Self-cleaning filters can optimize power output and heat rate for gas turbines operating in harsh, dusty environments, writes Jim Lenox

To operate efficiently, gas turbines require the cleanest possible air, yet at the same time consume vast amounts of it.

Any contaminants within this air stream will result in degradation in operating efficiency (output), fuel consumption (heat rate) and differential pressure.

The resulting lower MW output and increased heat rate come at a huge cost to the power generation sector.

In recent years operators have become far more aware of the cost associated with operational compressor degradation and, therefore, solutions to minimize this effect have become essential.

Proper air filtration is critical to the overall health of the compressor and reliability of gas turbines. Significant gains in output and heat rate are achieved through the proper use of air filters. These gains, combined with reduced maintenance costs of filters and the engine itself, mean that selecting the correct air filter solution is a vital decision.

A prime example of the importance of proper filter selection is dealing with the climatic conditions in the Middle East, which add challenges not experienced in many other power generation regions around the world. These challenges include high volumes of sand, dust and hydrocarbons that require filters to have a long life and be self-cleaning.

No filter is 100 per cent efficient, so some dust will get through. When a filter is used in an environment that is much dustier, a proportionate amount more dust will get through the filter, leading to an increased rate of compressor fouling and a greater need for downtime to off-line wash when compared to using the same filter in a lower-dust environment.

A typical high-dust coastal site in the Middle East, where proper filtration
A typical high-dust coastal site in the Middle East, where proper filtration is critical to sustained gas turbine performance
Credit: Clarcor

Selecting filters designed for optimized efficiency to remove from the airstream those particulates that lead to higher operational compressor degradation is critical to the performance of the turbine.

An inadequate solution will carry a heavy cost for power plant operators with a significant loss in revenue due to lower MW output, increased heat rate and shorter intervals between water washes.

Another environmental element that negatively impacts gas turbine performance is salt. Many gas turbines are located in coastal regions where salt, mist and fog are a problem. Sodium in salt will combine with sulphur in the fuel within the hot section of the turbine to cause accelerated corrosion known as sulphidation.

There are two types of sulphidation corrosion: Type I takes place where temperatures are above the melting point of the sulphate (825°C-950°C) and Type II corrosion occurs between 700°C and 800°C.

The chemical reaction between the sodium sulphate and turbine metal at these temperatures attacks the base metal of the turbine blades and can cause catastrophic turbine failure.

The lower-temperature Type II hot corrosion is characterized by pitting, but the higher-temperature Type I sulphidation corrosion produces a devastating accelerated attack on turbine metals.

This is a particular concern in coastal Middle East regions where much of the fuel used is sour fuel that has high sulphur content. The mean time between failures in these conditions is highly dependent on the corrosion level, which is directly related to the protection that is provided to prevent salt from entering the gas turbine inlet.

Salt can freely change between dry particulate and liquid depending upon the relative humidity. In the Middle East, airborne salt will most often be in dry particulate form and, therefore, will be filtered by traditional dust filters. However, when the humidity is high, such as during the frequent fogs that are present in many Middle Eastern coastal locations, salt can turn to liquid and migrate through the filter and into the engine of the turbine.

This can then leave a sticky salt residue on the compressor blades, which makes them more likely to foul with fine particulate that has passed through the filters and, more importantly, leads to accelerated corrosion. The sticky salt will also compromise the blade aerodynamics, leading to operational degradation including lower MW output and increased heat rate.

Some suppliers are introducing new, high-efficiency technology such as ePTFE membrane filters into power generation applications, but those filters have been developed for installation environments found in areas such as HVAC and the pharmaceutical sector.

Therefore, the robustness of ePTFE solutions has proved unsuitable for the harsh environments and demanding operating conditions of the power generation sector, where hydrocarbon contaminants and moisture are real challenges that lead to unexpectedly high pressure losses and unplanned turbine shutdowns.

This industry sector requires filters with proven media technology that are very robust, very strong in terms of rated airflow, and are assembled in such a way that will last for years despite harsh conditions.

Utilizing clearcurrent Pro and BP NanoPleat technology, cartridge filters from Clarcor
Utilizing clearcurrent Pro and BP NanoPleat technology, cartridge filters from Clarcor Industrial Air help ensure proper gas turbine filtration in harsh environmental conditions for increased power output and heat rate
Credit: Clarcor

Advanced glass-fibre media and properly engineered nanofibre which has been utilized within the industry for many years provides efficiency, proven-through-life performance and hydrophobic properties equivalent to, and in many instances better than, ePTFE membrane without sensitivity to moisture, mist, fog and hydrocarbon fouling.

Clarcor Industrial Air has supplied in excess of 200,000 gas turbine filters with this advanced media over the past decade. These advanced media filters have been selected in preference to ePTFE filters on various new unit coastal systems for power generation applications.

Clearcurrent Pro and clearcurrent BP NanoPleat are new gas turbine inlet air filtration products from Clarcor Industrial Air, specifically designed for power generation applications, which offer two solutions for varying harsh environmental conditions.

The clearcurrent Pro filters are engineered to remove particulates from the air stream that are most likely to adhere to a compressor blade. BP NanoPleat is nanofibre technology engineered through optimized in-house nanofibre capabilities specifically for the power generation sector. The resulting proprietary nanofibre medium has smaller pores for more efficient filtration and is more durable than ordinary nanofibre media. These improvements lead to better filter cleaning and longer life compared with traditional alternatives.

Clarcor Industrial Air’s comprehensive range of final filters covers efficiency ratings from F8 to E12. Extensive field trials of clearcurrent technology show significant reductions in gas turbine degradation between compressor washes, leading to significant operational benefits with greater overall generated output and increased time between offline washes. Comprehensive studies with several of these new filters in high-moisture, high-salt coastal sites also show significant improvements in turbine output and heat rate compared to using conventional filters on the same sites.

The variance in the requirements and operating conditions of each gas turbine installation means that one filtration solution does not fit all. In many power generation installations a mixture of dust, salt and moisture leaves turbines vulnerable to degradation that will reduce efficiency and availability while increasing risk of failure.

The cost of an ideal gas turbine inlet filtration solution needs to be assessed against all lifetime operational and maintenance costs as well as lost power output from reduced efficiency and unscheduled downtime.

Jim Lenox is Senior Product Manager at Clarcor Industrial Air