The Challenge

The integrity of flanged connections is critical to containing fluids in piping systems. Loss of containment, whether in chemical or water distribution lines, will have an environmental, operational and commercial impact, and could pose a serious safety risk. Flanges present a unique corrosion protection challenge because solutions must both prevent corrosion and allow access to fastenings for inspection or maintenance. In addition, due to the complex geometry of a flanged connection, problems such as crevice corrosion found within the void between the two flange faces, and galvanic corrosion found where dissimilar metals are used, are common and can prove detrimental to the integrity of the piping system.

Corrosion Issues And Inspection

Effective monitoring and inspection techniques are required to minimise unscheduled shutdowns due to leakage in order to meet ever demanding production requirements. Usually, leakages occurring between the flange faces are the primary concern, leaving fastenings and pipe external protection unaddressed, leading to potential problems especially under adverse conditions. Insufficient external protection can lead to rapid and more extensive damage to the flanges and fastenings which will in turn accelerate the deterioration of the sealing system as a whole. The consequences of such a situation are not easy to predict since they are dependent upon prevailing conditions: depending on the severity of these there may, at best, be a slow continuous degradation of the substrate. However, under extreme conditions, the external corrosion process may rapidly reach a point where the structural integrity of the system is adversely affected, resulting in a catastrophic loss of containment. Since visual inspection of the sealing faces of flanged connections can only be done during a system shutdown, it is vital that this process is made as simple as possible and therefore elimination of external corrosion must be a high priority. Alternatively, if shutdown is not an option, ultrasonic techniques can be used for the inspection, but this process can be more complex and inaccurate if external corrosion is not controlled.

Existing Solutions

Due to the complexity of flange geometry, it has become a challenge to design efficient solutions to protect flanges against corrosion. The most common solutions available on the market are maintenance paints and mechanical solutions. Maintenance paints are epoxy- or urethane-based hard coatings bonded directly to the substrate. As flanges involve lots of edges, paint systems will struggle to coat effectively due to thinning when applied over an edge: thicker layers may address the issue, but it may also lead to seizing of fastenings. In addition, accessing the bolt will be difficult without cracking the coating, therefore necessitating re-application after the inspection. Mechanical solutions such as covers and clamps encapsulate the flange or the void between the flange faces. Constructed from stainless steel or plastic, and incorporating a rubber seal, this protection is rigid and requires having the correct cover or clamp size in stock to cater for each size of flange. Another solution is to use semi-solid tapes, for example Petrolatum tape. Supplied on a roll, they are wrapped around the surface to be protected and provide reliable corrosion protection, thanks to their water-repellent nature.

However, these materials can be time-consuming and difficult to apply to complex shapes. If access to bolts is required, these relatively soft materials can be cut away, but cannot be easily resealed and normally must be replaced. Hot-melt thermoplastics are relatively new: they are wax-like meltable polymers which are heated to high temperatures and then sprayed onto the surface with specialist hot-melt equipment. The main advantage is that the coating can be melted and recycled, which offers a cost effective solution. However, it requires specialist equipment and contract application, and while the coating can be reused it cannot be easily opened and resealed to facilitate inspection. (NB. Belzona has developed a new liquid-applied flange protection system that can be peeled and resealed.)

Another relatively new form of flange protection are polymer bags, containing vapor corrosion inhibitors (VCI), which entirely cover the flanges and are composed of a low permeability polymer with internal VCI film (and drying agents). They are quick and easy to install but the ends of the bag are sealed using normal adhesive tape – not the most durable or effective method. The large vapour space inside the bag means that a relatively large amount of moisture may be trapped inside the system and the VCI’s are consumed over a period of time.

Application

The system requires the use of Belzona 8411 to achieve the optimum level of corrosion protection and to allow access to bolts and flanges in the event of required maintenance

Application of Belzona 8411

  • Spray or brush apply Belzona 8411 onto the flange, pipe and fastenings ensuring the film coverage is even and complete
  • Allow Belzona 8411 to become touch dry in accordance with the Belzona IFU
  • Once Belzona 8411 is touch dry, remove the masking tape and fit plastic caps over nuts/bolts

Application of Belzona 3411

Belzona 3411 is a moisture cured system, therefore to ensure cure is not retarded and the correct overcoat times are observed, the temperature and relative humidity must be measured before application of Belzona 3411 begins ensuring the relative humidity is above 30% and the temperature is above 5°C.

Prepare strips of Belzona 9311 (Reinforcement sheet) as follows:

  • One strip approximately the same length as the Bridging Tape applied earlier.
  • Two strips approximately the same width as the bond area.

Empty the entire contents of the Belzona 3411 Base container into the mixing bowl provided. Pour the contents of the Solidifier container over the Base and mix immediately.

NOTE: It is recommended that the first coat of Belzona 3411 is beige to provide a contrast with the substrate.

  • Using a short bristle brush apply the mixed Belzona 3411 over the area to be protected.
  • Ensure that the Belzona 3411 extends 1.5” - 2” (35 - 50mm) beyond the Belzona® 8411 onto the prepared steel or coated area of pipe, i.e. the bond area.
  • Apply Belzona 3411 on the Belzona8411 applied surface. Ensure application does not extend outside of the prepared area.
  • While the first layer of Belzona 3411 is still wet, bed strip of Belzona 9311 into the Belzona 3411 around the flange circumference and at both ends of the repair where it bonds to the pipe in order to provide additional strength.
  • The strip of Belzona 9311 applied around the flange circumference MUST be applied within the boundary of the outer edge of the flange and MUST NOT impinge onto the bolting faces of the flanges.
  • Belzona 9311 is left exposed at this stage. Belzona 9311 will wet through as the Belzona 3411 cures and it will be fully encapsulated after the second coat of Belzona 3411 is applied.
  • Allow Belzona 3411 to cure in accordance with Belzona IFU.
  • After the touch dry time of the first coat is reached, mix the entire contents of a further unit of Belzona 3411 base with solidifier and brush apply onto the touch dry first coat.

Physical & Mechanical properties

 

Sl.No Parameter Method Results
1 Tensile strength (Adhesion) ASTM D412 Unreinforced 290 psi/2.0 Mpa
Reinforced with belzona 9311 520 psi/ 3.6 Mpa
2 Tear strength ASTM D 624 Unreinforced 22pli/3.9 N/mm
Reinforced with belzona 9311 69Pli/12.1 N/mm
3 Corrosion resistance ASTM B117 No corrosion after 1000 hours exposure to a salt-fog atmosphere at 33 °C
4 Elongation Strength ASTM D412 Unreinforced : 69 %
Reinforced with Belzona 9311 :31%
5 90 °C Peel adhesion ASTM D429 Manually Prepared solvent- borne polyurethane paint: ?6.4 psi / 1.12 Mpa
       

Inspection

  • Visual Surface prep – Clean from dirt and contamination.
  • WFT – 750 to 1000 microns.
  • DFT – 750 to 1000 microns.

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