(H)EPA filters for gas turbine

(H)EPA - Why, when and how?

A short story of HEPA filters for gas turbines

(High) Efficiency Particulate Arrestance - or (H)EPA -  filtration for gas turbines is nothing new, but it is becoming more popular with many gas turbine operators. Camfil has been supplying (H)EPA filters for the gas turbine industry since 1986. So what has changed? 

As gas turbine technology evolved and turbine efficiency went up, their sensitivity and thus the importance of maintaining blade integrity also rose. With new alloy permitting higher firing temperature, smaller tolerance on profile parts, higher efficiency and reliability requirements as well as better tools for performance monitoring, operators realized that the air entering the turbine needed to be as clean as possible to maintain top performance. They were also better equipped to make a cost / benefit evaluation than before.

In the early years of the gas turbine history, adding some protection in front of the intake to protect the turbine had quickly become an industry standard to avoid foreign object damage and eventually blade erosion. As filter technology also developed and fouling consequences were better understood, gas turbine inlet final filter stage efficiency also started to raise over time to high efficiency (per EN779) in the 80's and (H)EPA in the 90's (per EN1822). 

To understand the differences between the EN:779, EN:1822 and ASHRAE 52.1 and 52.2, see our comparison chart below.

EN vs ASHRAE comparison (pdf)

As shown below, especially in offshore, coastal and other corrosive environments, where salt, water and/or hydrocarbons are important, gas turbine operators were prompt to adopt a multiple stage system with higher final filter efficiency, especially where corrosion had caused significant damages.      

 

Evolution in Static Filtration System 

Evolution in Pulse Filtration System 

1960's
  • Typical Roll-o-Matic
  • M80, Duracel 60/90
  • First pulsejet bag house (in dust collection)
1970's
  • Camfil FARR Dynavanes - Inertial separators type filters
  • Oil bath type filters
  • First pulse-jet self-cleaning air filtration system applied to a commercial gas turbine  - (Solar Turbines, Inc. Saturn gas compressor set, Union Oil application, Kenai, Alaska, 1976)
  • Camfil Farr Tenkay flange-style cartridge invented in 1977 
  • Typical efficiency F6-F7
1980's
  • Typical panel G4 pre-filter, such as the Camfil 30/30GT with final filter F7-F8
  • Water handling with coalescer pads
  • 1986: First 3 stages with final EPA Filter for gas turbine offhore
  • Camfil introduce aluminium inlet offshore
  • First horizontal pulse filter system for GT introduced in 1985
  • Cross flow brings smaller foot print

 

1990`s
  • Camfil introduces the CamGT: High efficiency F9 and (H) EPA filters with vertical pleating for better water drainage
  • Water handling with droplet separator vanes
  • 1995: Camfil introduces it`s first E12 for GT
  • 1998 Camfil introduces the FARR Gold Cone Tenkay cartridge
  • Typical efficiency up to F9 
2000's
  • Typically inland F9/E10 is the minimum standard
  • Offshore is (H)EPA E10 or E12
  • 2003: Camfil patents the HemiPleat for better dust release (longer life, lower dP at the same efficiency)
  • Membrane (H)EPA filter introduced as an aftermarket retrofit for pulse system in coastal environment 
 2010's
  • Camfil introduces the CamGT 3V600 for lowest dP at (H)EPA efficiency 

 

The benefits of (H)EPA filtration for gas turbines

Turbine efficiency:

A gas turbine's rotating part profiles have very small tolerance to keep their maximum working efficiency and maximize power output. Raising to (H)EPA filtration increases fuel efficiency, ensures continuous high power output, and can eliminate the need for compressor washings, reducing maintenance cost and shut down time. Typically, with an E10 final filter, operators will wash once a year, but won't observe any power degradation from E12. 

Reliability:

Fouling raises the temperature of the engine and that negatively effects the life of the internal components. Clean air ensures the maintaining in top conditions of critical turbine parts such as:

  • Air compressor rotor and stator blades, variable stator vanes and labyrinths
  • Burners, combustion chambers and transition ducts
  • Center bearing, labyrinths and buffer ports
  • Compressor turbine rotor and stator blades
  • Cooling passages and labyrinths
  • Power turbine rotor and stator blades

(H)EPA filtration combined with the appropriate pre-filtration stages to handle salt particles has also a proven track record to protect gas turbines against corrosion, reducing the risk of unplanned shutdown. 

Which (H)EPA filter to use in a gas turbine application?

EPA, HEPA and ULPA filter classes

(H)EPA definition has evolved over time. Originally HEPA meant 99.97% efficiency on 0.3 µm particle, but the latest EN1822:2009 standard has different classifications. It starts with EPA, E10 to E12 (previously H10 to H12) at 85% efficiency on the most penetrating particles (MPPS) for E10, 95% for E11 and 99.5 for E12. HEPA H13 and H14 demands 99.95 to 99.995, while Ulpa, U15 to U17, ranges from 99.9995 to 99.999995%. 

It is important to understand the order of magnitude in efficiency between an F9 filter (per EN779:2012) and an E10. Per the EN779 standard, F9 filters grade efficiency at 0.4 micron, which often is NOT the most penetrating particles. By example, our CamGT F9 meets the required 70% efficiency at 0.4 microns, while the E10 offers 85% efficiency at the most penetrating particle and 98% efficiency at 0.4 microns - reducing particles penetration by a factor of 15:1.  

Read more on the latest standard EN1822:2009 

If higher efficiency is usually better, why not U15 filters on a gas turbine?

It all comes down to life cycle cost consideration. An E12 filter offers over 99.5% efficiency, already eliminating water washing and power degradation. Going to U15, we raise efficiency to 99.9995% and particles penetration diminished by a factor of 1000:1, but the E12 performance already captured all the operation benefits.  U15 will add pressure drop and filter cost, as filters need to be individually tested from H13, but no incremental advantages. Also, the filter house themselves are not built to guarantee that level of filtration past the final filter stage. In clean room or pharma applications, where such filters are usually used, gel is used on all seams of the room to guarantee airtightness. 

Life cycle cost considerations

When looking at gas turbine (H)EPA filtration, it is important to consider the whole system, the environment and usage of the turbine. The number of stages, type of filter (Static: panel, bag, compact; or Pulse), the type of media (hygroscopic considerations), the quantity of media (in static filter system) and the filter construction (burst strength, pleating type, overall geometry) will impact the efficiency, pressure drop and life of the final, thus the life cycle cost. The pre-filter(s) choice become important, as it adds to the total efficiency, can help coalesce water to reduce re-entrainment of particles, and will prolongs life of the final filter. Filtration is always about protecting what's behind the filter, even if it's another filter! 

Systems recommendation per environment

(H)EPA filtration has many benefits, but before making the change to an existing installation, it is important to consider a few other factors. Verify the integrity of your filter house and transition after the final filter - if you have corrosion or by-pass, they will be worth repairing. The size of your filter house will also affect the efficiency as velocity through the filter media is important. Also note that the gas turbine ventilation filtration standard have raised too over the years, up to F9 as a minimum.  

When buying or retrofitting an inlet filter house, you should consider asking for a life cycle cost calculations. When you specify which level of filtration is ideal for your situation, always refer to the latest standard in your specification - by example EN779:2012 now includes a minimal efficiency for discharged filter. Many synthetic filters now have a lower filter class than before as they had an electrostatic charge that latest only a few weeks in the field - and in case of doubt, ask for test reports.      

More on the standards: Specifications and regulations 

 

 

 

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