HEPA GT inlet filters can make good sense in unlikely locations

Gas-turbine air inlet filters are like most other powerplant components in that one size doesn’t fit all. A debate among users over which filter medium is “best” could go on until the participants are exhausted. What does “best” mean, anyway: lowest cost, longest life, highest compressor efficiency, lowest pressure drop across the filter medium, highest capture rate of the smallest particulates, etc?

Many variables come into play if filter selection is based on a proper engineering analysis, as it should be—including cost, characteristics of the filtration medium, duty cycle, engine age, and ambient conditions, plus others. But there are exceptions to every rule and sometimes it may make good sense to consider only price—as might be the case if there is no clear guidance from the grid on how the plant will operate in the future.

High-efficiency particulate air (HEPA) filters have captured the lion’s share of attention at industry meetings for the last several years. One reason for their popularity: Most brands of HEPA filters actually delivered the performance promised—in some cases, better than promised.

But keep in mind that all HEPAs are not created equal; there are differences among suppliers and an engineering evaluation to identify the optimal filter for a specific application is recommended. Example: At a seacoast location, or in an area with a great deal of precipitation, a filter with hydrophobic properties (the ability to prevent water and dissolved salts from entering the compressor) might be the best choice.

CCJ has closely tracked industry experience with HEPA filters for the last eight years, first learning of their use on gas turbines in both Mexico and Europe. Performance details were difficult to pin down at the time, and with a dozen brands of “conventional” filters already on the market, GT owner/operators were reluctant to talk about, much less consider, an unproven (to their satisfaction) alternative with a luxury sticker price.

It wasn’t until Alliance Pipeline began sharing its HEPA experience with the industry did attitudes begin to change. The company’s Rob McMahon has carefully recorded operating data for more than five years that clearly show the considerable economic benefit offered by the use of HEPA filters on compressor drivers. However, the service profile and maintenance associated with remote base-load aero engines are quite different than those for large frame engines in powerplants required to respond as grid operators see fit.

Frame experience. Rick Shackelford, plant manager of Green Country Energy LLC and vice chairman of CTOTF’s™ Combined Cycle Roundtable, offered to share with the industry details on long-term frame experience with HEPA filters. His three 1 × 1 combined cycles in Jenks, Okla, powered by 7FA.03 engines (Package 3), have been operating for nearly three years with hydrophobic HEPA filters protecting the compressors. Green Country, operated by NAES Corp, is owned by J-POWER USA Development Co Ltd, one of the world’s largest IPPs and Japan’s largest wholesale power producer.

Most plant managers would be challenged by owners to justify a switch to HEPA filters. Shackelford and his team were fortunate in that J-POWER had positive experience with HEPAs at other plants, making cost the major concern in the decision-making process. Discussions with personnel from a utility with HEPA experience led engineers from J-POWER USA and the plant to conclude the filters would meet expectations for a least the two years the owner believed necessary to achieve an acceptable payback. W L Gore & Associates Inc HEPAs were installed in fall 2012 and are on track for more than three years of service based on performance (figure).

HEPA Green Country Fig 1

Selection of E12 hydrophobic HEPA filters for an Oklahoma plant that sees only 30 to 40 in. of precipitation in a typical year piqued the editors’ interest based on what they had been hearing at industry meetings. The selection seemed “over the top,” so to speak. A conference call with Shackelford, Operations Manager Danny Parish, and Makoto Kaneko, engineering manager for J-POWER USA, answered questions other users might also ask.

Compressor inlets were downwind of the cooling tower and summertime prevailing winds pushed the plume in the direction of the gas turbines. The chlorine/bleach used in tower water treatment attacked the Custom 450© stainless steel compressor blades, creating pits. One of those corrosive pits was identified as the cause of a second-stage vane liberation in 2006—an expensive lesson learned. Hydrophobic filters would protect against a recurrence.

The plant’s power-sales agreement could penalize Green Country if heat rate was not maintained within a narrow performance band. Before HEPA installation, the plant would schedule a shutdown every other month for an offline water wash to maintain efficiency. This is no longer necessary. However, Green Country still performs an offline water wash in the fall to maximize efficiency. But Shackelford concedes there’s not much heat-rate benefit in doing that.

The huff ’n puff system incorporated into the filter house to serve the original conical/cylindrical filters was turned off when the HEPA replacements were installed. The filter house was in good condition: No need for repairs, sandblasting, etc. This was confirmed by Gore, which provided a technical advisor assure the new filters were installed correctly. Shackelford stressed the need for attention to detail during installation to be sure filter alignment is correct and inlet air cannot bypass the HEPAs.

Green Country now is operating as a cycling facility at about 60% capacity factor. The HEPAs have accumulated about 15,000 hours of service, and the pressure drop across them has increased by only about 0.5 in. H2O in three years of service. Pressure drop out of the box was 1.2 in., today it is about 1.6. There are the expected delta-p spikes during driving rains, but the pressure drop returns to normal when the filters dry out. Ongoing tracking of data will determine when the filters should be replaced—perhaps next spring or fall. Shackelford said he is pleased to see more suppliers are offering HEPAs today than three years ago.

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