Coatings gaining acceptance in process industries

Dec. 18, 2006
Turbomachinery runs smoother, longer, more economically.

CPI turbomachinery operators face increasing pressure to get better performance from their turbines and axial compressors. Cut energy costs. Extend service intervals. Get more life from their turbomachinery.

A key to meeting these goals is using protective coatings to retard fouling and corrosion.

Coatings, proven for decades in jet engines and ground-based turbines, are gaining increased acceptance in the process industries as well. They demonstrably lengthen service life and service intervals, reduce part replacement repair costs, and improve net efficiency over time. Here is how the time-tested metal coating strategy embraced by progressive turbomachinery operators in the CPI.

Turbomachinery operators are coating steam turbines and centrifugal compressors in CPI plants with special protective coatings. These coatings pay off in efficiency retention, equipment life extension, lengthened service shutdown intervals, and reduced repair and replacement costs. It's a maintenance strategy proven in the aerospace and power generation industries decades ago. Many turbine and compressor OEMs, in fact, offer such coatings as options or standard on new equipment. Coated turbomachinery components run longer and more efficiently between shutdowns and last longer. The long-term savings--especially energy savings--more than offset the initial costs. Higher retained efficiency gives users in the Chemical Process Industry paybacks in as short as a month. The timing is right.

The problems

Operating conditions in chemical processing subject turbomachinery to fouling and corrosion. The relative severity of these conditions varies with the process and equipment.

Corrosion takes its toll on steam turbines by degrading the efficiency. Silica, salts, sulfur, chlorine, heavy metals, and other particulates carried in the steam pit, pockmark, and abrade the surface finish and airfoil shapes in the gas path. Loss of airfoil geometry and surface smoothness degrades efficiency, sometimes dramatically. It also increases component repair costs. Figure 1 shows the relationship between surface roughness and efficiency for three turbine airfoil surfaces in one mechanical drive steam turbine. Note that the efficiency begins to drop off dramatically at about 50 microinches of surface roughness.

From that point, efficiency plummets from a one percent loss to as much as an eight percent at 1,000 microinches of roughness. Note also that the impact of airfoil roughness is most acute in the high-pressure section of the turbomachinery where flow areas are the smallest.

Lost efficiency inevitably translates into lost fuel dollars.

Efficiency, though, is not the only performance standard that suffers. The cost of maintenance work and lost production time can mount, too. Costs rise across the board.

Process economies plummet. Compressor fouling cuts efficiency and contracts service cycles. Fouling degrades the efficiency of axial, barrel, and centrifugal compressors.

Organic and inorganic deposits on the airfoils throw compressor rotors out of balance.

Fouling also compromises axial and radial clearances between rotors and stationary components such as diaphragms. Abrasive wear and corrosion can damage impellers and labyrinth seals severely.

At the same time that efficiency is dropping, fouling is contracting service cycles. To cite just one example, operators of ethylene plants face the impact of compressor fouling on a regular basis--premature declines in efficiency, shortened service intervals, and costly shutdowns. These problems increase interest in prevention strategies. Coating the compressor rotors with a three-part polytetrafluoroethylene-impregnated coating, for instance, slows fouling and significantly lengthens production runs.

In many industries, it is common practice to schedule turbomachinery shutdowns on the basis of condition rather than operating hours. One of the principal indicators of condition is efficiency. It's on the basis of protective coatings that turbomachinery operators extend the service intervals of their equipment and point to the resulting higher throughputs and lower repair costs.

On close examination, the dual benefit lies in longer service intervals and in the higher retained efficiency that made that interval extension possible. Every dollar gained in throughput and reduced repair costs stemming from longer operating intervals is accompanied by savings in fuel. Protective coatings cut energy costs big-time. The more severe the operating conditions, the bigger the potential payoff.

The new solution

Decades ago, the aviation industry recognized the benefits of using specialized protective coatings to inhibit the corrosion, erosion, and fouling gas turbine airfoils. They had the same enemies as operators of ground-based turbomachinery. Turbine life extension, time on the wing (service interval), and lower repair and replacement costs were key concerns. Fuel efficiency was not an issue originally, but eventually evolved into one. Today, treating jet engine gas-path surfaces is standard practice. The power generation industry has been using coatings to extend the life and enhance the efficiency of their gas and steam turbines for at least a decade.

But in the Chemical Process Industry, these benefits were not realized because there was no technology to apply coatings to base metals with sufficient bond strength to withstand the severe operating environment. It wasn't until recently though, that coating technology advanced enough to answer these concerns. Engineers then began to identify significant productivity gains achieved simply by improving smoothness of turbomachinery airfoil surfaces. Today, CPI field experiences demonstrate that the payoff from this strategy can be as big as the payback is quick.

Aluminum-ceramic and impregnated PTFE coatings fare best

If the service demands combine corrosion and fouling, aluminum-ceramic and special impregnated PTFE coatings fare better than conventional polymer-only coatings. The newer aluminum-ceramic coatings excel at temperatures from 400 to 1,500 degrees F, typical of conditions inside turbomachinery.

One excellent example is a two-layer coating system that protects steam turbines from corrosion at service temperatures of up to 1,050 degrees F. The base layer of this coating system is a galvanically active aluminum-filled ceramic composite. It is sacrificial to corrodents and that ensures excellent adherence to iron, steel, and nickel alloys. A chemically inert, glassy ceramic topcoat seals this sacrificial basecoat and produces a smooth surface that is resistant to corrosion.

Another type of three-layer coating system protects centrifugal compressors from excessive fouling and wet chlorides. The coating consists of a layer of high-temperature polymeric resin filled with metal powder and applied to an aluminum-phosphate basecoat. The two layers are then sealed using a high-temperature polymer containing particulate PTFE. This produces a non-reactive smooth finish of less than 20 microinches in thickness.

Upgrading your equipment is not difficult

Chemical Process Industry turbomachinery users typically ship rotors, impellers, and other components to a service center for coating during a scheduled maintenance shutdown. The service center repairs and coats the parts and ships them back for reinstallation. The benefits of protective coatings are impressive, and expenditures for them can be paid back quickly.

Aluminum-ceramic and impregnated PTFE protective coatings for steam turbines and industrial compressors--especially centrifugal compressors--are growing in popularity within the Chemical Process Industry. The reason why is simple to understand: they significantly extend the service life while reducing part replacement and repair costs. Increased reliability and profitability has been proven over decades in the protection of aviation gas turbines, and for at least a decade in the protection of the utility industry's gas and steam turbines.

Today the proving ground is the CPI, where the success of energy savings and other efficiency measures utilizing aluminum-ceramic protective coatings has been impressive for the last half decade or so.

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