Ballot: 581-XXXX-XXX
Modifications to Carbonate Cracking Module (Section 11 of Part 2)
Title:
Date:
Contact:
April 15, 2011
Source:
Name: Kim Galante
Company: Equity Engineering
Phone:
484-442-8030
E-mail:
kkgalante@equityeng.com
This ballot is to update the Carbonate cracking module for calculation of
damage factor based on the latest technology. Also, have added a VERY
LOW susceptibility category
Action Log
Revision:
0
Impact:
The addition of the VERY LOW susceptibility provides better flexibility
for inspection planning.
Rationale:
This ballot is to update the Carbonate cracking module for calculation of
damage factor based on the latest technology.
Purpose:
Changes: See attached red-lined version of Section 11.0
11 SCC DAMAGE FACTOR – CARBONATE CRACKING
11.1 Scope
The damage factor calculation for components subject to alkaline carbonate stress corrosion cracking
(ACSCC) is covered in this paragraph.
11.2 Description of Damage
Carbonate cracking is the common term applied to surface-breaking cracks that occur at or near carbon
steel welds under the combined action of tensile stress and in the presence of an alkaline sour water
containing moderate to high concentrations of carbonate (CO 3). On a macroscopic level, carbonate
cracking typically propagates parallel to the weld in the adjacent base metal, but can also occur in the
weld deposit or heat-affected zones. At times, ACSCC may be mistaken for SSC or SOHIC, but further
review will show that carbonate cracking is usually located further from the toe of the weld and contains
multiple, parallel cracks. At times, the pattern of cracking observed on the steel surface is described as a
“spider web” of small cracks, which initiate at or interconnect with weld-related flaws that serve as local
stress risers. Finally, from the microscopic perspective the intergranular, oxide-filled cracks are similar in
appearance to caustic stress corrosion cracking and amine cracking.
Historically, carbonate cracking has been most prevalent in fluid catalytic cracking unit (FCCU) main
fractionator overhead condensing and reflux systems, the downstream wet gas compression system, and
the sour water systems emanating from these areas. Based upon recent survey results, sour water
strippers with side-pumparound designs, Catacarb/CO2 removal facilities for hydrogen manufacturing
units and delayed coker light ends units have been added to the list of affected units. In all instances,
both piping and vessels are affected.
Assuming the presence of a free water phase containing H2S, three key parameters are used to assess
the susceptibility to carbonate cracking; pH of the water, carbonate concentration of the water, and the
residual stress level of the exposed carbon steel. Typical pH’s are in the 8-10 range. Regarding
contaminant/additive concentrations, no threshold for H2S has been identified; no evidence exists to
indicate cyanides or polysulfides have any impact; and increased ammonia (NH3) concentrations will
directionally increase ACSSS likelihood. As for residual stress levels, they typically emanate from
welding, but can be associated with cold worked areas. Studies have concluded that the sour water’s
corrosion electrochemical potential can be used to assess the likelihood of ACSCC. However, accurate
measurement in a field environment is difficult. Therefore, further discussion of the electrochemical
potential is outside the scope of this document.
With regard to mitigation techniques, the application of a post fabrication stress-relieving heat treatment
(e.g. postweld heat treatment) is the most commonly used method of preventing carbonate cracking. A
heat treatment of about 649°C - 663°C [1200°F-1225°F] in accordance with WRC 452 is considered
effective. The heat treatment requirements apply to all exposed welds as well as any external welds with
heat affected zones (HAZ) in contact with the service environment. Other mitigation techniques include:
process barriers (either organic or metallic), alloy upgrades (solid or clad 300SS, Alloy 400 or other CRA),
effective water washing and inhibitor injection.
11.3 Screening Criteria
If the component’s material of construction is carbon or low alloy steel and the process environment
contains “free” sour water at pH > 8 in any concentration, then the component should be evaluated for
susceptibility to carbonate cracking.
11.4 Required Data
The basic component data required for analysis is given in Table 5.1 and the specific data required for
determination of the carbonate cracking damage factor is provided in Table 11.1.
11.5 Basic Assumptions
The main assumption in determining the damage factor for carbonate cracking is that the damage can be
characterized by a susceptibility parameter that is designated as high, medium, or low based on process
environment, material of construction, and component fabrication variables (i.e. heat treatment). Based
on a susceptibility, a Severity Index can be determined that a measure of the susceptibility of the
component cracking (or the probability of initiating cracks) and the probability of a crack resulting in a
leak.
If cracks are detected in the component during an inspection, the susceptibility is designated as High, and
this will result in the maximum value for the Severity Index. Cracks that are found during an inspection
should be evaluated using Fitness-For-Service methods in API 579 [10].
11.6 Determination of the Damage Factor
11.6.1 Overview
A flow chart of the steps required to determine the damage factor for carbonate cracking is shown in
Figure 11.1. The following paragraphs provide additional information and the calculation procedure.
11.6.2 Inspection Effectiveness
Inspections are ranked according to their expected effectiveness at detecting carbonate cracking and
correctly predicting the rate of damage.
Examples of inspection activities for carbonate cracking that are both intrusive (requires entry into the
equipment) and non-intrusive (can be performed externally), are provided in Table 11.2.
The effectiveness of each inspection performed within the designated time period must be characterized
in accordance with Table 11.2. The number and category of the highest effective inspection will be used
to determine the damage factor. If multiple inspections of a lower effectiveness have been conducted
during the designated time period, they can be equated to an equivalent higher effectiveness inspection
in accordance with paragraph 4.4.3.
11.6.3 Calculation of the Damage Factor
The following procedure may be used to determine the damage factor for carbonate cracking, see Figure
11.1
a) STEP 1 – Determine the number of inspections, and the corresponding inspection effectiveness
category using paragraph 11.6.2 for all past inspections. Combine the inspections to the highest
effectiveness performed using paragraph 4.4.3.
b) STEP 2 – Determine the time in-service, age , since the last Level A, B, C or D inspection was
performed.
c) STEP 3 – Determine the susceptibility for cracking using Figure 11.1 and Table 11.3 based on the
pH of the water and CO3 concentration, and knowledge of whether the component was subject to
PWHT. Note that a HIGH susceptibility should be used if cracking is known to be present.
d)
STEP 4 – Based on the susceptibility in STEP 3, determine the severity index,
e)
STEP 5 – Determine the base damage factor for carbonate cracking,
SVI , from Table 11.4.
Dcarbonate
, using Table 7.4
fB
based on the number of, and the highest inspection effectiveness determined in STEP 1, and the
severity index, SVI , from STEP 4.
STEP 6 – Calculate the escalation in the damage factor based on the time in-service since the last
inspection using the age from STEP 2 and Equation (2.21). In this equation, it is assumed that the
probability for cracking will increase with time since the last inspection as a result of increased
exposure to upset conditions and other non-normal conditions.
f)
D carbonate
 D carbonate
 age 
f
fB
1.1
(2.1)
11.7 Nomenclature
is the in-service time since the last Level A, B, C or D inspection was performed
age
Dcarbonate
f
D
carbonate
fB
SVI
is the damage factor for carbonate cracking
is the base value of the damage factor for carbonate cracking
is the severity index
11.8 References
1. R. D. Merrick, “Refinery Experiences with Cracking in Wet H 2S Environments,” Materials
Performance 27, 1 (1988), pp. 30–36.
2. J. H. Kmetz and D. J. Truax, “Carbonate Stress Corrosion Cracking of Carbon Steel in Refinery FCC
Main Fractionator Overhead Systems,” NACE Paper #206, CORROSION/90.
3. H. U. Schutt, “Intergranular Wet Hydrogen Sulfide Cracking,” NACE Paper #454, Corrosion/92 (see
also “Stress Corrosion Cracking of Carbon Steel in Amine Systems,” NACE paper #187,
Corrosion/87) (see also Materials Performance 32, 11 (1993), pp. 55-60).
4. E. Mirabel et al, “Carbonate-type Cracking in a FCC Wet Gas Compressor Station”, Materials
Performance, July, 1991, pp.41-45.
5. NACE Publication 34108, “Review and Survey of Alkaline Carbonate Stress Corrosion Cracking in
Refinery Sour Water”, NACE International, Houston, TX, 2008.
6. M. Rivera et al, “Carbonate Cracking Risk Assessment for an FCCU Gas Plant”, Paper #04639,
NACE International, Houston, TX, 2004.
7. D. Milton et al, “FCCU Light Ends Plant Carbonate Cracking Experience”, Paper #07564, NACE
International, Houston, TX 2007.
8. WRC Bulletin 452, “Recommended Practices for Local Heating of Welds in Pressure Vessels”,
Welding Research Council (WRC), Shaker Heights, OH.
11.9 Tables
Table 11.1 – Data Required for Determination of the Damage Factor – Carbonate Cracking
Required Data
Comments
Susceptibility
(Low, Medium, High)
The susceptibility is determined by expert advice or using the
procedures in this paragraph. This type of cracking may be sporadic
and may grow rapidly depending on subtle changes in the process
conditions. Periodic monitoring of process pH and CO3-2 in FCC sour
waters should be done to determine cracking susceptibility.
Presence of Water
(Yes or No)
Determine whether free water is present in the component.
Consider not only normal operating conditions, but also startup,
shutdown, process upsets, etc.
Presence of H2S in the Water
(Yes or No)
Determine whether H2S is present in the water phase in this
component. If analytical results are not readily available, it should be
estimated by a knowledgeable process engineer.
pH of Water
Determine the pH of the water phase. If analytical results are not
readily available, it should be estimated by a knowledgeable process
engineer.
CO3 Concentration in Water
Determine the carbonate concentration of the water phase present
in this component. If analytical results are not readily available, it
should be estimated by a knowledgeable process engineer.
Age
(years)
Use inspection history to determine the time since the last SCC
inspection.
Inspection Effectiveness Category
The effectiveness category that has been performed on the
component.
Number of Inspections
The number of inspections in each effectiveness category that have
been performed.
Table 11.2 – Guidelines for Assigning Inspection Effectiveness – Carbonate Cracking
Inspection
Effectiveness
Category
Intrusive Inspection Example
None
A
Highly
Effective
Wet fluorescent magnetic particle
testing (WFMT) of 100% of repair
welds and 50-100% of other
welds/cold bends. Alternating
Current Field Measurement
(ACFM) can also be used to
detect and size surface breaking
cracks.
See Notes below.
B
Usually
Effective
WFMT or ACFM of 20-49% of
welds/cold bends.
See Notes below.
Shear wave ultrasonic testing of 50100% of welds/cold bends; or Acoustic
Emission testing with follow-up shear
wave UT.
WFMT or ACFM of less than
20% of welds/cold bends; or Dry
magnetic particle testing of 50100% of welds/cold bends; or
Shear wave ultrasonic testing of 20-49%
of welds/cold bends.
Dry magnetic particle testing of
less than 50% of welds/cold
bends.
Shear wave ultrasonic testing of less
than 20% of welds/cold bends; or
Radiographic testing; or Visual
inspection for leaks.
Visual inspection. Although
cracks may be seen visually in
more progressed damage, WFMT
or ACFM is best for crack
detection.
No inspection
Inspection
Category
C
Fairly Effective
D
Poorly
Effective
E
Ineffective
Non-intrusive Inspection Example
NOTES:
1. When cracking is identified, it is suggested that metallographic techniques, in-situ or destructive,
be used to confirm the cracking mechanism.
Table 11.3 – Susceptibility to Cracking – Carbonate Cracking
Susceptibility to Cracking as a Function Residual Stress
and of CO3 Concentration in Water
Effective PWHT,
possible cold working
pH of Water
CO3 < 100 ppm
< 8.0
Very Low
≥ 8.0
Very Low
Unknown or Ineffective PWHT,
possible cold working
CO3 ≥ 100 ppm
CO3 < 100 ppm
CO3 ≥ 100 ppm
Very Low
Low
Medium
Low
Medium
High
Table 11.4 – Determination of Severity Index – Carbonate Cracking
Susceptibility
Severity Index –
High
1000
Medium
100
Low
10
Very Low
1
SVI
11.10 Figures
STEP 1: Determine the number of inspections
and the corresponding inspection
effectiveness category for all past
inspections. For all past inspections, combine
inspections to the highest effectiveness
performed.
STEP 2: Determine the time in-service,
age, since the last inspection.
Cracks
present?
Yes
No
pH of Water
STEP 3: Determine the susceptibility for
cracking using Table 11.3
High
Susceptibility
CO3
Residual
Stress
STEP 4: Determine the severity index from
Table 11.4.
STEP 5: Determine the base damage
factor for carbonate cracking using Table
7.4.
STEP 6: Calculate the escalation in the
damage factor using Equation 2.21.
Figure 11.1 – Determination of the Carbonate Cracking Damage Factor