Sea salt concentration in air depends on elevation, wave height, wind speed, temperature, relative humidity (RH) and distance from the shore:
- In high RH environment the plants up to 20 km´s from the shoreline are most affected by the droplet form of salt.
- In low RH environments the plants up to 50 km´s from the shoreline are most affected by the smaller sizes dry salt crystals.
Why is salt difficult to filter from the air?
As mist droplets in the air closer to shoreline are small in comparison to raindrops, they are penetrating through non-hydrophobic filters.
Once mist droplets have penetrated the filters’ upstream surface and have adhered to the fibers, the air speed between the fibers rises causing an increase in pressure drop (dp).The pressure drop/air velocity increases as more water is absorbed by the filters.
Finally, the increase in air velocity between the fibers become too much for the mist droplets to resist and they are dragged through the filter material, transferring water, dirt and salt to the clean side plenum. Water is more likely to penetrate the upstream surface when filters have low efficiency and/or used for too long reducing the water repellent properties, especially for membranes.Once RH reduces the droplet dries leaving the salt crystal on or inside the filtration material.Salt crystals are even smaller than mist droplets and are carried with the wind to even further from the shoreline and are even more difficult to catch without E-class filtration.
Majority of the salt crystals travelling in the air are below 3,5μm and it is recommended that the filter should be efficient in the sub-micron area, which means at least E11-class filtration to ensure high initial efficiency (>95%) @ 0,3μm particles.Salt crystals captured with the filtration media are causing problems to filter media when the RH rises due to hygroscopic behaviour of the salt crystals.
Salt crystals transforms into supersaturated droplets when relative humidity (RH) rises above 75% and remain in this form until RH decreases below 45%. When a salt crystal becomes a droplet, its volume increases by almost 100%. As RH increases the droplet grows further to several times larger than the original crystal causing weak materials and flexible ePTFE and polymer membranes to deform under pressure.
When RH reduces the crystal shrunk back to original size leaving the filter material weakened with holes. Punctured filter may have lower dp making damage hard to detect while allowing more impurities to pass reducing the filtration efficiency.The hygroscopic phenomena of crystal growth and transformation to super saturated droplet dramatically increases dp of the final filter as the droplets block the free flow surface of the filter media. dp spike can be high especially when the filter is already partly blocked by dirt or has a high salt content.
The reduced free filtration surface increases air velocity through the material causing the largest salt droplets to break - and if the filter material isn’t of sufficient quality, the flow will either drag fragments through the filter media or will burst the filter due to high differential pressure.Most problematic feature of the Salt crystal is that as long as it is captured within the filter media it will expand every time RH increases and dry back to crystal once the RH decreases – or until it makes its way through the filter material to the compressor.
How to Prevent Salt induced problems
It is possible to prevent salt contamination by means of Inlet air filtration by taking into account all different salt transfer phenomena when selecting the correct Inlet air filtration system.
Main features of filtration when protecting GT from Salt:
- Prevent small droplets from entering the compressor - requires hydrophobic filtration media in all stages
- Prevent tiny particles penetrating into the compressor – requires min E11 filtration grade as your final stage
- Withstand high dp and material stress of swelling salt crystal even with wet material – requires a high tensile strength material
- Staging of filtration – the more cascading filtration stages you have in your filtration system the better chances you have preventing the salt problems from occurring – as long as all your stages are working as a defence line.
Eagle Filters´ unique 3-stage hydrophobic high tensile strength filtration solutions ensure you the best protection against Salt induced fouling and Corrosions.
To cope with Salty environments the Eagle Filters filtration system is designed to:
- Pre and Fine filters are water repellent removing in cascade most of the moisture and salt in order to protect the final water repellent EPA filters from excessive water and salt burden.
- Fine filter is designed to remove majority of the Salt loading hence protecting the EPA stage
- EPA Filter is designed to prevent the remaining salt loading from entering the compressor
- The depth loading EF Filter materials have better resistance against salt crystal swelling ruptures.
- Filter materials are exceptionally strong tolerating increased pressure drops without bursting and coping with salt crystal swelling