HEPA Filter Integrity Test (ISO 14644-3 Part B.6)
Absolute air filters (those having an efficiency of at least 99.97% or greater for particles 0.3 nm and larger) or HEPA filters are of paramount importance in obtaining viable and nonviable cleanliness levels in controlled macro- or microenvironments. These filters are rated for their efficiency and operational characteristics by the manufacturer before delivery, typically using the method recommended in Military Standard Mil-std- 282 “DOP Smoke Penetration and Air Resistance of Filters”, IES RP-CC-001-83 and European Standard EN 1822-1. Test results are reported in an attached label or certificate provided for every filter.
These filters are delicate, even though modern design makes them less susceptible to damage. Potential damage can occur during shipping and installation. Because of that, filters must be tested after installation to guarantee integrity. For this purpose, an air-generated or mechanically generated aerosol challenge to verify the integrity of the filters by using a light-scattering photometer has been devised and accepted by the industry as adequate to determine defects in HEPA filters or HEPA filter systems. In general, the condition under which this aerosol is generated provides a consistent concentration of 10 Hg/L of air, which is considered as an adequate challenge. This concentration is equivalent to approximately 3X1010 droplets per cubic meter (109 droplets per cubic foot) when the air is generated by a Laskin nozzle.
The use of a challenge aerosol is justified because, in general, the recirculation levels (number of times the same volume of air is passed through the filter) are so high that it is practically impossible to detect minor leaks using the room air as a challenge. Until recently DOP has been considered the primary aerosol choice. However, owing to its nature and origin, a possibility exists, even though it has not been proved, that DOP may represent some health risk. Substitutes have been developed that have the same particle distribution as DOP when a cold aerosol is generated. A synthetic oil, EMERY 3000, has been selected as an approved replacement.
The quantities of aerosol needed as a challenge for this test are minimal. A single nozzle can provide enough airflow to accommodate approximately 28 m3/min (1000 ft3/min). Normally, a compressed air source capable of delivering 75 L/min of air at standard conditions at 138 kPa (2.7 ft3 min at 20 lb/in2) is required for each nozzle. Thermally produced aerosols may have greater capacities.
Usually, no more than 100 g/L of air are used to obtain the full-scale response in the scanning photometer, which is the device typically used for this test.
The use of outside air as a challenge is possible, as long as particle concentrations beyond 10 million/ft3 of air are present. The process of challenging HEPA filters with outside air can be more difficult and lengthy owing to the variations in the challenge concentration and the need for a particle counter instead of a simple light- scattering photometer.
Occasionally, particle counters are used instead of light-scattering photometers. The procedure is implemented by using dissolution chambers and other devices that minimize the exposure of the delicate optical part of the particle counter to the challenge aerosols and to within the response capabilities of the instrument. When the intent is to use a particle counter as the detection device, it is advisable to consult the manufacturer of the unit before the implementation of the test method because improper settings may permanently damage these units. On the other hand, the scan probe needs to be designed such that it assures that the scanning velocity is adequate to obtain the proper response on the unit.
The HEPA filter integrity-testing procedure is clearly described under:
- Test objective: To provide evidence of the integrity of the HEPA filters and seals in situ.
- Acceptance criteria: An unacceptable leak is defined as a penetration of 0.03% or more of particles, 0.3 nm and larger than the reference calibration curve for 99.97% efficient filters, or as penetration of 0.01% or greater of particles 0.3 nm and larger than reference calibration curve for 99.99% efficient filters.
- Test procedure
- Introduce the challenge aerosol upstream from the filter to be tested at an adequate distance to assure proper air and aerosol mixing, a minimum of 10 to 15 cm (4-7 in.) from the filter face.
- Calibrate the aerosol photometer as given in the manufacturer’s procedures.
- Scan the filter face at an appropriate and approximate rate.
- Locate and repair the filter leaks in accordance with the approved procedure.
- Retest after repairs have been completed.
- Replace HEPA filters if necessary, and retest. 4. Reporting requirements
- Report results on the appropriate forms.
- Analyze the test data collected and write a conclusion for the acceptability of the test results based on the acceptance criteria specified.
refeernce by: Validation of Pharmaceutical Process by james and frederick