API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology
2-50 API RECOMMENDED PRACTICE 5817 SCC DF—Amine Cracking7.1 ScopeThe DF calculation for components subject to amine cracking is covered in this section.7.2 Description of DamageAmine cracking is defined as cracking of a metal under the combined action of tensile stress and corrosion inthe presence of an aqueous alkanolamine solution at elevated temperature. The cracking is predominatelyintergranular in nature and typically occurs in carbon steels as a network of very fine, corrosion-product-filledcracks. Low alloy ferritic steels are also susceptible to amine cracking. Amine cracking is typically observedin amine treating units that use aqueous alkanolamine solutions for removal of acid gases such as H 2 S andCO 2 from various gas or liquid hydrocarbon streams.Four available parameters are used to assess the susceptibility of steel fabrications to amine cracking. Theyare the type of amine, amine solution composition, metal temperature, and level of tensile stress.Results of a NACE survey indicate that amine cracking is most prevalent in monoethanolamine (MEA) anddiisopropanolamine (DIPA) units and to a somewhat lesser extent in diethanolamine (DEA) units. Cracking ismuch less prevalent in methyldiethanolamine (MDEA), sulfinol, and diglycolamine (DGA) units.Studies have concluded that the cracking occurs in a narrow range of electrochemical potential, which is verydependent upon the amine solution composition. Carbonate is a critical solution contaminant, and othercontaminants such as chlorides, cyanides, etc. have been shown to affect cracking susceptibility. Despitethis mechanistic understanding, the electrochemical potential of in-service components may not be readilyavailable. Amine concentration is a factor in cracking susceptibility in MEA solutions, where crackingsusceptibility has been shown to be higher in the 15 % to 35 % concentration range. There is not sufficientunderstanding of this relationship in other amine solutions, but it is noteworthy that cracking susceptibility islower in MDEA and sulfinol units that typically utilize higher concentration amine solutions.With regard to the amine solution composition, cracking typically occurs in the lean alkanolamine solutionthat is alkaline and contains very low levels of acid gases. Amine cracking does not occur in fresh aminesolutions, i.e. those that have not been exposed to acid gases. Amine cracking is not likely to occur in richalkanolamine solutions, which contain high levels of acid gases. In rich amine solutions, other forms ofcracking are far more prevalent.Amine cracking susceptibility is generally higher at elevated temperatures. A key consideration is the actualmetal temperature and not just the normal process temperature. Cracking has occurred in components thatnormally operate at low temperatures, but were heat traced or steamed out prior to water washing to removeresidual amine solution.As-welded or cold worked carbon and low alloy steel fabrications are susceptible to amine cracking becauseof the high level of residual stress remaining after fabrication by these methods. Application of a postfabricationstress-relieving heat treatment (e.g. PWHT) is a proven method of preventing amine cracking. Aheat treatment of about 621 °C (1150 °F) for 1 hour per inch of thickness (1 hour minimum) is considered aneffective stress-relieving heat treatment to prevent amine cracking of carbon steel.It should be noted that other forms of cracking have been reported in amine units. In most cases, crackingoccurred in components exposed to rich alkanolamine solutions and have typically been forms of hydrogendamage such as SSC, HIC, and SOHIC. These are not included here but are dealt with in other sections ofthis Part.
RISK-BASED INSPECTION METHODOLOGY, PART 2—PROBABILITY OF FAILURE METHODOLOGY 2-517.3 Screening CriteriaIf the component’s material of construction is carbon or low alloy steel and the process environment containsacid gas treating amines (MEA, DEA, DIPA, MDEA, etc.) in any concentration, then the component should beevaluated for susceptibility to amine cracking.7.4 Required DataThe basic component data required for analysis are given in Table 4.1, and the specific data required fordetermination of the amine cracking DF are provided in Table 7.1.7.5 Basic AssumptionsThe main assumption in determining the DF for amine cracking is that the damage can be characterized by asusceptibility parameter that is designated as High, Medium, or Low based on process environment, materialof construction, and component fabrication variables (i.e. heat treatment). Based on the susceptibilityparameter, a Severity Index is assigned that is a measure of the component susceptibility to cracking (or theprobability of initiating cracks) and the probability that the crack will result in a leak.If cracking is detected in the component during an inspection, the susceptibility is designated as High, andthis will result in the maximum value for the Severity Index. Cracks or arrays of cracks that are found duringan inspection should be evaluated using FFS methods in API 579-1/ASME FFS-1 [10] .7.6 Determination of the DF7.6.1 OverviewA flow chart of the steps required to determine the DF for amine cracking is shown in Figure 7.1. Thefollowing sections provide additional information and the calculation procedure.7.6.2 Inspection EffectivenessInspections are ranked according to their expected effectiveness at detecting amine cracking. Examples ofinspection activities that are both intrusive (requires entry into the equipment) and nonintrusive (can beperformed externally) are provided in Annex 2.C, Table 2.C.9.1.If multiple inspections of a lower effectiveness have been conducted during the designated time period, theycan be equated to an equivalent higher effectiveness inspection in accordance with Section 3.4.3.7.6.3 Calculation of the DFThe following procedure may be used to determine the DF for amine cracking; see Figure 7.1.a) STEP 1—Determine the susceptibility for cracking using Figure 7.1. Note that a High susceptibilityshould be used if cracking is confirmed to be present.b) STEP 2—Based on the susceptibility in STEP 3, determine the Severity Index, S VI , from Table 7.2.c) STEP 3—Determine the time in service, age, since the last Level A, B, or C inspection was performedwith no cracking detected or cracking was repaired. Cracking detected but not repaired should beevaluated and future inspection recommendations based upon FFS evaluation.d) STEP 4—Determine the number of inspections and the corresponding inspection effectiveness categoryusing Section 7.6.2 for past inspections performed during the in-service time. Combine the inspectionsto the highest effectiveness performed using Section 3.4.3.
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RISK-BASED INSPECTION METHODOLOGY, PART 2—PROBABILITY OF FAILURE METHODOLOGY 2-51
7.3 Screening Criteria
If the component’s material of construction is carbon or low alloy steel and the process environment contains
acid gas treating amines (MEA, DEA, DIPA, MDEA, etc.) in any concentration, then the component should be
evaluated for susceptibility to amine cracking.
7.4 Required Data
The basic component data required for analysis are given in Table 4.1, and the specific data required for
determination of the amine cracking DF are provided in Table 7.1.
7.5 Basic Assumptions
The main assumption in determining the DF for amine 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 the susceptibility
parameter, a Severity Index is assigned that is a measure of the component susceptibility to cracking (or the
probability of initiating cracks) and the probability that the crack will result in a leak.
If cracking is 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 or arrays of cracks that are found during
an inspection should be evaluated using FFS methods in API 579-1/ASME FFS-1 [10] .
7.6 Determination of the DF
7.6.1 Overview
A flow chart of the steps required to determine the DF for amine cracking is shown in Figure 7.1. The
following sections provide additional information and the calculation procedure.
7.6.2 Inspection Effectiveness
Inspections are ranked according to their expected effectiveness at detecting amine cracking. Examples of
inspection activities that are both intrusive (requires entry into the equipment) and nonintrusive (can be
performed externally) are provided in Annex 2.C, Table 2.C.9.1.
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 Section 3.4.3.
7.6.3 Calculation of the DF
The following procedure may be used to determine the DF for amine cracking; see Figure 7.1.
a) STEP 1—Determine the susceptibility for cracking using Figure 7.1. Note that a High susceptibility
should be used if cracking is confirmed to be present.
b) STEP 2—Based on the susceptibility in STEP 3, determine the Severity Index, S VI , from Table 7.2.
c) STEP 3—Determine the time in service, age, since the last Level A, B, or C inspection was performed
with no cracking detected or cracking was repaired. Cracking detected but not repaired should be
evaluated and future inspection recommendations based upon FFS evaluation.
d) STEP 4—Determine the number of inspections and the corresponding inspection effectiveness category
using Section 7.6.2 for past inspections performed during the in-service time. Combine the inspections
to the highest effectiveness performed using Section 3.4.3.
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