CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.
Issue link: http://coatingspromag.epubxp.com/i/72303
Table 2: Comparison of Typical Physical and Performance Characteristics of Linings on Buried Steel Pipelines Characteristic Corrosion Control Method of Control Water Absorption Experience Design Life - Durability Physical Damage During Delivery/ Installation Maintenance Thickness Cracking or Pinholes Pipe Stiffness Flow Velocity Mortar Lining (AWWA C205) Very Good to Excellent Passivation Negligible oxygen dif- fusion Enhances Passivation >175 years >50 to 100 years Easily Repaired in Field Virtually None 0.50" (1.2 cm) mini- mum Passivation & Autogenous Healing Contributes Significantly Up to 20 ft/sec (6 m/sec); May vary depending upon special circumstances Empty Buried Pipeline Maintain Humidity Excellent Seismically Active Areas Water Flow, Flow Friction (Surface Smoothness) High Hazen-Williams C-Factor – 140 to 150 Barrier – Isolation Detrimental Varies by Material, See Table 1 20 to 30 years Specialized Equipment & Material Inspection Every 5 to 7 years Recommended 1/16"-1/8" Coal tar enamel Min. 0.012" - 0.020" Others Steel Corrodes No Contribution Varies; Up to 20 ft/sec (6 m/sec) Not Affected Excellent High Hazen-Williams C-Factor – 140 to 150 than 5°F (2.7°C) above the dew point. These fast-curing formula- tions must be spray-applied, and this can result in a more difficult satisfactory application when the irregular surfaces of fittings and special pipe sections are considered. The estimated material and shop-applied costs at a minimum thickness of 0.020" (0.51 mm) range from $2.75 to $3.95 per square foot of steel surface area. Estimated field-applied costs range from $5 to $8 per square foot with estimated maintenance costs ranging from $1 to $4 per square foot. In addition, polyurethanes have the same concerns regarding long-term bonding when in a stressed condition, as was described for painted liquid epoxy linings previously. Some formulations are better suited to handle these long-term stresses. Due to its more recent introduction to the water pipe market, less than 2% of large-diameter steel water pipelines are estimated to be lined with polyurethane. Use of polyurethanes as linings on Dielectric Lining (AWWA C203, C210, C213, C222) Variable large-diameter steel water pipelines has increased in the past 10 years but still has a relatively limited service history. Lack of formu- lation standardization has resulted in varying quality performance expectations from this family of linings. Maximum service life for polyurethane linings receiving periodic maintenance is estimated at 20 years. COMPARISON OF LINING THICKNESS Figure 1 shows a comparison of the thickness of several of the lining systems. The minimum thickness of the portland cement lining is ½" (1.2 cm), which is substantially thicker than any of the dielectric lining systems. The minimum thickness of the four dielectric lining systems, except for the coal tar enamel system, ranges from 0.012" to 0.020" (0.30 to 0.51 mm). METHODS OF CORROSION PROTECTION The primary purpose of linings on steel water pipelines is to protect the steel from corrosion and the resulting leaks that occur. The dielectric linings (CTE, liquid epoxies, FBE, and polyurethanes) protect steel from corrosion by isolating the electrolyte (the trans- ported water) from the metal. They are intended to act as a barrier to the corrosive effects of the water. Water and oxygen diffusion through these dielectric linings is detrimental to the protection of the steel surface. Mortar linings protect steel from corrosion through passivation and retarding oxygen penetration to the steel substrate. In contrast to dielectric linings, such as epoxies and polyurethanes, water diffusion through mortar linings enhances passivation of the steel. PHYSICAL AND PERFORMANCE CHARACTERISTICS OF LININGS Various physical and performance characteristics of the lining systems used on large-diameter steel water pipe are summarized in Table 2. Dielectric linings are more prone to damage during trans- portation, installation, and backfilling than mortar linings, which are more durable and easier to repair in the field. MAINTENANCE It is reported (Helsel, et. al., 2008) that for fresh or potable water immersion, the estimated service life of liquid epoxy systems ranges from eight to 17 years before first maintenance re-lining. The dry film thickness (DFT) of the epoxy systems listed ranged from 6 mils to 40 mils (150 to 1000 microns) with a minimum near-white metal blast required. It is also reported (Helsel, et. al., 2008) that the estimated service life of 100% solids 20-mil-thick (500 micron) polyurethane lining systems range from 14 to 16 years before first maintenance re-lining. Portland cement mortar linings require virtually no maintenance under most installation conditions. CP References Helsel, J. L., Lanterman, R., and Wissmar, K. (2008). "Expected Service Life and Cost Considerations for Maintenance and New Construction Protective Coating Work." CORROSION 2008 Paper No. 08279, NACE International, Houston, TX. July 2012 J www.coatingspromag.com 53