Tube corrosion, fouling and leakage in heat transfer equipment at chemical plants creates unfavorable conditions that can seep across the entire business. Indeed, inefficient heat transfer is a common bottleneck at sites around the world, often adding significant expense to operations.
The bare carbon steel tubing used in heat exchangers, condensers and other heat-transfer equipment is a key culprit. However, applying a polymer coating to the inner diameter (ID) of the tubes can cut plant and equipment downtime, extend intervals for routine maintenance, lengthen the life of capital assets and improve energy efficiency. Today, many companies consider ID coatings a best practice for extending the performance and lifecycle of a heat transfer system.
After coating the tubular systems in its heat exchangers, One Gulf Coast refinery estimates $30,000 savings per day from more reliable production, reduced downtime and lower energy costs — or nearly $9 million per year. Additionally, the site saves an extra $3 million annually in retubing costs compared to its historical retubing cycles.
The power of polymers
Tubular heat exchangers that run cooling water are the most likely candidates for protective coatings. Water is corrosive to the metallic tubes and can promote fouling and bacterial contamination that eats away at the tubes’ inner and outer surfaces. Leaks and damage, pitting and obstructive build-up all require frequent maintenance. Stoppages for tube cleaning occur roughly five times more often, on average, if tubes remain bare or uncoated. Additionally, the bare tubes wear down faster than coated tubes, diminishing their useful life and necessitating costly retubing of the heat transfer equipment as it ages.
At chemical plants, it’s vitally important to preserve these systems and prolong their working lives through good maintenance practices. Cleaning traditionally involves water treatment and periodic hydroblasting but doesn’t always provide optimal results. Because the cost of entirely retubing a large piece of heat transfer equipment climbs well into the tens of thousands of dollars, it’s far better to extend the life of the tubes through coating their ID with phenolic or epoxy materials especially suited for the base metal and the function of the heat exchanger. This practice isn’t new but remains under-utilized.
Over the years, coating has evolved and matured into a cost-effective remedy to reduce typical fouling and corrosion problems intrinsic to this equipment. It has benefited from improvements in materials, surface preparation, application techniques and thermal conductivity, plus from insights from owner-operator data collection and analysis.
A German chemical company first developed phenolic materials for tube ID coatings in the 1950s. Applied by a fill, drain and rotate method in a specialized shop, this was the industry’s best option until the mid-1980s. Around that time, companies in Italy began experimenting with air-atomized spray applications of epoxy phenolic developed by their engineers. By coating the tube ID with the compound, the Italians achieved excellent results and improved fouling and corrosion resistance, actually restoring condensers to their normal operating capacity.
Now, thanks to decades of experience, it’s possible to pinpoint the right coating for each ID, type of metal and application.
The chemistry of corrosion
Microorganisms that draw nutrients from cooling water inside tubes cause bacterial build-up and fouling, and represent the most common way that corrosion cells are created inside condensers and heat exchangers. The bacteria breed quickly in the nutrient-rich environment, enhanced by certain chemical processes and the lack of light inside the tubes.
Each corrosion cell creates a pit — a place for bacteria to further multiply and hide — that leads to intricate bacterial structures that can rapidly cause blockages that cut efficiency and can even be dangerous. Such bacterial pits can cause electrochemical changes inside tubes that exacerbate the damage.
Another common culprit for tube pitting and corrosion is activated sulfide- or manganese-containing films. If their potentials differ from the base metal, a galvanic reaction can lead to pitting, which invites severe biologic decay and faster deterioration of the tubing. Polymer coatings are inert to these types of chemical and biological attack and can restore the integrity of tubes that have suffered substantial wall loss.
Figure 1 shows an uncoated tubesheet and tubes after one year in cooling water service.
In contrast, Figure 2, shows a coated tubesheet and tubes after one year in the same service being supplied by a common header.
Generally, tube pitting is corrected in one of three ways:
Cleaning. Because of the bacterial component of pitting, removal of tube deposits and adoption of better methods for tube cleaning can halt deterioration or at least stabilize the rate. Deep pits can be identified through eddy testing and then plugged prior to cleaning. Rough cleaning can be accomplished by brushes and scrapers or by using hydrolyzing or sponge balls. Tubes need to be decontaminated as well. Low-chloride potable water that has been demineralized or water conditioned with a chloride neutralizer can flush out contaminating chlorides. Finally, abrasive blasting should be employed to remove the last vestiges of contamination in the tubing.