Flange corrosion protection
Published: 25 June, 2015
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. 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 posing serious and costly problems. ODEE reports.
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.
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.
Crevice corrosion has long been the ‘Achilles heel’ of stainless steel in seawater 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.
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 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.
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.
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.
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).
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.
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.
For further information please visit: www.belzona.com
Flange repairs at a North Sea FPSO
In November 2014, a deck water seal of inert gas generator system on an FPSO that handles seawater 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.