ISO 29461-2 the first international standard for filter performance in fog and mist environments

ISO 29461-2 is the first international standard for reporting an air intake filter's endurance in fog and mist environments, as well as its resistance to water penetration.

Using the data provided by ISO 29461-2, turbomachinery operators have the information needed to select the filter that would perform the best in wet environments.

Why ISO 29461-2?

A turbine that is protected by an EPA filter lacking hydrophobicity features will still suffer from degradation due to fouling, erosion, and corrosion. When water is present on the filters, some contaminants captured in the filter will dissolve in the water (such as salts). Other contaminants may be pushed through by the water. If the final filter is not hydrophobic and lets this water through to the clean air side, these contaminants will make their way to the turbine. Measuring a filter‘s hydrophobicity is therefore critical for turbomachinery applications.

ISO 29461-2:2021 is the first official standard that evaluate a filter’s endurance in fog and mist environments, including both pressure drop and hydrophobicity. Previously, filter manufacturers applied in-house testing methods with different test parameters to measure hydrophobicity since a common standard did not exist; some measured only the filter media hydrophobicity according to the EN20811/ISO811 and AATC 127 standards; and some filter manufacturers did not test for hydrophobicity at all - making it difficult to compare filters. ISO 29461-2:2022 is the first universal standard that measures the hydrophobicity and pressure drop of the entire filter element, and can be applied to all turbomachinery air intake filters.

Poor filter drainage will allow water to build up on the filter‘s media. This can lead to spikes in filter pressure drop, which in turn, can cause the turbine to trip, in worst cases. High pressure drop can also increase the risk of contaminant migration through the filter media. ISO 29461-2 measures the endurance of the entire filter, against its resistance to pressure.

ISO 29461-2 Test method

Does the filter pass or fail the fog & mist endurance test, and is it hydrophobic?

ISO 29461-2 requires filters to be measured and rated according to the following test parameters:

The test procedure is 3 hours.

The filter must have a pressure drop less than 1000 Pa (4" w.g.).

There cannot be any measurable water downstream of the filter.

Optional test method if a filter is to be labeled "Hydrophobic": use a water dye to confirm that there is no water downstream of the filter.

A filter passes the fog & mist endurance test if it meets criteria 1-3. A filter is considered as ”hydrophobic” If meets all 4 criteria.

ISO 29461-1 & ISO 29461-2

When selecting filters, it is important to consider both the ISO 29461-1 efficiency standard and the ISO 29461-1:2 hydrophobicity standard. For most applications exposed to heavy water concentrations, Camfil recommends final filters that are high efficiency T10+ according to ISO 29461-1:2021 and hydrophobic with a low and stable pressure drop according to ISO 29461-2:2022.

The graph and table below shows the importance of considering both standards. All filters are equivalent in efficiency, however, Filters A, B, and C failed either by water bypass, pressure drop or both. Filter D passed the fog endurance test, however, the water dye test showed water droplets downstream of the filter, whereas Filter E showed no bypass. The results show that Filter E would perform the best in high humidity, fog conditions.

Graph: Fog & mist endurance test final filter comparison

Table: Fog & mist endurance and water dye test final filter comparison

Final Filter	Test Duration (min)	Water Sprayed (liters)	Water Bypass (liters)	Max. dP During Test (inch w.g. / pascal)	Pass or Fail	Optional dye test for water droplets: Is the filter hydrophobic?
Filter A T10	35	15	0	4.0/ 1000	Fail	-
Filter B T10	65	18	14	4.0 / 1000	Fail	-
Filter C T10	180	74	20	2.9 / 720	Fail	-
Filter D T10	180	76.5	0	2.6 / 645	Pass	No
Filter E T10	180	76.5	0	1.2 / 300	Pass	Yes