TURBINE AIR FILTRATION: Filter review process critical to improved turbine run-times

The CamGT filters - the company’s prime offshore filter - have been installed on three GE10 turbines delivered to Exxon’s Rovigo development in the Adriatic Sea. The delivery also included chilling coils to reduce the temperature of the air flow on hot days, a means of getting more power from the turbine.

“This was the first offshore application of chilling coils for us,” says managing director Peter Sandberg. “We could not use evaporative coolers as we normally would due to the high humidity. Behind the chilling coil is a droplet eliminator which takes out the condensation.”

The company also recently completed three FPSO deliveries which illustrate the global project range. The latest concerned acoustic and inlet systems forBW Enterprise, the FPSO which Bergesen Worldwide Offshore is converting for duty on Pemex’ Ku-Maloob-Zaap field complex in Mexico. The systems, supplied through Dresser Rand, are installed on two LM2500+ turbines.

The other projects were Marathon’sAlvheim FPSO in the Norwegian North Sea - in this case, the company supplied inlet systems for two Nuovo Pignone PGT25+ turbines; and inlet systems for four Nuovo Pignone PGT25+ turbines for the Petrobras P54 production floater. This was also the company’s first-ever delivery to a Brazilian project.

Uninterrupted for years

“We find that customers are more and more expressing an interest in better turbine protection so the turbines can run longer between washes,” says Sandberg. “Some customers are looking to keep their turbines running for several years without stopping. To achieve that requires a very high level of protection, with the removal of all pollutants and sub-micron particles.”

Given that the normal period between washes is 1,000 operating hours, equivalent to around 40 days, several years of uninterrupted operation calls for a real step-change in improvement. Technically it can be done, Sandberg says. Of course there is a cost involved, so at the end of the day, there must be a net benefit. Here Camfil Farr’s life-cycle cost - LCC - program has proved useful in providing customers with a guide to the long-term costs of different types and combinations of filters.

The two key features to consider when assessing a filter system are pressure drop and efficiency. The greater the pressure drop, the greater the fuel consumption to achieve the same power output from the turbine - and with fuel costs rising, this factor takes on greater significance. Similarly, the less efficient the filter system, the greater the deterioration in turbine performance induced by fouling.

Camfil Farr’s investigations suggest that the benefits of upgrading to a higher efficiency filter such as a three-stage system outweigh the disadvantages. Increasing the surface area of the media 50% leads to a doubled lifetime, says area sales manager Tord Ekberg. Although the investment is greater and the pressure drop a little bigger, the LCC calculations indicate an overall cost benefit for a three-stage system compared with a simpler protection system, he says.

Camfil Farr also has compared the efficiency of its glass-fiber media with the synthetic fibers favored by some other vendors. When synthetic fibers are manufactured, they acquire an electrical charge which makes them efficient at attracting particles. However, when exposed to moisture in a turbine plant, the media lose their charge and their performance suffers drastically. Glass-fiber media, which consist of very fine fibers, keep their performance without electrical charging.

Multi-stage filtering

The company has looked into high-velocity systems as an alternative to multi-stage filters. While the industry standard is filtering with a phase velocity of between 2.5 and 3.5 m/sec, some suppliers claim to achieve the same performance with a smaller filter by speeding up air flow. Camfil Farr’s analysis suggest such an approach sacrifices efficiency while also entailing higher fuel costs to compensate for a greater pressure drop.

The company ran calculations for a 25-30 MW turbine to compare a Camfil Farr three-stage system with a typical high-velocity system. It concluded that over a three-year period, the operating costs for the three-stage system would be €200,000, and €3 million for the high velocity system. The findings have proved of interest to many operators, Ekberg says.

A truly efficient filter must be able to capture the smallest particles - particles of two microns and less are also those most responsible for fouling. In this respect Camfil Farr has drawn on its experience delivering high-efficiency air filtration systems to hospital clean-rooms. This has resulted in the development of the HEPA super filter for gas turbines, which has delivered very successful results, says Ekberg. Designated CamGT H10 and H12, the HEPA filter is designed especially to capture sub-micron particles in a gas turbine application.

HEPA filters are installed on a number of large turbines in the UK as replacements for the original filters from another vendor, which had performed poorly. In addition to minimizing the occurrence of fouling, they have enabled the period between turbine washes to be extended.

For more information contact Tord Ekberg, Camfil Farr Power Systems. Tel +46 3317 8538, fax +46 3317 8555,[email protected], www.camfilfarr-gt.com