the inspector (API
510 par 5.5.3.1)
•Inside of vessel inspected from outside vessel. (API 510 par 5.5.3.2)
63
Types of Inspection
API 510 par 5.5.1, page 5-4
D. External Inspection
• Performed by inspector or qualified others (qualified with appropriate
training). (API 510 par 5.5.4.1.1)
• Interval is lesser of 5 years or the internal interval.
• External inspections check; (API 510 par 5.5.4.1.2)
•
•
•
•
•
•
•
•
•
Condition of Outside surface of vessel
Condition of Insulation system
Condition of Coating system
Condition of Supports
For leaks
Hot spots
Vibration damage
Allowance for expansion
Bulging, misalignment, distortion, etc
• Conditions discovered by others, must be reported to inspector. (API 510 par 5.5.4.1.3)
64
Types of Inspection
API 510 par 5.5.1, page 5-4
E. Thickness Inspection
• Performed by inspector or examiner. (API 510 par 5.5.5.1)
• No required interval.
• Inspector should consult with corrosion-specialist when short term corrosion-rate changes
significantly. (API 510 par 5.5.5.3)
• Owner-user is responsible for assuring individuals taking thickness readings are trained and
qualified (API 510 par 5.5.5.4)
Types of Inspection
API 510 par 5.5.1, page 5-4
F. CUI Inspection
• Performed by inspector or other qualified personnel (i.e. same as external)
• Shall be considered for; (API 510 par 5.5.6.1)
• Carbon steel and low alloy operating between 10oF and 350oF.
• Stainless steel operating between 140oF and 400oF.
• Usually causes localized corrosion damage (API 510 par 5.5.6.2)
• Susceptible locations include; (API 510 par 5.5.6.2)
• Insulation or stiffening rings
• Nozzles and manways
• Structural penetrations (ladder clips, pipe supports, etc)
• Damage insulation
• Insulation with failed caulking
• Top and bottom heads
• CUI inspection may require some or all insulation (API 510 par 5.5.6.3)
• Insulation may not need to be removed if; (API 510 par 5.5.6.3)
• Insulation is in good condition and there is no reason to suspect damage behind the insulation;
• CUI inspection can be performed with UT from ID of vessel.
66