12-Oct-2006

Filters under test

Abstract

Jason Kelly, a director of OptiCal Sciences Ireland, outlines the current leak testing methods for HEPA filters and the relevant standards to follow

High Efficiency Particulate Air (HEPA) filters are used to provide clean air for cleanrooms throughout many diverse industry sectors semiconductor, pharmaceutical, medical device, nuclear and biotechnology to name a few. Capable of removing 99.97% of particulates 0.3_m in diameter, the HEPA filter is constructed with many pleated layers of filter media paper. This design prevents particles from freely passing through the filter instead they become trapped and stick to the filter fibres.

In such a design, there are three mechanisms at work:

  • Straining/Sieving where a particle is too large and becomes trapped between two filter fibres
  • Impaction where a particle of relatively greater mass is unable to follow the curved streamline around the fibre and, as a result of momentum, travels in a straight line into the filter fibre and sticks.
  • Interception which occurs when a section of a particle "runs into" a filter fibre.
  • Diffusion where capture occurs when particles leave the streamline due to random collisions with the surrounding fluid molecules and strike the fibres, where they again stick.
Why test these filters if they work so efficiently? Regulated Industries such as Pharmaceutical, Medical Device and Nuclear require cleanroom certification so filter testing is a necessity. There are many standards that outline the frequency of testing and guidelines for the actual tests to be carried out. The minimum requirements for cleanroom certification are :

  • Filter Leak/Integrity Test
  • Room Pressurisation
  • Airflow Areas for exchange rates
  • Airbourne Particle Count Test
  • Airflow Pattern Visualisation Test (Optional)
Cleanroom Certifier/Validation companies carry out this testing to the required standards. When choosing such a company, the training records and experience of the engineers and type of test equipment used to verify they are up to date with the standards and equipment used are important factors to consider.

Recommended standards to consult include: ISO144-1 ISO 14644-2 ISO14644-3, IEST-RP-C001 IEST-RP-C007 IEST-RP-C034 and EN1822 (Using Particle Counter and Polystyrene Latex Spheres PSL)

This article will concentrate on the procedue and requirements for filter leak/integrity testing. Once the HEPA filters are installed in the cleanroom they require validation. The leak test will pick up any leaks that may be present due to the filter being incorrectly installed, damaged during transit or defects in the filter media. Routine testing can pick up other related problems which would otherwise go unnoticed such as filter seal leaks. (there has been documented instances of the HEPA filter seal failure due to siloxane gel degradation).

Leak testing

ISO 14644-2 outlines the frequency of cleanroom validation according to cleanroom classification. Part of this validation includes leak testing of the HEPA filter, while ISO 14644-3 outlines the testing procedure to follow.

The equipment required to carry out leak testing includes an aerosol generator, photometer and particle counter (if following EN1822) (see Figures 1-4) .

The aerosol generator is used to produce an aerosol upstream of the HEPA filter. It also produces a stable test aerosol providing particles with the following distributions:

  • More than 20% by mass of particles less than 0.5m
  • More than 50% by mass of particles less than 0.7m
  • More than 75% by mass of particles less than 1.0m
An annual output test should be carried out to verify that the aerosol meets this distribution criteria.

The photometer is used to measure the upstream aerosol concentration as well as the downstream penetration of the HEPA filter by the aerosol. It should also be calibrated regularly, at least annually. (Some companies recalibrate every 6 months depending on how often the equipment is used).

Finally the particle counter is used to measure the concentration of particles of different sizes downstream of the HEPA filter. This too should be calibrated at least annually.

Polystyrene Latex Spheres (PSL) are used instead of the usual PAO (Polyalphaolefin - Emery Oil ) or DOP (Dioctylphthalate- Shell Ondina Oil ) aerosols associated with photometers. This type of filter testing with PSL is not common in the pharmaceutical Industry but looks like it could be gaining some suppor, as noticed in the recent CETA conference last April. However, it has been noticed that the FDA do not yet approve of this method in the pharmaceutical Industry and so it may not take off.

Output test

The aerosol generator output test carried out by the supplier annually verifies that the aerosol generator is capable of producing a stable distribution (see figure 5).

The photometer, also calibrated annually, verifies that the sensor/optics and flow rate are within tolerances. Some new digital photometers now on the market have the ability to verify on power-up the condition of the sensor/optics and flow rate as part of a self diagnostics operation. This improves the reliability of the unit because instead of waiting once a year for a calibration test and hoping it passes, photometer operators can be satisfied that the photometer is free from contamination around the sensor/optics and the flow path is unobstructed. This means that the unit is performing correctly and the operator can be confident that the results are accurate.

Test method

A linear photometer may be used to measure up-stream concentrations provided it is calibrated. This upstream concentration (X) may be used as the 100% reference, enabling percentage penetration down-stream to be measured directly.

The linear photometer may be used to test filters Y to either 0.01 or 0.001%. Each filter requires an up-stream challenge of 20 100 g/l for the best results.

The test requires the following steps:

-

  • Find the area of the filter
  • Find the airflow through the filter
  • Calculate throughput of the filter
  • The product throughput and concentration required then gives the aerosol output required
  • Alternatively adjust aerosol to reference photometer at 100%.
Test procedure

Disperse the test aerosol upstream of HEPA filter to produce a uniform challenge concentration in the region of 20-100g/l. Then, using the photometer measure the upstream challenge concentration. Maintain this concentration throughout the test.

Adjust the aerosol generator such that the challenge concentration at the upstream filter face is at a level such that the photometer can be set and maintained at a stable reading of 100%.The photometer is then set at 100%.

Using the same photometer, scan all of the down-stream face and perimeter inclusive of the sealing device with the sampling probe. The probe should be held approximately 25mm away from the area being tested and passed over the entire area in slightly over lapping strokes at a rate of 5cm/sec. Record the location of any steady repeatable reading of the photometer that exceeds the value given in table 1 for the relevant class of environmental cleanliness (Refer to ISO 14644-3 section B.6.2.5 and IEST-RP-C007.1,IEST-RP-C034.2 for more detailed procedures).

Additional guidelines

Care should be taken when generating the up-stream aerosol, as too much aerosol will over concentrate the filter and filter replacement will be necessary; if too little aerosol is generated there may not be enough aerosol to effectively scan the filter sufficiently. The testers should discuss and reach an agreement with the customer regarding the upstream concentration.

Once a leak has been detected, repair of the filter will be necessary. It is advisable to follow manufacturers procedures regarding the type of repair media and guidelines on resealing the filter, and after-repair tests.

After successful testing a particle counter positioned under the filter can evaluate the condition of the filter and can be used as a back up to the photometer.

Footnotes:

For further information on testing procedures or testing equipment required go to www.optical-sciences.ie.


Illustrations provided by OptiCal Sciences Ltd, ATI International, CliMet Instruments and Biological Controls Inc.

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