Wed, 16 November, 2022
TWI engineering team is requested to conduct the corrosion assessment of the blind flange, and investigate the reason for general and pitting corrosion in the ring groove of the blind flange. Figure 1 illustrates a general photo of the requested flange, and figure 2 shows the schematic of the ring groove base on reference (API-6A).
According to table "D.4" of the "API 6A" (Twenty-First Edition, November 2018), the dimension of the blind flange is as mentioned characteristics in figure 3.
- Methodology of evaluation
In order to identify the material, determine the state of corrosion and also check the hardness of different parts of the flange, the following tests and inspections were performed by TWI on the sample.
- Hardness test (HT)
- Positive material identification (PMI)
- Closed visual inspection (CVI)
The results of the hardness test for the blind flange are shown in Figure 4.
The results of the PMI for the ring groove and old ring of the blind flange are shown in Table 1.
Table 1 - Results of PMI for the ring groove and old ring of the blind flange
Low alloy carbon steel
According to the CVI report, the depth of external corrosion in the ring groove is in the range of 0.2 to 0.4 mm. (as shown in Figures 5 & 6).
In this section, the following two important issues are discussed.
- Corrosion mechanisms review
- Corrosion assessment of the blind flange face
4.1 Corrosion mechanisms review
The hardness of Ring Type Joint (RTJ) gaskets is a critical factor when sealing RTJ flanges. It ensures the rings deform and affect a seal without damaging the RTJ grooves on the flanges.
The following statement was referred to in the API B16.20 (2017), Part RJ-3.1:
“It is recommended that ring-joint gaskets be of a lesser hardness than that of the mating flanges.”
Therefore, to investigate the cause of corrosion in the ring groove of the blind flange, the following two issues should be considered.
- According to PMI results, the old ring used in the blind flange was in stainless steel, and the material of the mating flange is low alloy carbon steel (LACS). Therefore, according to API 571 (2020), Section 3.31, this blind flange is susceptible to galvanic corrosion, subject to the following statements:
“There must be two (or at least two) different metals with different electrochemical potentials”
“The dissimilar metals must be electrically coupled together”
“Both metals must be immersed or in contact with the same, continuous electrolyte”
According to the API 571, a typical listing of the relative position on the active to the noble scale of alloys is shown in Table 2.
- The old ring was not softer than the mating flange. Therefore, the ring didn’t deform in the ring groove of the blind flange and so water or moisture was trapped in the crevice between the ring groove and mating flange, this can lead the ring groove to crevice corrosion and also galvanic corrosion can accelerate crevice corrosion.
Therefore, the result of corrosion in the ring groove is directly related to the two statements mentioned above.
4.2 Corrosion assessment of the blind flange face
According to ASME PCC-1, APPENDIX D, Section D-2, the type of gasket should be considered to evaluate the imperfection of the flange surface. There are two different types of gasket.
- Hard-faced gaskets (Ring gasket)
- Soft-faced gaskets
The gasket discussed in this issue is a hard-faced gasket (ring gasket). The following Three main parameters should be measured to assess the corrosion in flange faces:
- Radial width of gasket seating surface (This parameter is denoted by "w" in ASME PCC-1.). (As shown in Figure 7).
- Projected radial distance across the seating surface (This parameter is denoted by "rd" in ASME PCC-1.). (As shown in Figure 7).
- Radial measurement between defects (This parameter is denoted by "d" in ASME PCC-1.). (As shown in Figure 7).
According to ASME PCC-1, APPENDIX D the following table should be used to evaluate the flange face corrosion.
According to the closed visual inspection (CVI) and also considering the above table, the following finding was measured in the ring groove of the blind flange:
- w/2 ˂ rd ˂ 3w/4 & rd ˃ 3w/4
- Maximum measured pitting depth: 0.4 mm
According to the above findings during the inspection and taking into account Table 3, the depth of pitting corrosion and the corrosion area in the ring groove of the blind flange is not within the acceptable range based on the ASME PCC-1, APPENDIX D.
According to the ASME B16.20, table RJ-3.2-1, a new ring (Gasket ring) should be prepared according to the part specified (Red mark part) in the following table:
Considering the galvanic series (Table 2), there is no significant potential difference between the suggested ring gasket in the aforementioned table and the mating flange, therefore, there is no probability of galvanic corrosion between the ring gasket and ring groove.
For further information about TWI CS’s Engineering Services please contact email CRM@twi-cs.com, or +994124043200/01
- API specification 6A, “Specification for Wellhead and Tree Equipment”, Twenty-first edition, November 2018.
- ASME B16.20, “Metallic Gaskets for Pipe Flanges”, December 2017.
- ANSI/API recommended practice 571, “Damage Mechanisms Affecting Fixed Equipment in the Refining Industry”, Third edition, March 2020.
- ASME PCC-1, “Guideline for Pressure Boundary Bolted Flange Joint Assembly”, September 2019.