Maintaining optimal performance of carbon steel tubes demands careful attention to cooling water flow rates, pH levels and temperatures throughout a unit that may have many pieces of equipment tied to the same loop. Not surprisingly, aggressive fouling and corrosion often impact operations. So, plants frequently consider an alloy upgrade for exchangers. However, tube ID coating offers a lower cost alternative.
Polymer linings such as high baked phenolics, phenol epoxy and novolacs epoxy are commonly used in “product” environments like tankage, transport containers and vessels. At such ambient temperature conditions, these linings resist a wide range inorganic and organic acids, solvents and hydrocarbons. Typical applications call for coating films to be applied at greater than 20 mils, and materials often are loaded with glass or ceramic pigments to inhibit porosity over time.
However, heat exchangers are a tougher application; their operating temperature and pressure must be taken into account in choosing coatings. The limits of high performance epoxy coatings in immersion service (with neutral pH water) are about 300°F and about 400°F in “dry” service.
A lining material for thin-film applications suitable for cooling water service must provide the following characteristics:
- homogeneous coverage at films less than 12 mils thick;
- adhesion resistance exceeding 2,500 psi;
- ability to withstand thermal cycling and occasional excursions beyond normal operating temperatures; and
- superior release of foulants such as calcium and sulfite deposits.
Heat-curing — up to 400°F for baked phenolics and 250°F for epoxies — can improve the chemical- and heat resistance properties of these linings.
While the tubeside of exchangers in cooling water service typically has operating temperatures well within the limits of the coating, also always consider product-side temperature conditions and flow rates. Plant maintenance “steam out” will impact coating integrity if temperatures exceed out-of-service or dry tubeside coating limits.
Widely used ¾-in. and 1-in. outer-diameter tubing sizes are amenable to coating; the general limits of coating and full inspection of tube insides is about 0.40-in. ID. Tubes up to 60 ft. long have been fully coated using a airless spray technique; “U” tube and hairpin design exchangers can be coated.
Have all coated surfaces inspected for discontinuities or “holidays.” NACE has a procedure for a low-voltage spark test for thin film (<20 mils) coatings. This procedure uses a “fish tape” to wipe a wet sponge through each exchanger tube, a “beep” signals a holiday, which the applicator should repair and recheck using the same procedure.
Ed Curran is president of Curran International, Dickinson, Texas. E-mail him at email@example.com.