Internal coating of natural gas pipelines is employed to reduce friction and improve flow efficiency when conveying non-corrosive natural gas, and to offer adequate corrosion resistance during subsequent storage and transportation of coated pipe. By reducing the surface roughness the coating reduces the friction factor of the pipe wall.

The use of thin film (less than 100 microns) epoxy resin based coatings for this purpose has an extensive track record with many pipeline operators. Such coatings have typically been formulated around solid ‘1-type’ epoxy resins (with a molecular weight of approximately 1,000) in conjunction with either polyamine adduct or polyamide curing agents.

The solid/semi-solid nature of the epoxy resin and curing agent necessitates the use of substantial levels of organic solvents in order to provide a suitable liquid coating composition. A typical commercial coating product would therefore contain 40–45 per cent by weight of solvent, equating to a volatile organic compound (VOC) content of 400–450 grams per litre.

Performance requirements The performance attributes required for an internal flow efficiency coating are detailed in a number of internationally recognised performance specifications and standards – API RP 5L2 (API), TRANSCO CM2 (British Gas) and more recently ISO 15741.

Whilst there are differing requirements within each, many common requirements exist:

• adhesion
• hardness
• flexibility
• corrosion resistance
• water resistance
• chemical resistance
• resistance to gas pressure variations

The overall properties required from the cured flow coating presents a number of challenges to the formulator seeking to reduce VOC content.

The use of liquid epoxy resin, rather than solid ‘1-type’ resins, enables solvent contents to be reduced. However, the lower molecular weight of liquid resin results in the formation of polymer networks with an increased crosslink density, yielding coatings of limited flexibility. The use of ‘flexibilising’ agents generally leads to reductions in corrosion, water and/or chemical resistance, and the use of non-reactive diluents or plasticisers must be avoided to prevent out-gassing from the coating as a result of in-service temperature/pressure fluctuations.

Despite these constraints, appropriately formulated flow efficiency coatings can now be produced with VOC contents ranging from 225 grams per litre down to zero.

Comparison of VOC emissions for different flow coating technologies

Solvent emissions, and associated carbon emissions, for a range of coating technologies are illustrated below, calculated on the basis of a nominal 200 km, 36 inch inner diameter internal coating project. The reduced environmental impact of high solids/solvent free formulations can be clearly demonstrated.

a) Conventional solvent based flow coating with a VOC content of 440 g/litre

• Practical applied coating film thickness (wet) = 200 microns
• Coating consumption = 120,000 litres
• VOC emissions = 120,000 x 0.44 kg = 52.8 tonnes
• Assuming typical aromatic hydrocarbon/alcohol solvent blend, carbon emissions = 45 tonnes

b) High solids solvented flow coating with a VOC content of 225 g/litre

• Practical applied coating film thickness (wet) = 125 microns
• Coating consumption = 75,000 litres
• VOC emissions = 75,000 x 0.225 kg = 16.9 tonnes
• Assuming typical aromatic hydrocarbon/alcohol solvent blend carbon emissions = 15 tonnes

c) 100 per cent solids, solvent free flow coating with a VOC content of 0 g/litre

• Practical applied coating film thickness = 75–100 microns wft
• Coating consumption = 45–60,000 litres
• VOC emissions = nil
• Carbon emissions = nil
• Effect of internal flow coating on surface roughness

According to Koebsh et al (2005), a number of roughness/profile parameters can be utilised to characterise pipeline surfaces, including:

• Average roughness (Ra)
• Root mean square roughness (Rq)
• Maximum height of profile (Rt)
• Average maximum height of profile (Rz).

It might be assumed that dry film thickness is the principal driver in reducing the surface roughness of a blast cleaned surface. However, study of the roughness parameters obtained from a range of flow coating compositions, at equivalent dry film thickness, reveals the volume solids of the liquid coating to be highly significant in reducing surface roughness.

Roughness parameter plots for three flow coating variants applied to blast cleaned steel line pipe (Rz = 40 microns) at a dry film thickness of 75 microns are shown in Figure 1, with a summary of the data detailed in Table 1.

Solvented, thin film epoxy flow efficiency coatings have served pipeline operators well for many years. However, their VOC content may be considered environmentally undesirable and ultimately unsustainable. The advent of a new generation of reduced solvent content (high solids) and solvent free (100 per cent solids) flow coatings enables the environmental impact of internal coating processes to be minimised without compromising coating performance. Furthermore, hitherto unexpected benefits in reducing the surface roughness of internally coated pipe are realised by the adoption of these new coating technologies, without any increase in applied coating thickness.