Not all HEPA Filters are the Same

By Steve Hiner

Ineffective gas turbine inlet filtration will cost a power plant a lot of money through reduced turbine efficiency, increased maintenance costs and lower turbine availability. In the face of these impacts on the bottom line, many turn to (High Efficiency Particulate Air) HEPA filters to increase the efficiency of their inlet filtration system and, in turn, their gas turbine. Unfortunately a higher filter efficiency rating alone will not give an operator a complete picture and may even cause more problems than it solves. This article will look into some of the main points operators should consider to ensure they get the best from their filtration solution.

Because of the volumes of air a gas turbine consumes, the choice of inlet filtration system has a huge impact on turbine performance. Selecting an inappropriate solution will leave the turbine exposed to contaminants that can corrode and impede its performance. The wrong type of filter for an installation can also lead to sudden restrictions to the inlet air flow causing pressure spikes and unexpected turbine shutdown. Selecting a solution that will protect the turbine as well as optimize maintenance and performance requires an understanding of the local environmental challenges a filter needs to face.

Harsh conditions make filter choices tougher. HEPA-rated filters will perform differently depending on design and construction. Make sure your filter choice is right for your specific site conditions.

Gas turbines are often installed in harsh conditions. The local environment can contain high volumes of sand; salt, if near to the coast; pollution and hydrocarbon mists; snow; excessive rainfall; elevated levels of moisture from mist, fog or high humidity levels – or, of course a combination of any or all of these with seasonal variations. To effectively protect the gas turbines, a filtration solution needs to handle the local, real-world conditions in which it is installed.

Fine particles that reach the turbine blades can stick to them and, as they build up, affect aerodynamic performance. This will cause a reduction in output power and increase in heat rate that will ultimately require an offline wash. The more frequently this maintenance procedure needs to be carried out, the greater the cost impact through lost MW output and increased rates of fuel usage. Filters use media that captures particulates and prevents them reaching and harming the internal parts of the gas turbine. The higher the efficiency of the filter, the finer the particles it captures.

Does salt threaten your turbine?

If a power plant is located within 12 miles of the sea, salt may be a particular contaminant that puts turbine performance in peril. While dry salt will be captured in the same way as other dry particulates, the affinity salt has for absorbing water and moisture means it needs special consideration.

As with other contaminants, salt can stick to turbine blades and reduce aerodynamic efficiency. Its stickiness, however, can increase the rate at which this occurs. Salt is also particularly harmful because, if allowed into the turbine internals, the sodium in the salt can combine with sulfur in the fuel in the hot section of the turbine to cause accelerated corrosion. In the cold end, the chlorine in the salt will additionally act as a pitting corrosion initiator. The overall impact salt can have on a gas turbine can lead to exceptionally high maintenance levels and premature failure of the turbine.

Operating Data Comparison

Turbine operating data can help show how different HEPA filters perform in the same environment.

Will a HEPA filter better protect a turbine?

To improve turbine protection some may conclude that a high efficiency particulate air (HEPA) filter will be the best option. To capture finer particles, however, means finer media is used which, in the presence of moisture or hydrocarbon mists, may be prone to sudden blockages that can have a serious impact on turbine operation. This is because moisture caused by mist or fog consists of tiny droplets that can work their way into the media matrix and become stuck – blocking pathways for the air to pass through. The presence of captured hydrocarbons from pollution in the media generally concentrates this effect further, making it even harder for tiny water droplets to escape, increasing the filters sensitivity.

This same effect does not generally happen with the larger droplets of rain, as these coalesce on the media surface and drain, being too big to get into the media matrix itself. Similarly, as high humidity does not contain droplets, this will generally pass through with little effect.

Standard filter efficiency ratings are based on laboratory testing in a controlled, dry atmosphere- an environment that is a long way from that of a power plant’s. Just relying on filter efficiency rating alone, therefore, does not give a complete picture of how a filter will perform once installed in a specific installation environment. For example, simply selecting a fine filter media may not allow for the effects of moisture which can result in shortened filter life expectancy, bypass of corrosives, increased maintenance cycles (and cost), and the need for fast operator reaction times to deal with sudden pressure spikes. Indeed, the hydrophobicity of a filter, it’s ability to keep liquid contaminants out should be a major consideration for the power industry, where unexpected turbine outage can be a costly occurrence.

HEPA filters are typically constructed from two media choices – PTFE membrane or microglass. The thickness of the media can play a large role in filter operating performance.

Not all HEPA filters are the same…

When it comes to HEPA filtration, there are two main options to select from: a filter made from Microfiber glass (glass fiber) or PTFE membrane. Both can provide HEPA level filtration and filter out fine particles from the gas turbine inlet. To achieve this, both use fine media but they are constructed and perform very differently.

PTFE is a simple polymer composed of carbon and fluorine. Expanded polytetrafluoroethylene (ePTFE) filter membranes consist of a single, very thin layer of finer media that creates a ‘sieving’, surface filtration effect. While highly efficient at capturing fine particles, this means the membrane has a relatively low filtration surface area. Any moisture droplets that become trapped, therefore, can quickly cause a complete blockage of the inlet filter. In real-world installations, the complete blocking of these filters can occur in as little as three weeks after installation.

Microfiber glass filters use a media layer that is ten times thicker than ePTFE, greatly increasing the filtration surface area. Rather than using the ePTFE model of a thin layer of finer pores, it uses its depth to capture particles as they travel inside its matrix. Even if moisture droplets block pores within the media, the volume of pores means the media will take much longer to become completely blocked. Any deterioration will happen slower than with ePTFE membranes; extending the life of the filter and giving operators plenty of warning that cleaning or replacement is required. It is this predictable performance that, for the time being, continues to make microfiber glass the preferred choice in heavy industrial applications.

Robustness of filter design

Many filters are designed for use in HVAC or laboratory-style conditions. To withstand the rigors of a power plant, a filter needs to be designed to handle a tougher environment and greater operating pressures.

Particular areas of weakness to consider when reviewing a filter’s construction include the gasket that seals the filter to its holding frame. Any joins in this gasket may be potential break points. The protection provided to the media pleat packs, i.e. does it have supporting mesh downstream to enable the pleats to cope with the increased working pressures in this application? For cartridge filters, the way the media pleats are glued to hold the pleats is important as any glue beads formed in the assembly process if not done correctly may break off the filter once it is installed. The overall materials of construction, including the compatibility of any adhesives also needs to be appropriate for the site conditions to avoid brittleness or cracking and to ensure the filter frame does not become warped over time or under the wider extremes of operating temperature.

Optimizing a filtration solution

Selecting a filtration solution will depend on a number of factors beyond filter media and unit design. The best solution may not be the most expensive and, ultimately, it is how the turbine performs that shows the suitability of a filter for its application. The level of output and heat rate of the turbine and how this varies over time are the best forms of data in establishing the effectiveness of a filtration solution. If an operator sees unacceptable increases in heat rate and decreases in output, then the filtration solution should be re-evaluated and other types considered.

Although testing standards continue to be researched and developed as environmental factors become more clearly understood, one of the problems operators face today is that the standard efficiency rating of the filter will not necessarily equate to the performance of the installed system. Recent comparative studies carried out by a major gas turbine OEM on the performance of equivalent ePTFE membrane and microfiber glass HEPA filter products covered overall efficiency, pressure loss, hydrophobic performance, wet performance and the dust holding capacity of each technology. The tests showed the microfiber glass filters to produce equal or better performance than the ePTFE equivalents, and at a lower cost.

This article is not trying to say microfiber glass media is better than ePTFE. CLARCOR manufactures ePTFE membranes which are successfully installed in many other filtration applications and future developments may well deliver improved resilience to moisture and hydrocarbons of this media type. As the technology stands today, however, the unpredictable response ePTFE membranes have to the wide-ranging air contaminants found at gas turbine installations means CLARCOR does not recommend them for use on turbine installations.

Summary

Filter test standards and materials for use in gas turbine filtration solutions continue to develop as we learn more about environmental impacts. What is clear is that operators need to take a wider view of filter and turbine performance rather than considering only standard efficiency ratings.

Both microfiber glass and ePTFE membranes can provide HEPA filtration levels. While ePTFE membranes can deliver a typical lifespan of two years, however, this is still much shorter than microfiber glass equivalents for power plant gas turbine installations. Where moisture levels are high or there is likelihood of fog or mist weather events, the thicker microfiber glass media will give a more predictable response. ePTFE solutions will require close monitoring and quick change out to protect machinery if they suddenly become blocked.

As technology and understanding develops, it is recommended that power plant operators consult filter manufacturers to understand the options available to them. By working with filter suppliers that appreciate and understand the varying needs of different environments, a turbine can be better protected and this may not be the most expensive option. By eliminating unnecessary maintenance or shutdowns and improving turbine performance, the right solution can give a very quick return on investment.

Author:
Steve Hiner is chief engineer of Gas Turbine Inlet Systems at CLARCOR Industrial Air.

Tags PE Volume 121 Issue 5