Determining the leakage in HEPA filter according ISO 14644-3: Aerossol Photometer test versus Optical Particle Counter test. JOSE AUGUSTO S. SENATORE, PMP
1. Rationale This report aims to present the methodology of the test for the detection of leakage points in filter system, as presented in ISO 14644-3, using the methodology of scanning with OPC (optical particle counter). This procedure is described in the standard reference is used for situations where the test aerosol residue deposited on the filter or in ducts can not be tolerated or where the use of solid aerosol is recommended. Moreover, because of its greater sensitivity and accuracy, it is recommended for situations where filter efficiency is higher.
2. Premises, equipment and testing instruments In order to allow an evaluation under controlled conditions of operation, the test was performed in a vertical clean bench in top operating condition. In addition, we used the following measuring instruments: • Optical Particle Counter, 1 CFM, Lasair II; • Aerosol Generator, ATI brand; • Aerosol Photometer, ATI brand; • Flow meter type Vel-grid; • Thermo-Anemometer; • Micro-manometer; • Aerosol diluter;
3. Test Procedure The first step in the test was the installation of the e-PTFE filter in unidirectional flow device available to users. The dimension of the filter was 457x610x78mm (2 inches pack).
3.1 As found situation The clean bench was pre certified by the client with the original HEPA (new) class type H-13 (ISO 35H), efficiency of 99.95% MPPS, made of micro-fibers glass. Initial pressure loss was approximately 105 Pa at average face velocity of 0.45 m/s. In this condition, the frequency inverter presented the regulation of 48Hz.
3.2 Installing e-PTFE filter After installation of the e-PTFE filter HEPA, grade H-13 (ISO 40H), efficiency of 99.99% MPPS the air flow was adjusted for the average face velocity of 0.45 m/s. In this condition the filter presented the initial pressure loss of 40Pa and adjustment of frequency inverter presented 39 Hz.
3.3 Test for detection of leakage points in filter installed - scanning procedure for OPC (optical particle counter).
3.3.1) Determination of the upstream aerosol concentration Initially it was necessary to adjust the concentration of aerosol PAO upstream of the filter, considering that the air volume of the unidirectional flow device was much lower than the amount of aerosol generated by a single Laskin nozzle of the aerosol generator. In this regard, after repositioning and adjustment in the generator it was possible to obtain the amount of aerosol challenge suitable for the test according to calculations described in ISO 14644-3 section B. The actual upstream concentration of the HEPA filter was calculated, considering the dilution factor of the aerosol diluter, as provided in the standard. The following are the results concentration upstream of the filter: Number of particle read (with dilutor – particle / ft³) Period: 1 minute
Dilution factor
Real number of particle upstream (particle / ft³) Period: 1 minute
560 560 560 560
12.980.800 12.454.400 11.900.000 11.698.400
1a. 2a. 3a. 4a.
23.180 22.240 21.250 20.890
Average
21.890
12.258.400
Std. Deviation
1.032
578.022
Relative Deviation
5%
5%
Table 1
NOTE: The ISO 14644-3 does not determine the minimum aerosol concentration to filter challenge, however it states that this can be high enough to permit adequate sensitivity for detecting small leaks. The example provided in the standard considers appropriate the value of Cc (concentration of aerosol challenge) 30 particles / cm ³. In this case, the actual concentration read in 1 ft ³ / minute particles will be 849,600. On the other hand, the NEBB document “Procedural Standards for Certified Testing of Cleanrooms”, also based on ISO 14644-3, recommends that the upstream concentration exceeds 3,000,000 particles / ft ³, however suggests that the value exceeds 6,000,000 particles / ft ³ to increase the sensitivity of detection of minor leaks.
The table 1 shows that the concentration of particles obtained upstream of the filter during the test was higher than the values recommended by current standards and thus ensures proper sensitivity for the detection of leaks.
Cc Calculation: Particle counter flowrate: 1ft³/min = 472 cm³/s Period: 1 minute = 60 seconds Cc (particle/cm³) = 433
3.3.2) Calculation of speed of probe – Sr (according ISO 14644-3) Sr ≤ Cc x PL x qVs x (Dp/Np) Where: Sr = Scanning speed (cm/s). Cc = Challenge concentration (particle/cm²). PL = Allowable penetration. In this case used 0,01% qVs = Particle counter flowrate = 472 cm³/s (28,3 l/min). Dp = Dimension Probe – parallel to scanning direction (cm). In this case, 1.5 cm. Np = Number that designate a leakedge. Ca = Count acceptable (particle). For situations where false counts are not negligible, according to ISO 14644-3, Ca ≥ 1 and, as Poisson distribution (ISO 14644-3 Table B.2), for Ca = 1, Np = 5.6.
Sr = 433 x 0.0001 x 472 x (1.5/5.6) Sr = 5.47 cm/s
3.3.3) Calculation of time Ts (according ISO 14644-3) Ts ≥ (Dp/Sr) sec. Ts ≥ (1.5/5.47) Ts≥ 0,274 sec. If one or two counts increase over a period of time less than 0.274 seconds is recommended that a measurement is made using the stationary method to check for a possible leak. NOTE: Since the particle counter presents the results of reading every 1 second (at least) should be considered the maximum permissible particle to every second, following the same statement, that is, 1/0. 274 = particle allowable per second this case: = 4 particles.
3.3.4) Test results – OPC method After the calculations of scanning speed and maximum variation of particles per second read, the test was started. For operational characteristics were performed prints labels from particle counter every 5 seconds, and in this case, each label would be capable of registering maximum variation of 5 periods of 4 particles per period, a total of 20 particles. For the tests, was considered the target particle of 0.3 microns as it is the one that is closest to the filter’s MPPS (as recommended by ISO 14644-3). 49 measurements were made of 5 seconds each, covering entirely the filter media and seals, including overlaps between them. The maximum read, for particles of 0.3 microns was 4 particles (a period of 5 sec.), that means the system (filter, seals and gasket) was free of leaks.
3.4 Test for detection of leakage points in filter system installed - procedure using Aerosol Photometer After the tests with the OPC, it was conducted another test, this time using the procedure with the aerosol photometer, also described in ISO 14644-3.
3.4.1) Adjust the concentration of the aerosol upstream of the filter Due to the low flow of equipment unidirectional flow, even the aerosol generator operating with just a Laskin nozzle open, the average upstream concentration was about 35 µg / l. This concentration is within the range admitted in the ISO 14644-3 (between 10 and 100µg / l).
3.4.2) Test using aerosol photometer After adjusting the concentration and operating procedures in the equipment (photometer), the test was performed with a scan rate appropriate to the size of the isokinetic sampling probe (approx. 5 cm / s). The integrity of the filter media, seals and sealant, was checked and no significant leaks or penetration greater than 0.01% was found.
4. Conclusions The tests performed demonstrate that the use of both methods – Optical Particle Counter (OPC) and the aerosol photometer have similar results with respect to detecting leaks. However, it is important to note that the test using the particle counter permits to adjust the variables to make the procedure even more sensitive and accurate and, therefore, it is applied when there is a need to check for leaks in higher efficiency filters. On the other hand, the test using the OPC is only possible with the use of dilutor, because even low aerosol concentrations generate excessive amounts of particles capable to lead to read errors.
E-PTFE filters a) Energy consumption The installation of the e-PTFE filters in clean bench showed one of the major benefits associated with this technology: the low pressure loss. Below is the power consumption calculated according to the difference in the adjusted frequency inverter. Full load condition - rotation 1760 rpm - 60Hz - Power 367 W (0.5 hp) Condition as found (traditional HEPA filter - NEW) - 48Hz Using the fan’s law, we have: P (48Hz) = 367 x {[1760 * (48/60)] ³ / [1760] ³} = 150W
Condition as left (e-PTFE filter - NEW) - 39Hz Using the fan’s law, we have: P (39Hz) = 367 x {[1760 * (39/60)] ³ / [1760] ³} = 65W Summary: 56% reduction in energy consumption compared to traditional HEPA filter.
b) Life (dust holding capacity) The concept of dust holding capacity is not applied to HEPA filters because the target particles in your application have very small mass. However it has been discussed the use of alternative ultrafine dust, to aid in determining the useful life of the HEPA filters. Preliminary tests have shown that for target particles (ultrafine particles between 0.1 and 0.7 microns) the e-PTFE filters are have identical saturation curve of the glass fiber filters, which leads us to say that when preceded good filtering (F-8 or F-9) or inside equipments installed in the cleanrooms (ISO Class 7, for example), their saturation rate should be equal to that of traditional HEPA filters. Therefore, as the initial pressure loss is lower, it may be considered that the lifetime of the ePTFE filter to the same point of substitution is higher than that found in traditional HEPA filters.
c) Tests with Aerosol Challenge The filters in e-PTFE, by its own features, have peculiar behavior when subjected to large amounts of aerosol challenge for long period. For this reason and in order to maximize all its benefits to the system, it is recommended that testing for the determination of leak points (commonly known as integrity tests or scan) are performed with low aerosol concentrations. At first, it was worth saying that they should be tested only through the method with OPC described in ISO 14644-3 and performed as described in part 3.3 of this report. However, the technological advancement of this filter media already allows it to be subjected to the normal aerosol concentrations for a period of time compatible, without a major damage to his performance. The test performed on the 3.4 of this report confirmed this fact. For approximately 15 minutes, the test was conducted with PAO aerosol at a concentration of 35µg / l in normal scan speed with the photometer, and besides filter maintain its integrity was verified only the addition of 2 Pa (0.2 mmCA ) in it pressure loss from 40Pa prior to testing to 42Pa after all tests. So, it is possible to say that there are several alternatives to the tests for leak detection, compatible with current standards, regardless of the media in which the filter is made with fiber glass or e-PTFE.
5. References ASSOCIACAO BRASILEIRA DE NORMAS TECNICAS - ABNT. NBR ISO 14644 – Partes 1-3: Salas limpas e ambientes controlados associados. PEREIRA, MARCOS ANTONIO V., 2012. Ensaios para a detecção de vazamentos em sistemas de filtragem instalados utilizando contador de partículas discretas e baixa concentração de aerossol. Revista da Sociedade Brasileira de Controle de Contaminação, São Paulo, Edição 57, p. 40-49. MOORE JR.,D.R. , MARSHALL, J.G., KENNEDY, M.A. - Comparative Testing of Challenge Aerosols in HEPA filters with Controlled Defects – Pharmaceutical Engineering March - April, 1994. BRYAN, E., KITCH, B., MEEK, J., MILHOLLAND, D., NANCE, N., 2011. Alternative Test Methodology for In-Situ Testing of e PTFE HEPA filters for Pharmaceutical Applications Pharmaceutical Engineering November – December. NATIONAL ENVIRONMENTAL BALANCING BUREAU – NEBB – 2009 – Procedural Standards For Certified Testing Of Cleanrooms, third edition.
6. About the author José Augusto S. Senatore has mechanical engeneering degree from Mackenzie University, certified Project Management Professional – PMP from PMI since 2006 and MBA in USP University. Acts as filtration reseacher since 1999 and now is a member of SBCC (Brazilian Society of Contamination Control) and ABNT CEE-138, the group that represents Brazil in ISO TC-142 committee.