Why GT filter selection should be largely determined by air quality from the surrounding environment
Water is present in our environment in bulk form/droplets, like rain, sea spray, snow, mist and fog. It is also present in vapor form, the amount being measured as relative humidity. Bulk water and large water droplets have a direct impact on erosion of the compressor blades. Small droplets or vapor in the air can also be damaging as they promote or accelerate other factors that result in potential gas turbine damage and sub-optimal performance.
Moisture can also have a detrimental effect on the filters designed to protect a gas turbine, which can also result in lower performance levels. We’ll explore the different ways in which water affects these filters and what can be done to alleviate or even remove these effects. We’ll take a look at some of the basic facts and how we can go some way to alleviate the issues faced.
Water will pass through many gas turbine filters, which is why filter selection is so significant in this area. There are filters in the market that prevent water ingress, but they should be seen as specialist products. If you have ever experienced a leaking pipe in your home, you know, unchecked, water damage, even a small amount over a prolonged period can have devastating effects. Bulk water passing through filters will carry soluble pollutants with it, which in turn damage turbine components. Typically, dissolved salt is the major ‘transgressor’ causing corrosion of vital and expensive components.
Water bypass can also occur with poorly designed or constructed filters and/or filter housings taking the path of least resistance around poorly designed seals for example.
The effect of water
Historically cellulose fibers have been used in the manufacture of filter media. These were particularly prone to swelling in the presence of high humidity, fog or mist due to high moisture absorption. When the fibers swell, the voids between them close and subsequently you get the associated and unwelcome rise in differential pressure (dP).
In this type of gas turbine filter dP rising is much more common and results in a reduction in power output. It should also be noted that dP spiking can lead to unscheduled shutdowns if the alarm limit is reached. This results in very costly unplanned downtime and may result in filters prematurely reaching the end of life, which is another expense to consider.
Different filter types cope very differently in the presence of water, and this should be discussed with any potential supplier.
Solution to fiber swelling
For fine filters and especially pulse filters, synthetic fibers have been widely adopted to alleviate the effect of water on filters as synthetic fibres have hydrophobic characteristics.
Another response has been to add a hydrophobic coating to either a base cellulose media or to a blend of synthetic and cellulose media. Depending on how this coating is applied, its thickness and composition, the hydrophobic effect can protect the fibers to varying degrees.
Although the hydrophobic characteristic can help avoid the negative dP effect caused when fibers swell, it does not follow that a hydrophobic media makes for a good water barrier. This is a complex issue as some synthetic media filters offer excellent water barrier properties, while others simply allow water to pass through.
The effect of water on particulates
Water droplets coalesce on filter media in the presence of moisture from high humidity, fogs and mists. This can saturate captured particles on the surface of the media, they absorb the water, resulting in particle swelling. This increases the size of the particles which blocks the path of the airflow and consequently differential pressure is increased.
This can occur with any type of media as it is the particulate composition that is affected, not the filter media itself. These particles may then dry and form solid ‘cakes’ either on the surface of the media for surface loading filters, or within the filter media itself.
If soluble the pollutants may be washed through the media which is equally undesirable as we’ve seen above.
Alleviating the effects of water on captured particulates
A potential solution is to simply reduce the amount of water that comes into contact with the filter by installing a pre-filter with water coalescing properties. Suitable pre-filtration will also reduce the amount of particles on a single filter. Thus, extending the life of the system.
We’ve seen that bulk water and water vapor will affect filter performance and subsequently gas turbine performance, efficiency, and component life. Interaction is complex, but there are solutions to alleviate some of the detrimental effects. Understanding the environment in which the turbine is operating is a crucial first step to delivering suitable filter solutions. We have only touched on some of the factors at play here, so it is always advisable to discuss your specific environmental conditions, efficiency requirements, filter life expectations and fuel efficiency goals with your filter supplier.
The key ‘take away’ is that the effects of moisture on gas turbine filters is a complex subject that will vary considerably depending on the surrounding environment and the filtration system. Simply specifying a media type may not deliver the desired results to protect the turbine from the effects of water. Therefore, it is important to speak to a supplier who is an expert in filtration and who can demonstrate a competence in alleviating the effect of water on filters and gas turbines.
AAF has a wealth of experience in this area and we would be more than happy to demonstrate this, please contact us if you would like specific support related to your gas turbine installation.