Gas turbines (GT) are used to drive electric generators on power stations, ships, large industrial plants, and the like. They are also used as compressors in areas such as pipelines, refineries and chemical plants, and to feed steam turbines to produce more power. Wherever they are used, however, they are high value assets and their performance and reliability is heavily based upon their installation. Large gas turbines consume huge amounts of air, which means inlet filtration is a critical part of the overall system.
As gas turbine technology develops it becomes increasingly complex and sensitive to contaminants and the inlet filtration technology is more vital than ever to the system's performance. The first stages of a gas turbine are the compressor blades. These can become fouled with any unfiltered contaminant, degrading the performance of the GT and eventually requiring it to be shutdown to carry out an offline compressor water wash. Reducing gas turbine degradation improves turbine efficiency, reliability and increases time between offline washes. This gives plant operators the potential to significantly increase system availability and operating profit.
Although operators of modern gas turbine installations are looking for higher efficiency filtration to protect system performance and availability, using finer media can shorten filter life and create other system issues such as sudden pressure spikes if blockages occur. More frequent shutdowns to change a filter can cost an operator many millions of dollars per day in applications such as LNG plants. Turbines used in areas such as offshore platforms in the oil and gas industry are also subjected to exceptionally harsh environmental conditions that require careful consideration when selecting filter media. While aiming for greater filter efficiency, the use of finer media layers tends to mean a less robust solution. Nanofibers may degrade or even simply blow away in some applications. So how can the nonwovens industry meet the demand for greater turbine protection while ensuring continued trouble-free operation?
Threats to turbine performance
Fouling due to contaminants of typically <1 micron can adhere to compressor blades and reduce system efficiency. A filtration solution therefore needs to either stop the particulates from reaching the turbine or stop them from sticking to it. (H)EPA filters can address dry contaminants but they also need to handle moisture. Use of hydrophobic and advanced fiber coated filters can stop sticky contaminants such as salt in liquid phase and hydrocarbons, from reaching the turbine blades.
Filtration systems also need to protect turbines from corrosion. Sodium in salt will combine with sulphur in the fuel within the hot section of the turbine to form sodium sulphate which causes Type I and Type II accelerated corrosion or “sulphidation”. Salt is hygroscopic and can easily move between solid and liquid phases and is a particular challenge in offshore or coastal gas turbine installations.
Media for the real world
In industries such as power generation and oil & gas, reliability is absolutely vital. As such, filters need to operate with predictable performance. The filter industry and the plant operators are becoming more aware of the fact that a standard filter efficiency rating does not always equate to a performance level. Particular issues are being seen with the use of synthetic media types where they do not appear to offer the same levels of performance in actual installations as they do in lab tests. Some of this relates to electrostatic discharge. Whether intentionally added or just residual charge left from the manufacturing process, electrostatic charge can boost filter efficiency in tests meaning that actual performance may not be as expected once installed.
The challenge for media manufacturers is to either produce a media that performs the same in the real world as it does in the laboratory or to revise laboratory tests so that they are realistic to the real world. Current test standards do not ensure this, which means that a standard filter rating does not necessarily provide what is expected.
For synthetic media to better meet the needs of gas turbine installations, it needs to improve its efficiency as dust collects. At the moment this has not proved to be the case and, where the dust itself acts as an additional filtering medium in media such as glass fiber, synthetics have not shown similar properties, instead typically degrading in performance over time.
To handle multiple environmental challenges, a filter often comprises several layers of different types of media laminated together. Such multi-layer composites require greater diversity from manufacturers or partnerships that bring together the different media types into the required solution in an economical way. To extend life, a greater volume of media is typically used and pleated into the filter element. This means that media pleat-ability and flexibility as to how it can be pleated are also important product characteristics.
In the offshore industry in particular, the overall size of a filter solution is a major consideration when designing the filter house. Greater pleat-ability to produce high-density media and designs that ensure air flow is optimized through it are, therefore, of significant benefit. With this in mind, some media utilize fine corrugations or strategic embossing. These hold the layers of media apart to enable the most effective use of the media area. To this end, the formability of media is another consideration. If corrugations and embossing can be added and the media is self-supporting this can reduce the cost of a solution – removing the need for additional meshes or supports to maintain form.
Simply installing higher efficiency, (H)EPA filters does not answer all the needs for a gas turbine installation. In many applications, the media used needs to be able to handle moisture and hydrocarbons without easily blocking. Coatings with hydrophobic or oleo-phobic properties can be used but manufacturers need to ensure that these don't just offer additional protection when first installed, but throughout the lifetime of the filter. Fluorocarbon coatings, for example, offer excellent performance at first but as dust covers the filter the coating becomes concealed and no longer does the job for which it was intended. ePTFE media has also shown to be more sensitive to hydrocarbons. When it is challenged with these hydrocarbons, and then mist or fog, it can quickly become blocked and produce severe, sudden pressure spikes in the system that can trip the turbine or cause damage to the installation.
In many parts of the world where there are high volumes of dust, self-cleaning pulse filters are installed. To add (H)EPA filtration to these sites requires the finer filter layer to be on the surface to maximize dust release and minimize depth loading. This means the use of laminated, multi-layer media is much less appropriate, which presents a particular challenge to filter manufacturers and anything the nonwovens industry can do to improve filter performance in such applications would be very valuable.
Single salt crystal.
Challenges for the nonwovens industry
Media for industries such as oil & gas need to offer high efficiency, compact size, predictable performance, and reliable operation. They need to effectively handle multiple environmental challenges that can include large volumes of dust, hydrocarbons, mist, fog and salt aerosols. They need to be robust and offer long life to increase system availability and reduce maintenance overheads. As we work to enhancing media performance in all of these areas, we also need to keep costs down. The benefits of such solutions do offer the industry a lot – but it is not prepared to pay a great deal more. Designs, therefore, need to optimize the use of the media within the filter and manufacturing logistics need to be considered. The gas turbine market is very much a global one and regional manufacturing capability offers competitive advantage to suppliers.
Gas turbine filter media needs to offer robustness, proven through-life performance and high efficiency with reduced sensitivity to mist and fog. Options for pulse cleaning systems also require high strength, surface loading media. Advancements in formability and pleat-ability will aid filter solution manufacturers to better utilize media, optimize costs and reduce installation footprints.
Overall, there is a growing need for cost effective, composite media using a combination of multiple nonwoven media types and advanced coating technologies. To be cost effective these need to be available as a single media from multiple global locations.
