Gas Turbine Inlet Installations in Coastal Environments

Coastal gas turbine inlet system installations present unique problems of turbine inlet air contamination. Salt, from seawater, will become airborne in significant quantity due to wind and wave action and can give rise to massive and catastrophic turbine damage when ingested. The salt concentration at any given time and place is a function of many factors, including elevation, wave height, wind velocity and direction, temperature, humidity, and the previous history of the local air mass.

The salt content of air above or near the sea surface can be thought of as being from two sources: the very fine droplets ejected from bursting bubbles (oxygenation), and the relatively coarse spray from whitecaps and breaking waves. These two effects are to a certain extent separable. Spray from whitecaps and breaking waves produces locally generated salt, while that from ocean oxygenation is transported over long distances by wind and thermal currents.

Investigations into the generation of sea-salt aerosol have found that it is a complex process of bubble film shattering on the sea surface. The ìRaleighî jet mechanism is responsible for the production of droplets ranging in size from 2 to 20 micrometer and bubbles, less than 50 micrometer in diameter. The larger droplets have a relatively high settling velocity and quickly fall back into the ocean. In addition, a substantial quantity of droplets in the smaller size range collide on with one another as they rise above the surface of the sea. In so doing they coalesce into larger droplets that settle back into the ocean. Those droplets that do escape into the atmosphere are substantially less than 10 micrometer in diameter.

Wave interaction generates sea-spray droplets, 150 to 200 micrometer in diameter. But the gravitational effect on these very large droplets allows them to quickly settle.

Whether salt particles exist as dry crystals or as saturated droplets depends primarily on the relative humidity. Airborne salt originates as supersaturated droplets and remains in suspension until the relative humidity falls to 60 percent or less. Once the salt is in crystalline form, it will not deliquesce until the relative humidity rises above about 73 percent. At this humidity the salt crystal will continue to absorb moisture until has formed a supersaturated saline droplet, five times larger than the original dry crystal.

Thus, the gas turbine inlet air filtration system for a coastal installation must be capable of filtering and retaining sea salt, in both the wet and dry phases, in order to protect the gas turbine. The wet phase presents no problem to the filtration system designer because current, state-of-the-art vertical eliminators and coalescers efficiently remove free moisture from the incoming air stream. Similarly, dry salt crystals can be removed from the air stream with high efficiency barrier air filters. Difficulty arises from the fact that sea salt exists either as a solid crystals or as a saline droplets. It will change from one state to the other as the relative humidity rises and falls above and below the critical humidity of NaCl.

During periods of low relative humidity, dry salt crystals in the size range 0.5 - 2.0 micrometer will be removed from the incoming air stream with an efficiency of almost 100% by a high efficiency barrier air filter. These salt crystals, along with other filtered particulate, will be retained on the air entering surface of the filtering medium. However, should the relative humidity rise above about the 70 percent level, the literally millions of salt crystals retained in the barrier filter will begin to absorb moisture (in the vapor phase) from the incoming air stream. As the humidity continues to rise, the salt crystal will reach the supersaturated condition and become saline droplets.

In the above situation, the high efficiency barrier filter quickly becomes saturated with saline droplets which, if the barrier filter is not completely waterproof, will leach through the filtering medium, into the clean air stream and into the gas turbine. It is therefore essential that the final stage filtering medium be hydrophobic and that the media packs be fully potted to prevent water bypass and leaching. As a further precaution, the final stage filter frame banks should be sloped (about six degrees) toward the air entering face of the system and provided with horizontal drain pans. These facilitate drainage of the deliquesced moisture to the outside of the filter house.

Material selection and joining is also of particular importance to coastal and offshore inlets. These inlets are fabricated from high-grade stainless steels, which contain chromium to make them corrosion resistant. When welding stainless material, chromium can leave solution and form chromium carbide at grain boundaries. This is defined as sensitization. Sensitization occurs when the base metal is kept at a temperature of >500 degrees C for over 60 seconds (these numbers vary with carbon content). Sensitized stainless is prone to corrode due to chromium depletion. However, low carbon stainless (316L) will not form chromium carbides at grain boundaries unless carbon is included in the weld via gas shield breakdown, surface contamination, or improper electrode selection.

Chromium carbide is not visible to the unaided eye, but oxides of chromium cause the deep blue appearance of welded stainless steel. When welds have a black color, carbon has been included in the weld and corrosion will occur. In fact, passivating a weld which includes carbon will bring on corrosion.

Passivation creates a "passive" zone of chromium oxide which resists corrosion. But, passivation does not put chromium back into solution if sensitization has occurred. Thus, passivating ìL gradeî stainless steels is simply an aesthetic measure which adds no corrosion protection.

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Pneumafil Gas Turbine Division PO Box 16348 Charlotte, NC 28297 Phone: 704-399-7441 Fax: 704-398-7528
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