Corrosion Control In Gas Pipelines Coating Protection Provides

While equipment operating in "tubeside" service typically has temperatures well within the limits of the coating, nevertheless "productside" temperature conditions and flow rates should also be taken into account. Pipelinemaintenance "steam out" will impact coating integrity if temperatures exceed out of service or dry "tubeside" coating limits.

Increasing pressure loss in the pipeline is often caused by leakage and corrosion scale or product fouling of the internal surfaces. This condition slows transmission and distribution capacity and can require more than just routine maintenance if it remains unaddressed. Interior corrosion, fouling and leakage in pipeline systems will adversely impact bottom-line operations.

If you see evidence of corrosion and fouling, chances are it is occurring throughout the length of the pipes. If the corrosion is left untreated, it can weaken the pipe walls to the extent that the entire pipeline will need to be completely replaced, a costly solution. Corrosion control is a surmountable problem if there has not been significant wall loss, thanks to new corrosion control technologies.

One of the best practices in corrosion technologies involves applying polymer coatings to the inner surfaces, or inner diameters (ID) of the steel tubes and larger diameter pipe. With this technique, it is possible to thoroughly clean the grubby buildup with a process of chemical cleaning and sandblasting.

If your pipelines use heat exchangers to control gas temperatures and re-pressurize the natural gas, these too may sustain corrosion and fouling. Condensers and heat-exchanger equipment running water or process fluids through tubular systems to achieve a temperature changes are also subject to corrosion and fouling problems.

Water is corrosive to metallic tubes; it also contributes to fouling and bacterial contamination that eats away at the piping surfaces. Leaks and damage, pitting and obstructive buildup all require frequent maintenance. Stoppages for pipe cleaning occur roughly five times more often, on average, if the pipelines remain bare or uncoated.

Additionally, uncoated pipelines erode many times faster than coated lines, and their useful life is diminished to no more than a handful of years in corrosive service, requiring costly rehabilitation. The advantages of coating metal piping rather than leaving it bare are numerous. Coating saves significant costs during the lifetime of thepipeline.

For many decades, existing pipeline ID has been coated or lined by "slip lining with PE/PP"- or by specialized epoxy loading and pigging. Both methods were successful in protecting and returning the deterioratingpipeline to service. However, these operations are uneconomical for smaller projects under 1,000 feet or, in the case of pigging applications, impractical with pipelines of 30 inches or more in diameter because of the pig's tendency to float over the coating.

New semi-robotic equipment can now address these particular cases and provide an economic solution. Pipes can be cleaned and coated in place in 300-500-foot sections. Several applications have taken place in refining operations where corrosive gases in flare-stack pipes often lead to very short pipeline failure.

A Caribbean base refiner had to replace a 30-inch flare-stack pipe every three years due to HCL, H2S and HF acid precipitating inside the horizontal flare pipe. The 1,800-foot pipeline was spooled every 300 feet to accommodate the robotic grit-blasting equipment, waste grit removal and coating application. The refiner selected a Viton-based fluoropolymer coating for this instance due to its excellent chemical resistance to HF acid. After two years of subsequent service, an inspection revealed the pipeline operating in perfect condition, saving the refiner millions of dollars in replacement costs and downtime.

Polymer Power

Pipelines perform heavy-duty transmission of natural gas throughout the contiguous United States. At each pressure station, approximately every 50 to 100 miles along a system, heat exchangers run cooling water to reduce the temperature of the gas after it is re-pressurized so it can continue its journey up the pipeline to its usage/destination. Heat exchangers that run cooling water are strong candidates for protective tubular coatings. Pipelines with small-diameter tubing, as well as pipes with diameters greater than 30 inches, are equally good contenders for corrosion protection with coatings.

In pipeline systems, it is critical to preserve the working properties of the tubing and pipe. This is largely handled through best maintenance practices. The cost of entirely re-tubing a large pipeline or section can climb well into the hundreds of thousands. Coatings represent a cost-effective, practical approach to resolvingcorrosion issues.

The latest phenolic or epoxy materials are especially suited for certain base metals typically used in pipelines. Tubular and pipeline coatings can also be readily applied right on location, in-situ, in a timely manner, precluding the need to disassemble and transport pipe to a shop at a distant location.

Corrosion Chemistry

Micro-organisms, SRB or sulphate-reducing bacteria are among the most common causes of corrosion, but many other corrosion mechanisms exist not related to bugs. Micro-organisms that draw nutrients from cooling water inside tubes cause bacterial buildup and fouling and represent the most common way that corrosioncells are created inside pipes. 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 rapidly advance corrosion to inefficient transmission and sometimes even dangerous gas buildups. Such bacterial pits can cause electrochemical changes inside tubes that exacerbate the damage.

Another common cause for pitting and corrosion is activated films, which can galvanize inside the tube from sulfide or manganese influences. If these potentials differ from the base metal, the reaction leads to pitting, which invites severe biologic decay and faster deterioration of the pipe tubing. Leaks and bottlenecks can seriously jeopardize the efficiency of the system. Polymer coatings are inert to these types of chemical or biological attacks and can restore the integrity of tubes that have had sustained substantial wall loss.

Best Practices And Considerations

Carbon steel is a lowest-cost material available for pipeline tubing, as much as a quarter of the cost of nickel and chrom/moly alloy materials, e.g., admiralty brass, 70/30, or duplex stainless steel. The inherent passivation of these high-alloy materials in cooling water provides advantages when compared to the oxidation of carbon steel.

For optimal performance of low-cost carbon steel tubes, cooling water flow rates, Ph levels and temperature must be maintained throughout an operating unit that may have many pipelines or pieces of equipment working within the same loop. It is common to promote an alloy upgrade as a replacement for carbon steel when aggressive fouling and corrosion impacts production units. The consideration of tube ID coating is a good, lower-cost alternative to "alloy upgrade" for many carbon steel tubes operating in cooling water service.

Polymer linings, such as high-baked phenolics, phenol epoxy and novolacs epoxy, are commonly used in "product" environments, such as tankage, transport containers and vessels. In ambient temperature conditions, these linings provide resistance in 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 are often loaded with glass or ceramic pigments to inhibit porosity over time.

Operating temperature and pressure are usually part of the equation when evaluating the range of potential coating applications. The limits of high performance epoxy coatings in immersion service - neutral Ph water - are about 300 deg F and about 400 deg F in "dry" service conditions. Low or high-Ph operating conditions will influence the operating temperature limits of polymer linings to less than 150 deg F in "immersion" service.

Selecting a lining material for thin-film applications suitable for service in pipeline systems requires a material that can provide the following mechanical characteristics:

• Homogenous coverage at films less than 10 mils,
• Adhesion resistance >2,500 psi,
• Resistance to thermal cycling and conditions exceeding operating temperatures, and
• Superior release of foulant such as calcium and sulphite deposits

The chemical and heat resistance properties of these linings is improved through a heat cure of the application - up to 400 deg F for baked phenolics and to 250 deg F for epoxy applications.

While equipment operating in "tubeside" service typically has temperatures well within the limits of the coating, nevertheless "productside" temperature conditions and flow rates should also be taken into account. Pipelinemaintenance "steam out" will impact coating integrity if temperatures exceed out of service or dry "tubeside" coating limits.

It is important to have all of the coated surfaces inspected for discontinuities, aka "holidays." NACE International has a procedure for a lowvoltage spark test for thin film coatings (<20 mils). This procedure uses a "fish tape" to wipe a wet sponge through each exchanger tube, a "beep" signals a "holiday" that the applicator should repair and recheck using the same procedure. To read about NACE, visit http://www.nace.org.

Prevention Pays

The famous Boy Scout motto - be prepared - certainly applies to corrosion control. The cost of coatings is easily a quarter of the cost of retubing, and once the tubes are recoated after their first ten-year period, they remain anti-corrosive in perpetuity, requiring minimal maintenance for the rest of their working life. Over a 12-year period, the costs saved by coating the tubes in a single system can start in the $8-million range or more, based on our experience.

Tube coating is well proven and utilized to control corrosion by many of the world's largest companies. There are now more approaches to produce the desired outcomes and reduce the losses incurred through inefficiencies caused by corrosion and fouling. The best place to start is by considering the various conditions and then identifying the most efficient method to clean and coat the problem tubes.

By taking care of the small details - watching for evidence of corrosion or fouling - you can save enormous expense in unnecessary replacements and additional maintenance. You can enhance operational efficiency over the lifetime of your pipeline. One small step in coatings is a giant step forward in corrosion-control technology today.

Sumber : Curran, Edward, CEO. "Corrosion Control In Gas Pipelines Coating Protection Provides". 29 Januari 2014. http://search.proquest.com/docview/197453432?accountid=31562