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HEPA Filter Leakage Rate Test: Is it necessary for the leakage rate of HEPA filter to be less than or equal to 0.01%?

2022-04-24

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Marya

High efficiency filter is mainly used to capture more than 0.5 um of granular dust and various suspended matter, as the end filtration of various filtration systems. Ultra-fine glass fiber paper is used as filter material, rubber plate paper, aluminum foil plate and other materials are folded as cutting plate, new polyurethane sealant is sealed, and the galvanized plate, stainless steel plate and aluminum alloy profile are used as outer frame.

1. What is the Acceptance Criteria of HEPA？

The approach most test standards take regarding acceptance criteria is that the acceptable leak size limits are ultimately determined by the customer and supplier. However, a scan test leak size limit of greater than or equal to 0.01% has generally been adopted for many applications utilizing HEPA filters or clean areas of different levels. Although the 0.01% leak size has been used historically and has its origins linked to early generation analog photometer test equipment, establishing a leak size limit of 0.01% as an acceptance criteria without performing a science and risk-based assessment can result in issues relating to leak testing and can contribute to significant operational costs if an out of tolerance or failed condition is identifed in a low risk areas. As previously noted in Section 6.5, filters are not 100% efficient and are expected to have some natural or integral penetration of particles near the MPPS. Test acceptance limits become more controversial or problematic when utilizing lower-rated HEPA filters where the acceptable factory penetration at or near MPPS can be comparable to or larger than the field test leak size acceptance criteria. This is especially true where the bleed through effect can occur (see Section 6.9).When purchasing a filter, it is therefore important to consider a filter rating as well as how it will be tested after installation order to avoid unnecessary field test failures.

ISO 14644-3 [33] gives guidance on how alternative leak acceptance criteria can be implemented. In a risk-based approach, it may be ideal to have acceptance criteria that trends with the efficiency of the filters being used or the cleanliness of the room being tested. ISO 14644-3 uses the factory filter efficiency rating as the basis of acceptance criteria negotiation. The leak acceptance criteria for a photometer leak test and a particle counter based leak test should be the same, as the theory and methodology behind leak sizing is identical for both methods. If performed properly, a leak test with a photometer and a particle counter will result in the same leak size (Meek, etc., 2011[121]).

 

 

A leak detected in excess of 0,01 % of the upstream mass concentration is deemed to exceed the maximum allowable penetration. However,for filter systems of an integral efficiency at MPPS ≥ 99,95 %and less than99,995 %, the acceptance criterion is 0,1 %.
If filter systems of an integral efficiency lower than 99,95 %at MPPS are to be tested, a different acceptance criterion are necessary, based on agreement between customer and supplier.

2. The Filters for Depyrogenation Tunnel

The filters fitted to a depyrogenation tunnel are subject to prolonged periods of high temperature operation, so standard filters are not suitable. Special filters rated to 350°C are available with a manufacturer guaranteed efficiency of 99.99% for 0.3 µm particles (note that the FDA defines of a HEPA is > 99.97% at 0.3 µm) at a temperature of 350°C) with a ceramic material used for the media to frame seal. Recently there have been advances in the materials available and flexible sealants are being introduced that can reduce the heat up time and reduce the risk of sealant cracking. Traditional filters need to have a controlled heat up and cool down time (typically not to exceed 1°C per minute; the rate should be confirmed with the filter supplier) to prevent heat stress damage to the seals. Often the systems are maintained hot during periods of non-use to reduce the heat cycling on the filter.

Filters used for depyrogenation tunnel are generally H14. These filters can be full face scan leak tested on installation, but after the initial heating cycle (burning in the filter, which usually results in the filter grade becoming equivalent to H13) traditional testing is not recommended. The oil aerosol (if it is Poly alpha olefin (PAO)) will load onto the filter and burn off,giving offunhealthy fumes, and may load the filter media; this is usually more fragile after burn in as the binder holding the media together has off-gassed.DEHS (Diethylhexyl Sebacate) is an alternative oil that may evaporate faster.