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Internal efficiency

ian robinson, 3M e Wood, UK, analyses the performance of high solids internal flow efficiency coatings for pipelines. First, Craig Thomas, 3M e Wood, provides an introduction to internal flow coatings.

The application of a two component epoxy coating to the internal surface of gas pipelines was first carried out in the 1950s. Gas transmission pipelines are a key element in the transportation of fuel, supplying energy and power

to many countries and providing sustained growth and development all over the world. The operation and pumping costs of a gas pipeline are substantial and the capacity of gas delivered by the pipeline depends to a large degree on key design parameters; those being diameter and length. In recognising these factors, the concept of internally lining gas pipelines was developed, providing enhanced flow and thereby engendering a reduction in operational costs.

International oil and gas companies such as Shell, BP, Exxon, Total, Transco, Statoil, Reliance and CNPC, to name but a few, have now recognised the many benefits of internally coating gas pipelines, which has become industry practice. For this application, 3M E Wood, Corrosion Protection Products, 3M United Kingdom

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Plc (formerly E Wood Ltd) pioneered a range of internal flow coatings under the COPON Pipelinings brand.

In addition to enhanced flow, there are many other economical and technical benefits available to pipeline operators in both an onshore and offshore environment, as well as to pipeline contractors and pipe coaters:

ËCorrosion protection in storage.ËReduced energy costs in pumping and compressor

stations.ËReduced energy requirements - reduced emissions of

CO2.ËLow capital costs.ËReduced commissioning costs.ËFaster commissioning (inspection).ËMore effective pigging/scraping.ËSealed surface - product purity.ËDiverse pipeline use - easier product switch.ËRapid payback.ËReduced valve maintenance.ËImproved application.

No review of the advances made in high solids internal flow coatings would be complete without using the experience of a coatings manufacturer that has supplied product for over 140 000 km of lined pipe worldwide during a period spanning more than 45 years and that has developed a range of flow efficiency coatings (48% solids, 75% solids and 100% solids - solvent-free) fully approved to API RP 5L2 and ISO 15741.

High solids internal flow efficiency coatings†Thin film epoxy coatings have long been known to reduce the internal roughness, and hence the friction factor, of natural gas flowlines. A new generation of high solids materials provide the pipe coater with a choice of environmentally sustainable solutions without increased coating thickness or loss of performance.

This article will illustrate the environmental benefits of high solids internal coatings, and present a study of the surface roughness parameters of internally coated pipe as a function of flow coating volume solids.

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. The coating

functions by reducing surface roughness and hence reducing the friction factor of the pipe wall.

The use of thin film (<100 microns) epoxy resin based coatings for this purpose is well known and has an extensive track record with many pipeline operators. By convention, such coatings have typically been formulated around solid ‘1-type’ epoxy resins (molecular weight approximately 1000) 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% by weight of solvent, equating to a volatile organic compound (V.O.C) content of 400 - 450 g/litre.

Performance requirementsGeneralThe 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, such as:

ËAdhesion.ËHardness.ËFlexibility.ËCorrosion resistance.ËWater resistance.ËChemical resistance.ËResistance to gas pressure variations.

Formulatory considerationsThe overall package of properties required from the cured

flow coating presents a number of challenges to the formulator seeking to reduce V.O.C

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.

Figure 1. Steel pipe coated with Copon EP2306 HF, creating a smooth, low friction internal surface.

†Article adapted from 'Advances in high solids efficiency coatings', a paper given at BHR's 17th International Conference on Pipeline Protection, Edinburgh, 17 - 19th October 2007.

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Despite these constraints, appropriately formulated flow efficiency coatings can now be produced with V.O.C contents ranging from 225 g/litre down to zero.

Comparison of V.O.C emissions for different flow coating technologiesSolvent emissions, and associated carbon emissions, for a range of coating technologies are illustrated below, calculated on the basis of a nominal 200 km/36 in. I/D internal coating project. The reduced environmental impact of high solids/solvent free formulations is clearly demonstrated.

Conventional solvent based flow coatingËV.O.C content = 440 g/litre.ËFor 200km, 36 in. I/D pipe.ËPractical applied coating film thickness (wet)

= 200 microns.ËCoating consumption = 120 000 litres.ËV.O.C emissions = 120 000 x 0.44kg = 52.8 t.ËAssuming typical aromatic hydrocarbon/alcohol solvent

blend, carbon emissions = 45.0 t.

High solids solvented flow coatingËV.O.C content = 225 g/litre.ËFor 200 km, 36 in. I/D pipe.ËPractical applied coating film thickness (wet) = 125

microns.

ËCoating consumption = 75 000 litres.ËV.O.C emissions = 75 000 x 0.225 kg = 16.9 t.ËAssuming typical aromatic hydrocarbon/alcohol Ësolvent blend carbon emissions = 15.0 t.

100% solids, solvent free flow coatingËV.O.C content = 0 g/litre.ËFor 200 km, 36 in. I/D pipe.ËPractical applied coating film thickness = 75 - 100

microns wft.ËCoating consumption = 45 - 60 000 litres.ËV.O.C emissions = nil.ËCarbon emissions = nil.

Effect of internal flow coating on surface roughnessGeneralA number of roughness/profile parameters can be utilised to characterise pipeline surfaces,1 including:

ËAverage roughness (Ra).ËRoot mean square roughness (Rq).ËMaximum height of profile (Rt).ËAverage maximum height of profile (Rz).

Impact of flow coating volume solidsIt 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 linepipe (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.

ConclusionSolvented, thin film epoxy flow efficiency coatings have served pipeline operators well for many years. However, their high solvent (V.O.C) 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% solids’) flow

coatings enables the environmental impact of internal coating processes to be minimise 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.

References1. KOEBSHetal,Measuring roughness of blasted steel pipe surfaces: a case study,(16thInternationalConferenceonPipelineProtection,2005).

Table 1. Roughness parameters for a range of flow coatings @ 75 microns dft

Flow coating composition

Volume solids (%)

VOC content (g/litre)

Roughness parameters (microns)

Ra Rq Rz

Conventional solvent based

45 440 1.38 1.64 5.90

High solids solvented

75 225 0.65 0.81 3.88

Solvent free 100 0 0.16 0.20 0.83

Figure 2. Roughness plots for a range of flow coatings @ 75 microns dft. (Left to right - blast cleaned steel, conventional solvented coating, high solids coating, solvent free coating).