By Joshua Kohn

It is not uncommon for gas turbines in coastal areas to suffer from corrosion due to poor inlet air filtration. In environments where salt and other hydrophilic (moisture absorbing) contaminants are present and humidity spikes are frequent, particles on some filter media can swell, causing pressure drop (dP).

High filter dP reduces compressor efficiency and ultimately power output. In an extreme case, it can set off a turbine alarm and force operators to derate.

Furthermore, high dP increases the risk of salt migration through the filter media. In the presence of salt, the risk of hot-end corrosion increases, leading to costly turbine repairs.

Filters with a high efficiency EPA class rating, good hydrophobicity (water resistant), and drainage capabilities will prevent salt carryover.

Salt and contaminent problems

Without them, water can dissolve salt and other contaminants present on the filter surface and carry them onto the clean side of the air inlet where they can be ingested by the gas turbine.

In the presence of salt, the risk of corrosion increases. This can cause the turbine to be less efficient, and lead to maintenance issues.

A one-piece, poured-into-place endless gasket instead of the typical four pieces will prevent bypass. Four-piece gaskets have the potential to allow water and particulates to migrate downstream due to the joints between the gaskets. Media is often glued to the filter header in two-to-four steps. A double-sealing design that uses a six-step glue technique to prevent leakage is recommended.

Depth loading with synthetic glass media offers an optimal balance between wet performance and dust holding capacity.

Vertical pleats and patented interrupted hot melt separators are designed for efficient water handling.

Horizontal pleats and uninterrupted hot melt separators, however, can trap water in the media, causing increases in pressure drop, which can force dissolved contaminates through the filter.

Take the case of a plant on the Pacific Coast in Los Angeles. It is situated in a deep hollow, where a thick marine layer (fog) comes in from the ocean. This leads to high humidity for long periods.

The engineering team conducted a borescope inspection on two Siemens 501F gas turbines, where standard F8 efficiency filters were installed on site.

The inspection found corrosion on both engines. This lead them to evaluate higher efficiency final filters to reduce ambient salt.

Standard F8 efficiency filters were installed on site. The team decided to evaluate a higher efficiency final filter to reduce ambient salt on the turbine air inlet and the gas turbines.

F8 grade filters are rated per EN779:2012 while E10 grade filters are rated per EN1822:2012. F8 filters at 0.4µm are required to meet a minimum 55% initial efficiency per EN779:2012, while a typical E10 grade filter rated per EN1822:2012 offers 98% efficiency at 0.4µm. Upgrading to an E10 efficiency class reduces particle penetration by a factor of 20:1.

A CamLab, a mobile laboratory for testing filters, was used to perform a blind test on site for 2,100 hours to compare four filters from different manufacturers. All had a 24 × 24-inch face area but slightly different filter element depths.

Measured parameters

During the test, the filters were exposed to on-site contaminants and atmospheric conditions. Measured parameters included ambient dust concentration, airflow, pressure drop, filter efficiency, temperature, and relative humidity.

Copper corrosion coupons were installed downstream of each filter duct to provide an additional reading of performance (measured according to ISA 71.04:2013 for change in thickness). After site testing, they were retested under controlled laboratory conditions.

Additionally, a deluge test on the dust-loaded filters determined resistance to water penetration and rate of pressure increase on the filters. Water was sprayed until the pressure reached over 4" w.g. (inches of water gauge) or one hour had passed, whichever had occurred first (Figure).

Effective filtration should be hydrophobic (to at least the typical operational dP) to prevent water and salt penetration. The right filtration for gas turbines in humid areas should also have efficient water drainage, a high rate of filtration efficiency, should demonstrate low dP in wet conditions, and have high burst strength.

Operational benefits of effective filtration include fewer maintenance shutdowns, increased availability and reliability, higher and more stable power output, extended turbine life, and reduced life cycle costs.

Author: Joshua Kohn is Chief Engineer, Camfil Power Systems.