The filtration of gas turbine inlet air has always been important. But its profile has been raised as more becomes known about filtration efficiency, the need to filter out small particulate and the role of filtration in overall turbine performance.
Vendors such as AAF, Donaldson, Camfil, Freudenberg Filtration, Mann+ Hummel, Gore and Eagle Filters have been working to improve the quality of their filters, to provide higher levels of protection in any environmental conditions, and to design complete systems that reduce maintenance requirements while filtering out harmful elements.
They note trends such as the development of better hydrophobic (tending to repel water) media, greater attention on the prevention of salt and hydrocarbon ingestion, and a wider range of filtration options to suit different budgets, performance objectives, and equipment or retrofit needs.
The Gas Turbine Systems division of Donaldson provides engineered filtration solutions for all environments and most air intake and lubrication systems for gas turbines, industrial compressors, and generator sets. Donaldson Turbo-Tek Cartridge Filters use an Er|W|P rating system instead of capture efficiency, as measured by MERV ratings. It also tests for watertightness and pulsability, and rates on a 0 to 5-point scale:
• Efficiency (Er0 to Er5) – what proportion of airborne contaminates does it capture?
• Watertightness (W0 to W5) – how does it handle high humidity and moisture?
• Pulsability (P0 to P5) – How readily do the filters in self-cleaning systems recover peak performance after pulsed with compressed air?
These attributes differ from one filter to another. In combination, they drive operating costs and performance under specific environmental conditions. Donaldson’s filter line can be divided into three categories:
Base medias are economical options that consist of cellulose and synthetic fibers. They are moisture-resistant with a high watertightness rating; however, they have a lower capture efficiency rating (Er1 – Er3) and do not provide (H)EPA-level efficiency.
Performance synthetic range of medias offer F9/MERV 15 efficiency, combined with a P5 pulse recovery rating. This option is for users not needing a high capture efficiency but are dependent on strong pulse recovery in desert or arctic environments, for example, where there is exposure to dust, snow, and ice buildup, or sudden episodes of heavy loading.
Premium (H)EPA filtration captures over 99.5% of 0.1p penetrating particle size. Turbo-Tek H20+ is the most watertight option, rated Er5|W5|P1. Its synthetic media acts as a barrier to water. It is suitable for coastal environments, stopping salts and other corrosive water-soluble contaminants. Turbo-Tek (H)EPA Pulse has surface-loading blended media with a rating of Er5|W1|P4. The P4 pulse recovery rating and high Er5 efficiency rating are best for deserts or areas with a high dust load that depend on a strong pulse recovery.
Cassandra Light, Manager of Gas Turbine Systems for the Americas and APAC, Donaldson Company, gave the example of a plant on the humid Australian coast using Turbo-Tek H20+ filters. The air was laden with sea salt and smoke from nearby brush fires, yet filter life rose from 18 months to 48 months with 0% loss in compressor efficiency, stable differential pressure across the filters (DP), clean blades, and elimination of on-line water washes of the filters.
“A site in a harsh environment in China with a high volume of hydrocarbons has increased filter life from one year to nearly three years and has eliminated water washes (on-line or off-line) altogether,” said Light.
In addition to filtration for gas turbine air intake systems, the Donaldson Gas Turbine Systems Quick Lock Yoke filter retention system ensures secure installation and reduces filter changeout time. Light said that in a test, the time required for filter replacement could be reduced by a little less than half.
AAF International offers a wide range of filters. It recently launched the DuraShield S filter cartridge to protect gas turbines and reduce compressor fouling, as well as cold and hot end corrosion. The DuraShield S has a high MERV 15 (F9) initial efficiency that provides sustainable performance over its operational life.
Other features include a high dust holding capacity, low pressure drop and cartridges that act as a water barrier, withstanding high humidity, mist, fogs or rain. This means differential pressure (dp) can remain predictable and stable and dp spikes are not a concern.
The company provides an online platform to help customers determine the best filtration approach for their plant. As well as the geometry of the filter, it takes into account climate, environmental and operating conditions, high humidity, extreme temperatures, coastal and offshore locations, and other variables such as the presence of hydrocarbons, dust and gas turbine mode of operation.
Stuart Blackburn, Product Management Director at AAF International, noted a change in attitude recently to address the disparity that often occurs between onshore and offshore filtration systems. In the onshore environment, higher grades of filtration are well adopted but are not found as frequently offshore. Yet the offshore environment must deal with salt, which if untreated will drastically reduce the efficiency, performance and life of any GT.
Camfil Power Systems offers a full range of pre-filters and final filters for turbomachinery applications. This includes bag filters, panel filters, compact filters and cartridge filters. Efficiency ranges are available up to H13. In addition, the company provides complete air inlet systems including filter houses, acoustics and ventilation (enclosures and silencers), environmental protection such as anti-icing systems, air treatment systems (inlet cooling and heating), weather protection (hoods, louvers/mist eliminators and insect/bird screens), ducting and retrofit services. The company also offers engineering laboratory services, site analysis, CamLab (on-site mobile engineering laboratory), remote monitoring, training, life cycle cost analysis (LCC), installation and filter change and performance optimization.
CamGT compact filters, for example, are engineered for extreme environments. They are designed for EPA efficiency to boost power by minimizing degradation while maintaining a low, stable pressure drop. New features increase equipment availability, reduce corrosion risk and improve fuel consumption and carbon footprint.
Hydrophobic media and a double-sealed construction eliminate the risk of dissolved contaminants passing through the filters. The filter construction offers a more open face geometry and the aerodynamic grid redirects exiting air to reduce pressure drop. In addition, drainage has been improved on the latest models.
The latest generation includes three models. The CamGT 4V-300 (12” depth), the New CamGT 3V-440 (17”), and the CamGT 3V-600 (24”). CamGT filters are offered from ISO ePM1 75% (F7) to E12 efficiency and fulfill standards for efficiency, burst strength and water repellency. Camfil offers a value rating feature on its website to help evaluate filtration effectiveness and economics.
“This line is designed for all environmental conditions including extreme conditions of humidity, salt, and heavy pollution that threaten gas turbine performance,” said Marc Van den Eynde, Vice President at Camfil Power Systems.
He offered some general advice on filter selection: Efficient Particulate Air filters (EPA) static filters are well-suited for wet, salt-laden or coastal/offshore environments. Pulse filters are recommended for dusty environments with low humidity. Depending on location, application, construction, ambient conditions, and particulate type/amount, the filter solution could vary from one stage to multiple stages with a variety of filter options.
Camfil is introducing a service that digitizes the coupled performance of the inlet system and GT. Known as PowerEye, it digitally monitors inlet system and GT metrics to provide trend analysis and forecasting. Ambient aerosol and weather conditions, current pressure drop, turbine load and heat rate are some of the metrics that can be analyzed to determine such things as future power output, and an optimized schedule for off-line compressor washes or filter replacement.
Gas turbine air filtration solutions from Freudenberg Filtration Technologies address areas such as particle removal, prevention of compressor fouling and corrosion. The Viledon hydroMaxx coalescing filter repels moisture at the pre-filter stage. The Viledon T-60 is a pre-filter used by turbine and turbocompressor OEMs for its dust holding capacity. The Viledon eMaxx is a final filter that protects equipment to the E10-E12 HEPA range. For systems using a cartridge or canister-type filter, the Viledon GTS double cylindrical filter offers high performance dust holding capacity and optimized pulse jet operation. It uses a nested design; one cylinder fits inside the other, reducing storage and shipping volumes. Besides filter elements, the company provides engineering staff for filter house construction, retrofitting, or modification to existing installations.
Mark Arsenault, USA Turbomachinery Sales Manager for Freudenberg Filtration Technologies, emphasized the capabilities of the Viledon hydroMaxx. It can convert any single-stage filter house into a multi-stage filtration system, is equipped with a clip-on system that allows close coupling to the intermediate or final filter in one filter wall, and drops out moisture before it causes problems in the inlet system. It also sheds snow and ice from its leading edge, preventing blinding and allowing air to pass through.
Mann+Hummel, headquartered in Germany, offers a range of high-performance air filtration solutions. It recently launched new products for all stages (coalescing stage, pre-filtration and final filters) and introduced pulse jet cartridges.
The Aircube Pro Power S and Nanoclass Cube Pro Power S for GTs are designed to meet the extreme operating conditions found, for example, on oil platforms at sea. They provide low pressure drop, high burst pressure and a high dust holding capacity (DHC). The media that Mann+Hummel developed in cooperation with a partner is water and salt repellent. Testing showed a burst resistance of 35.000 m3/h above a pressure of 8.000 Pa.
“The air filter market for this application has been dominated so far by micro glass fiber media, which has disadvantages such as sensitivity to mechanical stress and low water resistance,” said Mohammed Asif, Business Development Manager, Middle East & Africa, Mann+Hummel. “The synthetic media in the Aircube and Nanoclass Cube offer higher resistance to mechanical stress than glass fiber media and stable high efficiency during operation.”
He has noticed a trend toward the adoption of greater filtration efficiency on each stage, often using EPA filters at the final stage. This may increase pressure drop, but better air filtration efficiency improves overall GT efficiency of the gas turbine, he said. It also heightens performance in challenging environments such as offshore.
W.L. Gore & Associates offers Gore E-12 HEPA Turbine Filters. They are available in an array of pulse-cleanable and static operational mode cartridge and panel configurations to fit into existing filter houses. All Gore Turbine Filters contain Gore’s 3-layer synthetic composite with a depth loading layer, hydrophobic ePTFE membrane, and strong pleated backer for long life, low pressure drop, and protection from airborne contaminants, salt ingestion and hydrocarbons.
The company plays in multiple markets across the globe including power generation, oil & gas production, refining and LNG. In particular, it does well in environments where maintaining operational effectiveness is critical such as applications without backup options or built-in process redundancies.
“In applications where the user does not have the option to shut down their equipment, the filter’s job is to protect the turbine from particulate damage, to remove particles that would reduce its efficiency and to allow the turbine to remain online until the operator is able to perform their scheduled maintenance,” said Paul Vierra, Regional Sales Manager, at W.L. Gore & Associates. “All Gore solutions for gas turbines are all high efficiency E-12 HEPA filters.”
Gore Turbine Filters accomplish high particulate capture capacity. The filter must have the dynamic capability to function in varied applications which can be anything from coal dust from a nearby deposit to fine metal powder from a wrecking yard next door. HEPA E12 efficiency means that the media can pick up fine dust the eye cannot see. Studies show that sub-micron sized particles in the atmosphere account for a large majority of particulate count when compared to the number of larger than micron-sized particles. It is these sub-micron particles that are missed by traditional filtration technologies. By capturing these particles, the end user can keep its compressor clean and avoid performance degradation or shutdowns.
Vierra also touts the effectiveness of his company’s filters in water management. Poor water management leads to water build-up on the surface of a filter that will eventually blind the surface so air cannot pass. This results in inlet differential pressure spikes, load reduction and sometimes shutdown. Gore filters remove water from the inlet airstream, which can contain a number of dissolved elements including salt and other corrosives. Eliminating these from the airstream prevents harmful particles from contacting turbine parts.
“We have seen a push by OEMs to adopt HEPA technologies for this reason,” said Vierra. “Effective water management means filters must be able to continue to operate while blocking large amounts of moisture from rain, fog and mist.”
His advice to users is that the filtration selection process begins with knowing your environment. He also recommended a full return on investment analysis that includes an estimate of the total cost of filtration over the lifetime of the turbine. The plant must balance media costs, replacement costs, frequency of replacement and filtration effectiveness against risk such as lost production, increased operational costs and/or capital costs from damaged turbine parts.
Eagle Filters produces GT inlet air filtration products ranging from coalescers to HEPA final filters. They come in all forms and shapes with various connection methods to fit in any inlet house. The company can also upgrade filtration without having to modify inlet house structures using two-layer cartridges and snap-on attachment methods for square filters.
TurboMaster cartridge/canister type HEPA grade filters have a two-stage structure enabling filter houses to be upgraded to two-stage filtration. Both stages operate independently providing longer service life and predictable end-of-life behavior.
TurboMaster can be equipped with a prefilter sock that further improves service life by effectively making it a three-stage filter in a single cartridge/canister slot. The company produces its own filter media with no membranes in the filtration structure for better salt durability.
“Our fully synthetic product range provides excellent protection against humidity and salt,” said Pertti Sundberg, Director of Sales at Eagle Filters. “Our three-stage HEPA filtration offers excellent cleanliness and high GT efficiency without the need of on-line or off-line washing to regain the GT output lost to degradation.”
The company’s website features a GT degradation estimator. It takes into account the difference in turbine performance before and after off-line washing, as well as annual operating hours to determine the MWh lost each year. This helps users determine if their current method of inlet air filtration is sufficient.
For desert environments, Eagle Filters offers a way to address sand blockage of first-stage filters after sandstorms and the high average lifetime dp of pulse filters which can lead to loss of GT output. It’s large surface panel prefilters can be replaced quickly after a sandstorm, enabling a plant to run with static filters throughout the year at much lower lifetime dp.
The bulk of GT wear and tear may be caused by foreign particles entering via inlet air system as a result of poor filtration. But Sundberg noted the challenge inherent in convincing plants to invest in better filtration. Budgeting for HEPA-grade filter upgrades, he said, can be far less common than paying far more for the turbomachinery damage caused by particulate.
Environment impacts filtration
Filtration needs vary depending on the environment. Here are some of the site conditions to look out for:
High particulate, industrial areas: Refineries and areas of heavy traffic generate tiny hydrocarbon particles. HEPA filtration is recommended. In the presence of sticky hydrocarbons, prefilters are needed to extend the lifespan of HEPA filters.
High particulate, desert areas: Coarse sand can block first-stage filters after sandstorms. Pulse cleaning and pre-filters are among the options offered by vendors.
Salty coastal line: Anywhere within 50 miles of the coast can suffer from salt particles, but especially near the sea. Salt particles are small and demand adequate prefiltration to protect HEPA filters.
Offshore platforms: Salt and hydrocarbons are the primary challenges offshore. Recent studies show offshore facilities often require stronger filtration than has traditionally been implemented.
High humidity: Humid environments should opt for hydrophobic filter material, with the moisture removed in multiple stages.
“Organic material such as cellulose will absorb moisture and swell, increasing the dp as the media becomes blocked,” said Sundberg. “Also, the wet tensile strength of organic materials is typically much less than the dry tensile strength. Weak wet material with high dp is the most common cause of filter ruptures.”