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Flange Corrosion Protection: Isolating the Sealing Face

All pipelines and pipework incorporate flanges and welded joints of varying sizes, designs and materials. According to HSE Offshore External Corrosion Guide, flanges are one of the six main areas of concern. Evidence suggests that piping systems, including flanges and valves, collectively continue to be a major source of hydrocarbon releases, with piping being the single largest contributor [1]. Transmission of hydrocarbon products in the pipeline exposes flanges to corrosive action of sour gases (H2S, SO2) and chemically aggressive fluids at elevated temperatures, causing pitting to the pipeline internals and flanges. Thousands of flanges are affected annually on offshore platforms, process chemical facilities and water treatment plants posing serious and costly problems.

Corrosion Mechanisms
Corrosion may propagate from localised areas to the whole of the flange face through different corrosion mechanisms and therefore a lot of effort has been put into non-destructive methods of flange face corrosion identification. Traditional non-destructive inspection techniques do not identify the rate and type of corrosion whilst phased array flange inspection is a new and relatively expensive method. Vessel and pipe spool flange face damage becomes apparent only when adjacent pipe spools are removed or if a flange starts leaking either in service or during a leak test. In both cases the equipment’s integrity has been lost.
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Crevice corrosion has long been the ‘Achilles heel’ of stainless steel in sea water service, where corrosive materials concentrate between the crevice of the sealing surface and gasket material. This type of corrosion is accelerated by the presence of hydrocarbons with a high percentage of H2S and Cl¯. Flange corrosion will at some point cause subsequent leakage.
Pic 1. Corroded 24” flange face

Prevention and Repair Methods
Considering today’s economic and environmental climate where leaks are not only costly but can be hazardous to the environment, it is more important than ever to implement a sufficient corrosion prevention plan.

Gaskets under compression are a known and trusted method for corrosion prevention but have been known to fail when exposed to harsh chemicals and thermal deformation of the substrate. Once the flange face is damaged, the flange is no longer sealable by a gasket and requires a replacement or repair. There are four basic types of repair that can be considered:
1. Removing the corroded flange and welding a new one
2. Site machining of the seal face / ring groove within the flange tolerance
3. Weld buttering runs and site machining of the seal face / ring groove
4. Use of polymer composite repair materials to rebuild the flange face

Repeated cutting and welding may introduce more galvanic problems to the pipe joint and the use of heat can distort the substrate, leading to further stress cracking propagation that could cause accelerated flange corrosion. Site machining and weld buttering requires specialist equipment and hot work, necessitating a hot work permit for welding and cutting. In addition, when flammable materials are present, a plant shutdown may be required. Where possible it is advised to avoid hot work, thus eliminating health and fire risks and speeding up the turnaround.

Complete isolation of the flange faces from the operating environment with the use of epoxy composites that bond strongly to the sealing face can be a viable alternative. The 100% solids composite technology has been on the market for over 50 years, but is only now gaining acceptance for flange face repair applications. The material illustrated in this article is Belzona 1111 (Super Metal) manufactured and supplied by Belzona. The system is cold applied and does not require hot work or specialist equipment. Risk of sparks is furthermore eliminated by minimal requirements for surface preparation. Once mixed and applied, the epoxy paste grade composite acts as a permanent gasket, having excellent compressive strength as per ASTM testing. Tab 1 and Tab 2 illustrate typical values for Belzona 1111 when determined in accordance with ASTM D695 and its modified version, which was adapted to be more representative of in service operation by reducing the thickness of the Belzona material. Epoxy composites will also adhere strongly to a variety of metallic substrates, eliminating galvanic corrosion. Epoxy systems however do not add mechanical strength and would not be suitable in situations where the flange has corroded beyond the corrosion allowance.

Cure Temperature
Compressive Strength (Maximum)
Compressive Strength (Yield)
Compressive Modulus
68°F
(20°C)
12525 psi
(86.4 MPa)
9620 psi
(66.3 MPa)
1.77 x 105 psi
(1217 MPa)
212°F
(100°C)
16645 psi
(114.8 MPa)
10955 psi
(75.6 MPa)
1.75 x 105 psi
(1205 MPa)

Tab 1. Typical values of Belzona 1111 (Super Metal) when determined in accordance with ASTM D695 (1.0 in/25.4mm thick test pieces)

Cure Temperature
Thickness
Compressive Strength (Yield)
68°F (20°C)
0.24 in (6.0 mm)
13095 psi (90.3 MPa)
212°F (100°C)
0.24 in (6.0 mm)
16450 psi (113.4 MPa)
68°F (20°C)
0.12 in (3.0 mm)
19910 psi (137.3 MPa)
212°F (100°C)
0.12 in (3.0 mm)
23840 psi (164.4 MPa)

Tab 2. Typical values of Belzona 1111 (Super Metal) when determined in accordance with the modified version of ASTM D695 more representative of in service application


Pic 2. Abraded flange face, epoxy and former applied, former removed

Flange Face Forming Application Procedure
Application procedure includes manual surface preparation, mixing and applying a paste grade composite to a corroded or damaged substrate using a mating flange or a pre-fabricated former to form the flange face. Provided the necessary equipment is at hand, the repair can be delivered within hours with minimum interruption to the process flow.
Prefabricated formers can be metallic or plastic and would ideally be reusable in order to reduce application costs. Accessory kits containing formers and other relevant tools can be made available to simplify and streamline applications. (Pic 3)
Pic 3. Example of an accessory kit for flange face forming applications provided by Belzona

Put It to the Test
Vigorous laboratory and field testing was performed over the last decade. Results to date show that epoxy materials may be recommended for the protection or repair of weld neck flanges and ring type joint flanges. Testing carried out by Wood Group in 2003 confirmed that epoxy materials can be used for the repair of flanges for #150, #300, #600 and #900 pressure rating systems with temperatures up to 120°C (248°F) (Tab 3).

The largest independent crude oil and natural gas producers in the world have standardised the use of polymer materials for the repair of flange faces by forming technique. The use of polymer repairs, when undertaken following manufacturer’s guidelines, is effective in creating a suitable sealing face and preventing crevice and galvanic corrosion.


Test Number
Nominal Pressure (psi)
Tested By
Test Pressure (barg)
Test Temperature (°C)
Comments
1
150
OIS Limited
30 (start)
15
60 minutes soft gasket
30 (end)
15
No leaks
2
300
Motherwell Bridge Inspection Limited
81 (start)
19.5
30 minutes
81 (start)
19.5
Soft gasket, No leaks
3
600
Motherwell Bridge Inspection Limited
160 (start)
19.5
30 minutes
159 (end)
19.5
Spiral gasket, No leaks

Tab 3 Wood Group report data on flange face forming performance at different operating conditions

The Most Important Test: The Test of Time
Corrosion of the flange face is a common problem affecting pressure vessels, where the face needs to be completely isolated in order to prevent oxidation. Back in 2008, four newbuild pressure vessels, two desalters, a dehydrator and a separator, designed to handle hydrocarbons at 120°C (248°F) on a Brazil FPSO required corrosion protection. These vessels are critical pieces of equipment that remove high salinity formation water from the crude oil stream. After carefully evaluating design and operating temperatures and pressures as well as anticipating chemical resistance requirements, a total vessel corrosion protection solution was specified. The entire vessel was internally lined with a ceramic filled novolac epoxy coating. Difficult to access areas that commonly suffer from corrosion, such as small bore nozzles and flange faces, were isolated from the environment with the use of epoxy coatings and composites. Prefabricated formers were designed to form the material on the raised flange faces.

The vessels were then put in service on the FPSO operating in the Jubarte field for the next three years. In February 2013, one of the vessels was opened for inspection and the result was described as “flawless”. Lining, composite formed flange faces and small bore nozzles were all in excellent condition with no signs of deterioration.

Flange Repairs at a North Sea FPSO
In November 2014, a deck water seal of inert gas generator system on an FPSO that handles sea water at ambient temperatures suffered internal corrosion. Existing coating failure led to severe metal loss on the adjacent flanges. 2”, 4” and 24” flange faces were reformed with the use of formers and an epoxy composite material. The application was carried out over a weekend and the entire solution, from first notification, including former fabrication for the 24” flange, was completed in less than a week. The vessel was returned to service with minimum disruption to the production cycle.

Pic 4 and 5 – 4” flange before and after composite repair
Pic 1 and 6 – 24” flange face before and after composite repair
Summary

The use of composite materials for flange face repair and protection is a viable alternative where hot work is undesirable and shutdown may be too costly. Further innovations in polymer materials including faster cure times, surface tolerance and simplified surface preparation techniques make the composite technology even more attractive in both flange maintenance and protection situations.
Flange Corrosion Protection: Isolating the Sealing Face Reviewed by Admin Industri Migas on 12:00 PM Rating: 5

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