Types of Pipeline Coatings
There are various pipe coating materials and technologies available to suit diverse pipeline applications. Some common types include:
Fusion Bonded Epoxy Coatings
Fusion bonded epoxy (FBE) coatings involve melting a powder epoxy onto the steel surface forming a very dense, molecularly bonded layer that is impermeable to liquids and gases. FBE coatings offer excellent abrasion and corrosion resistance for buried onshore and offshore pipelines transporting oil, gas and water. They can withstand dissimilar metal corrosion and have long service lives of 25-50 years.
Three Layer Polyethylene Coatings
Three layer polyethylene (3LPE) Pipe Coatings comprise of fusion bonded polyethylene as the inner and outer layers sandwiching a adhesive bonding material in between. 3LPE coatings are often used as external coatings for buried oil and gas transmission pipelines. They have outstanding resistance to moisture, acids and bases along with good impact and wear properties. 3LPE provides cathodic disbondment prevention for up to 50 years.
Coal Tar Epoxy Coatings
Coal tar epoxy (CTE) coatings contain coal tar which is a byproduct of the coke manufacturing process. The coal tar imparts additional moisture resistance and barrier properties to the epoxy matrix. CTE coatings are economical and suitable as external protection for buried pipelines conveying corrosive products like sour gas and oil. They offer 20-30 years of effective corrosion control.
Polypropylene Coatings
Polypropylene (PP) coatings are made by extruding polypropylene onto steel pipe surfaces. Their density and crystallinity makes them highly impermeable to moisture and corrosive chemicals. PP coatings can protect pipelines for 25-40 years in most soil and environmental conditions. They find use as external coatings for small diameter pipeline networks for collection systems and product distribution.
Application and Installation of Pipe Coatings
For applying coatings in the field, the steel surface must be meticulously cleaned, abraded and preheated to remove mill scale, debris, moisture, oil and grease. This ensures optimum adhesion of the coating to the substrate. Several techniques are employed for different coating materials:
Hot applied coatings like FBE and 3LPE are applied by specially designed machines while the steel is still hot from production. This enables the coating polymers to directly fuse onto the steel molecularly.
Cold applied coatings like CTE and PP rely on adhesion promoters for bonding to steel. They are applied by spraying, brushing or rotating the pipe through application equipment.
Once coated, the pipe joints are also coated by either sleeve wrapping, in line/end sealing or heat shrink sleeves. Precise quality control checks the coating thickness, continuity and holiday detection to ensure complete coverage.
Proper field joint coating is critical for preventing corrosion under insulation or dissimilar metal corrosion in bimetallic joints. External field joints on buried and subsea pipelines require highly durable wrapping systems for long term protection.
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