Filter standard efficiency tests, although valid references, are performed under dry conditions. Poor drainage capabilities will allow water to build up on the media, causing increases in pressure drop, which can force dissolved contaminants through the filter. As a filter’s dust load increases, the filter’s sensitivity to rain and humidity increases, and can force premature filter change-out. In presence of salt, these issues are an even bigger concern due to the risk of corrosion.
What are the important design considerations for water handling?
- Louvers & rain hoods will remove some water from the airflow before reaching the filters
- Coalescing pre-filters turn smaller droplets into larger droplets that can then fall out of the airflow before reaching the next stage filter. Good pre-filters drain large droplets at the front of the filter, keep a low pressure drop when wet, and coalesce small droplets into larger droplets at the back.
- Hydrophobic final filters are then needed to prevent any penetration of water and should have enough drainage to keep a low pressure drop.
Although there are industry tests for media hydrophobicity, such as the EN 20811 / ISO 811 standard, there is no industry standard for filter hydrophobicity. Many turbine manufacturers and suppliers have developed their own custom standards for filter hydrophobicity, but performance is always limited by site dust and conditions (see the table below).
Signs that an upgrade is required:
During an outage, if you see salt crystals or brown water carry-over after your final filter, your final filter hydrophobicity is probably insufficient for your system water handling requirements. This will cause fouling or corrosion, and the consequences are reduced power output and increased heat rate.
Check out Part 2 of this newsletter to discover how to specify a filter’s construction for water drainage capabilities.
Hydrophobicity and Water, Typical Tests
| Existing Test |
Tests |
Results |
Limitations |
| Weather Hoods |
Hoods |
|
|
|
Droplet Separator
|
Louvres |
- Determines the effectiveness for removal of simulated rain, as well as pressure loss.
- Louvre effectiveness as a function of core velocity of air flow.
|
- Constant wind velocity and rain intensity
- Water droplet size is controlled
|
|
Hydrophobicity Tests
|
Media |
- Determines the filter media's resistance to water penetration.
- Measure in inches of water. Should match the highest filter dP see in operation.
|
- Does not consider dust load or filter construction such as drainage
|
|
Coalescer test
|
Pre-filer |
- Measures amount of water drained before and after the filter compared to what was sprayed.
- The result is drainage capability.
|
- Imprecise for very small measurements
- Variation in dust will affect hydrophobicity
|
|
Humidity Test
Cycling humidity
|
Filter |
- Preloaded filters are cycled to 95% humidity to predict dP sensitivity in storm conditions.
|
|
Spray Test
High pressure salt test
|
Filter |
- Determines the filter's resistance to water and salt penetration as it is dust loaded.
- Measures downstream bypass visually and in grams of water and salt.
|
|
Deluge Test
Long duration water spray
|
Filter |
- Determines the filter's resistance to water penetration.
- No water should be found downstream as a result.
|
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