Guide to in situ leak testing of HEPA filter configurations that cannot be conventionally scan tested

Transcription

1 DOP Solutions Technical Document: Guide to in situ leak testing of HEPA filter configurations that cannot be conventionally scan tested Issue 1: Prepared by Neil Stephenson and Tim Triggs and edited by John Neiger 4 July 2008 DOP SOLUTIONS LTD Unit 10 Protea Way Pixmore Avenue Letchworth Hertfordshire SG6 1JT United Kingdom Telephone: +44 (0) (0) Facsimile: +44 (0) sales@dopsolutions.com Web: Contents: Page 0 Introduction 2 1 Scope 2 2 Principal references 3 3 Testing principles 3 4 Method statements 5 5 Reporting requirements 5 Annex A Examples of test methods for filter configurations that 6 cannot be conventionally tested Annex B In situ leak test report sheet 9 Annex C Calculations of the effect of volumetric testing compared 13 with a scan test Annex D Theoretical calculation of the size of hole that can be 14 detected by means of the aerosol leak test Bibliography 16 DOP Solutions Limited July 2008 Page 1 of 17 Registered Office Pendennis Church Path Little Wymondley SG4 7JE Registered in England No. GB VAT Number GB

2 0 Introduction Where HEPA filters are fitted, it is important to ensure that the filters with their housing and sealing devices do not permit the passage of particles from the upstream side to the downstream side. If possible, this is checked by challenging each filter with an aerosol of particles dispersed upstream of the filter and scanning over the downstream face to ensure that there are no leaks that exceed a specified level of penetration. PD 6609: 2007 provides information supplementary to the provisions of BS EN ISO :2005 for such a test. However the scope of PD 6609:2007 is limited to the leak testing of HEPA air filters that can be conventionally challenged with aerosol and face scanned. There are many configurations for HEPA filters, notably in separative devices as covered by BS EN ISO :2004 and microbiological safety cabinets as covered by BS EN 12469:2000, where it is difficult to apply a uniform upstream challenge, or to carry out a full downstream scan-test. The guidance contained in this Technical Document is for the leak testing of HEPA filters that are installed in configurations that cannot be leak tested in accordance with BS EN ISO :2005 and PD 6609: 2007, either because the filters are not readily accessible for face scanning, or because the challenge cannot be applied in the specified manner. This Technical Document is for use by test engineers, to assist them with their testing, and by design engineers, to help them ensure that there is a suitable test method for all HEPA filters incorporated into the equipment that they design. The object of the tests described in this Technical Document is to determine if filters have been damaged between manufacture and installation, and during subsequent use. In addition, where the in situ test method to be used after installation is included in the purchase specification of a HEPA filter (which is strongly recommended), this method should be used after installation to check that the HEPA filter has been correctly supplied. All test methods should be properly documented and validated, with clear pass/fail criteria. 1 Scope This Technical Document provides information supplementary to the provisions of BS EN ISO :2005 and PD 6609:2007. In particular, it gives recommendations and explanatory guidance for in situ leak testing, using an oil aerosol challenge and photometer, of HEPA filters that cannot be conventionally DOP Solutions Limited July 2008 Page 2 of 17

3 scanned or challenged, either because the filters are not readily accessible for face scanning, and/or because the challenge cannot be applied in the specified manner. 2 Principal references BS EN ISO :2005, Cleanrooms and associated controlled environments Part 3: Test methods PD 6609:2007, Environmental cleanliness in enclosed spaces Guide to in situ HEPA filter leak testing 3 Testing principles 3.1 Preparation and safety Unlike HEPA filters that have the sole purpose of supplying clean air into cleanrooms or controlled environments, HEPA filters covered by this Technical Document might be operating in environments that are subject to chemical, microbiological or radioactive contamination. Due consideration must therefore be given to safety issues and a risk assessment carried out prior to testing or modification (for testing) in relation to each particular HEPA filter installation to be tested. 3.2 Filter challenge methods Principles of the challenge aerosol The challenge aerosol presented to the upstream side of the filter should be stable, homogeneous and have a concentration of between 20 µg/l and 50 µg/l in accordance with BS EN ISO :2005 and PD 6609:2007. In order to ensure that this is achieved this there should be provided: - a. a defined injection point b. provision for homogeneous mixing c. an upstream sampling point Provision for homogeneous mixing For ducted systems, it is generally accepted that that in order to ensure homogeneous mixing of the challenge aerosol, the aerosol should be injected into an upstream duct at a distance that is at least 15 duct diameters from the upstream face of the filter. There are other means of ensuring a homogeneous challenge: - DOP Solutions Limited July 2008 Page 3 of 17

4 a. Bends, dampers, baffle plates and sound attenuators in a duct all aid mixing and reduce the distance required between the injection point and the upstream face of the filter. b. Static fan blades have been fitted inside ducts to promote mixing. c. Where it is necessary to inject the challenge very close to the filter face, this can be done using sparge pipes. Some of these methods may require the use of an aerosol injection pump. 3.3 Access for downstream measurement Access can be: - a. Direct, where the filter face is accessible for scanning with a standard probe b. Through a glove port to allow scanning with a standard probe inside an enclosure c. Through a hatch d. Through a single scanning port (see 3.3 Scanning probes) e. Through a series of scanning ports f. By means of a permanently installed grid of sampling points g. By means of a single sampling point (for volumetric measurements) 3.4 Scanning probes Scanning probes can be in the following form: - a. Standard scanning probe b. L-probe c. T-probe d. Permanently installed grid arrangement 3.5 Detection of individual leaks Quantitative methods for the detection and measurement of individual leaks are strongly preferred but these are not always possible. Non-quantitative methods should only be used if there is absolutely no alternative and then only for noncritical applications. 3.6 Volumetric methods It should be noted that volumetric leak test methods, where the downstream samples give the overall penetration rather than the local penetration, are greatly inferior to scan methods. (See Annex C). Again, quantitative methods, with defined pass/fail criteria, are greatly preferred to non-quantitative methods. With DOP Solutions Limited July 2008 Page 4 of 17

5 non-quantitative methods, any detectable penetration at all indicates a leak. 4 Method statements Every test should have a full method statement consisting of: - Outline description of method Detailed description of method stating in particular: - a. The method whereby a homogeneous upstream challenge is achieved and maintained b. The method whereby the concentration of the upstream challenge is measured c. The method whereby the downstream challenge is scanned and measured d. Sketch or drawing of all relevant parts of the facility showing the location of the upstream aerosol injection point(s), the location of the upstream aerosol sampling point(s) and the location and type of the downstream measuring points e. List of test equipment to be used including any special test equipment f. Operating condition of the facility during testing g. Reference to the standard or guideline applicable h. Pass/fail criteria i. Risk assessment leading to the clear specification of: i. conditions for the granting of a permit to work ii. decontamination measures to be taken prior to the test iii. personal protective equipment to be used iv. handling and safe disposal of contaminated waste including the filter v. any other relevant safety measures vi. Requirement to give the name or job title of the person responsible for ensuring the facility is safe to test j. Requirement to give the name or job title of the person who will carry out the test 5 Reporting requirements A proforma test report form is shown in Annex B and follows the guidance on information to be recorded in BS EN :2005 and PD 6609: DOP Solutions Limited July 2008 Page 5 of 17

6 Annex A: Examples of test methods for filter configurations that cannot be conventionally tested A.1 Introduction The aerosol photometer is capable of being used in many different ways. It should therefore be possible to design a suitable alternative HEPA filter leak test for most filter configurations. Examples are given in Table A.1 below. Note: Whatever test method is provided, it must be fully documented. This applies to the possible tests suggested in this Annex and to any other test that might be devised. The test engineer has a basic obligation, which is to carry out a repeatable test that is appropriate for the filter to be tested. Table A1: Examples of HEPA filter configurations with alternative test methods Configuration Issues Possible alternative tests 1 Ducted exhaust filters Whilst it might be possible to apply and calibrate the upstream aerosol challenge, there may be no easy access to the downstream face of the filter (with the ductwork connected) in order to carry out a downstream scan-test. A sufficient number of downstream access points should be provided to allow the whole area of the filter to be scanned. The probe tube may be an L-probe to facilitate access to every part of the filter face. Alternatively, a T-probe may be permanently installed downstream of the filter face. For testing, this is connected to the aerosol photometer through a sealed access port and drawn slowly across the face of the filter at a rate defined by the formula in PD 6609:2007 (3.3). The location of the leak will only 2 Dual in line filters, where there is insufficient access to scan the downstream side of the first filter or to apply an even aerosol challenge on the upstream side of the second filter The choice of test depends on whether verification of the combined overall leakage is sufficient or whether it is necessary to check for leaks in individual filters. be defined by one coordinate. Where verification of the combined overall leakage is sufficient, consideration should be given to setting tighter pass/fail criteria for the combined filter than would be appropriate for either of the individual filters. Where it is necessary to check DOP Solutions Limited July 2008 Page 6 of 17

7 3 Filters where there is no provision for measuring the upstream challenge 4 Filters where there is no provision for providing an upstream challenge 5 Filters where it not possible to apply the aerosol challenge with the system running 6 Filters, such as cartridge filters where it is not possible to carry out a downstream scan in accordance with BS EN ISO B This may be a problem that faces test engineers on older installations and equipment that are not or cannot be equipped with suitable test ports. This may apply in older or poorly designed installations. In these situations, it is not possible to inject the aerosol challenge into the airflow. Therefore, the upstream challenge cannot be measured. The construction of cartridge filters is such that a scan-test is simply not possible. for leaks in individual filters, it might be necessary to remove the secondary filter in order to test the primary filter and then to reinstall the secondary filter and use a sparge pipe to ensure even distribution of the upstream aerosol challenge for the secondary filter. It is possible to apply a constant upstream challenge and then to calibrate the photometer against the worst case media penetration on the downstream side. The filter is then scantested in the normal way. A penetration of one or two times the media penetration would be considered to be a leak. An upstream challenge point should be installed by the test engineer. One solution would to use the air pressure generated by the aerosol generator system, assisted if necessary by an oil aerosol injection pump, to flood the upstream space with the aerosol challenge and then scan in the normal way. This test is not quantitative and therefore any leak that is identified is taken as a fail. Care must be taken not to over-challenge and wet the filter with the challenge aerosol. Depending on the feasibility of a suitable challenge concentration (which might be difficult with smaller sizes of cartridge filter), an overall (volumetric) test can be carried out in accordance with BS EN ISO B.6.4. Note: This test cannot detect individual local leaks. DOP Solutions Limited July 2008 Page 7 of 17

8 7 Filters, such as v-form filters where the actual downstream faces of the filter elements are inaccessible for conventional scanning. 8 Filters where there is easy access for the aerosol challenge on the upstream side but no access for scantesting on the downstream side 9 Filters in safe change filter boxes The challenge concentration penetrating a leak in a filter element is diluted by the time it reaches the plane in which it can be scanned. Certain designs of separative device have a return air or exhaust filter where the upstream side is readily accessible from the work space but where the downstream side leads to inaccessible plenums and airways. Systems that are designed to allow bagging of contaminated filters for safe disposal are often prone to leaks at filter seals. 10 Open-faced filters High efficiency filters are used as room extract or return air filters. In this case the upstream side of the filter is usually openfaced to the room so special measures are required to provide a homogeneous upstream aerosol challenge. More often than not the downstream face is in an inaccessible duct so downstream scanning is also difficult. The face of the filter casing is scanned. This will detect leaks, but at a sensitivity that is less than if the leak were detected at the face of the element itself. One possible solution would be to introduce the aerosol challenge using a scanning pattern that covers the whole face of the upstream side of the filter. The photometer probe is then set at maximum sensitivity and placed at a suitable point as far from the filter as possible (without the addition of dilution air) in the downstream airway. Note: This test gives a nonquantitative indication of leaks The filter and seal should be tested in the normal way with special emphasis on the seal. Where access is difficult, manufacturer s instructions should be followed. A uniform challenge may be achieved by attaching a temporary box and duct to the open upstream face of the filter. The box includes a sampling port for measuring the upstream challenge concentration. The length of the duct is at least 15 x the duct diameter. The challenge is introduced in the normal way at the opening of the duct so that by the time it reaches the filter it is fully mixed. It is usually more practical to scan a cross section of the downstream duct than the downstream face of the filter itself. This can be done through a series of scanning ports in the duct or through a permanently installed grid of sampling ports. DOP Solutions Limited July 2008 Page 8 of 17

9 Annex B: In situ leak test report sheet B.1 Introduction Table B.1 sets out a proforma test report form which has been prepared using the guidance provided in BS EN ISO :2005 (sections 5 and B.6.7) and PD 6609:2007 (Annex B). Table B.1 Name and address of the site where the facility to be tested is located: Name and address of testing organisation: Date of test:: Name of customer contact: Job title: Clear identification of facility to be tested, i.e. name of department or location on site: Name of test engineer: Testing qualification: Description of facility to be tested and specific designations/locations of all filters to be tested, all aerosol injection points and all upstream and downstream sampling points: Details of the test method that has been agreed between the customer and the supplier: ISO classification in operational occupancy state: Specific Standards/Guidelines on which the Method Statement is based (in order of relevance): State if by reference to sketch/plan: State if sketch/plan is attached: Special conditions: Method Statement reference number: State if Method Statement is attached: Departures from the Method Statement: DOP Solutions Limited July 2008 Page 9 of 17

10 Details of DOP Photometer: Calibration status (date of calibration certificate): Calibration certificate attached (tick): Serial Number: Details of probe: Serial Number: Details of aerosol generator: Service and test status (date of certificate): Certificate attached (tick): Serial Number: Details of sparge pipe or any other equipment provided to facilitate aerosol mixing: Details of aerosol injection pump: Serial Number: Details of any other test equipment: Service and test status (date of certificate): Service and test status if applicable (date of certificate): Certificate attached (tick): Certificate attached if applicable (tick): Filter 1: Filter 2: Filter 3: Rated air volume flow Actual air volume flow concentration at start of test mg/m 3 : Acceptance filter Rated air volume flow Actual air volume flow concentration at start of test mg/m 3 : Acceptance filter Rated air volume flow Actual air volume flow concentration at start of test mg/m 3 : Acceptance filter DOP Solutions Limited July 2008 Page 10 of 17

11 Maximum measured filter Acceptance frame and seal Maximum measured frame and seal concentration at end of test mg/m 3 : Maximum measured filter Acceptance frame and seal Maximum measured frame and seal concentration at end of test mg/m 3 : Maximum measured filter Acceptance frame and seal Maximum measured frame and seal concentration at end of test mg/m 3 : Filter pass/fail: Filter pass/fail: Filter pass/fail: Frame and seal pass/fail: Frame and seal pass/fail: Frame and seal pass/fail: Corrective action (if fail): Corrective action (if fail): Corrective action (if fail): Filter 4: Filter 5: Filter 6: Rated air volume flow Actual air volume flow concentration at start of test mg/m 3 : Acceptance filter Maximum measured filter Acceptance frame and seal Maximum measured frame and seal Rated air volume flow Actual air volume flow concentration at start of test mg/m 3 : Acceptance filter Maximum measured filter Acceptance frame and seal Maximum measured frame and seal Rated air volume flow Actual air volume flow concentration at start of test mg/m 3 : Acceptance filter Maximum measured filter Acceptance frame and seal Maximum measured frame and seal DOP Solutions Limited July 2008 Page 11 of 17

12 concentration at end of test mg/m 3 : concentration at end of test mg/m 3 : concentration at end of test mg/m 3 : Filter pass/fail: Filter pass/fail: Filter pass/fail: Frame and seal pass/fail: Frame and seal pass/fail: Frame and seal pass/fail: Corrective action (if fail): Corrective action (if fail): Corrective action (if fail): DOP Solutions Limited July 2008 Page 12 of 17

13 Annex C: Calculations of the effect of volumetric testing compared with a scan test This annex will be added at a later date. DOP Solutions Limited July 2008 Page 13 of 17

14 Annex D: Theoretical calculation of the size of hole that can be detected by means of the aerosol leak test It is possible to estimate, very approximately, the size of a leakage hole that the aerosol leak test can detect by using the formula for the velocity of a fluid through an orifice in a thin plate. This is particularly useful for the purpose of comparing the sensitivity of the aerosol test with other test methods such as the pressure decay test which is a standard test method for the pressure integrity of isolators (separative devices). Whilst the methods described in this Technical Document (and in PD 6609: 2007) apply to HEPA filters together with their housings and sealing devices, they can also be applied to separative devices. Where such separative devices rely on both HEPA filters and physical barriers to contain particulate contamination or to prevent the entry of particulate contamination, it is entirely relevant to be able to relate the extent of leakage through the filters with the extent of leakage through seals etc. in the physical barrier. The formula is: U = (2 P/ρ) Where: U is the velocity of the fluid through the orifice in m s -1 P is the differential pressure across the orifice in Pa ρ is the density of the fluid in kg m -3 (which for dry air is kg m -3 at atmospheric pressure (101.3 kpa) and 20 o C) The principal assumption is that the leakage hole has the characteristics of an orifice in a thin plate. In the case of a HEPA filter, this assumption is not unreasonable as a hole that is detected by the aerosol test is likely to have one particular place where the cross-section of the leakage hole is at its most constricted. The formula would apply at this point. For the purpose of the calculations that follow, it is assumed that P is 100 Pa. Therefore, using the above formula, U = m s -1. The calculations can be repeated for pressure differentials greater or smaller than this value. In the aerosol leak test, an upstream challenge is applied to the filter to be tested. A probe connected to an aerosol photometer is used measure the aerosol concentration of the upstream challenge and then to scan the downstream face of the filter for leaks. When a leak is detected, what is measured is the upstream challenge concentration at the volume flow rate of the leak diluted by the much larger sampling volume flow rate. DOP Solutions Limited July 2008 Page 14 of 17

15 If: V o is the volume flow rate through the leakage hole in m 3 s -1 V s is the sampling volume flow rate in m 3 s -1 which is typically 1 cfm (cubic foot per minute) = 4.72 x 10-4 m 3 s -1 C u is the upstream aerosol challenge concentration C d is the downstream sample concentration measured by the photometer Then: V o x C u = V s x C d Therefore V o = V s x C d /C u When the DOP photometer shows a leak penetration of 0.01%, C d /C u = 1 x 10-4 When the DOP photometer shows a leak penetration of 0.001%, C d /C u = 1 x 10-5 If: A is the effective cross-sectional area of the leakage hole in m 2 d is the effective diameter of the leakage hole in µm Then V o = U x A m 3 s -1 So A a = V o /U = V s x (C d /C u )/U m 2 = V s x (C d /C u )/U x µm 2 And d = (4A/π) µm Using these formulae and values, it is possible to calculate the hole area and hole diameter at various penetrations. These are shown in Table D.1. Table D.1 Penetration % C d /C u A a = effective area of hole d = effective diameter of hole x µm µm x µm µm By comparison, the single hole equivalent has been calculated for various classes of separative device (isolator). These are shown in Table D.2. Table D.2 Class of isolator b Hourly leak rate (h -1) Effective diameter of SHE (µm) c x x x a It should be noted that the effective cross sectional area of the hole (as calculated by the formula) is slightly smaller that the actual area. b As defined in BS EN ISO : c The values calculated are for an isolator with a volume of 1m 3. DOP Solutions Limited July 2008 Page 15 of 17

16 Bibliography Standards publications BS :1989, Environmental cleanliness in enclosed spaces Part 1: Specification for clean rooms and clean air devices (withdrawn) BS :1989, Environmental cleanliness in enclosed spaces Part 2: Method for specifying the design, construction and commissioning of clean rooms and clean air devices (withdrawn) BS :1992, Microbiological safety cabinets Part 1: Specification for design, construction and performance prior to installation (withdrawn) BS EN 12469: 2000 Biotechnology. Performance criteria for microbiological safety cabinets BS EN ISO :1999 Cleanrooms and associated controlled environments Part 1: Classification of air cleanliness BS EN ISO :2000 Cleanrooms and associated controlled environments Part 4: Specifications for testing and monitoring to prove continued compliance with ISO BS EN ISO :2001 Cleanrooms and associated controlled environments Part 4: Design, construction and start-up BS EN ISO :2004 Cleanrooms and associated controlled environments Part 5: Operations BS EN ISO :2007 Cleanrooms and associated controlled environments Part 6: Vocabulary BS EN ISO :2004 Cleanrooms and associated controlled environments Part 7: Separative devices (clean air hoods, glove boxes, isolators and minienvironments) BS EN ISO :2007 Cleanrooms and associated controlled environments Part 8: Classification of airborne molecular contamination BS 3928:1969, Method for sodium flame test for air filters (other than for air supply to I.C. engines and compressors) (confirmed 2003) DOP Solutions Limited July 2008 Page 16 of 17

17 BS EN :1998 High Efficiency air filters (HEPA and ULPA) Part 1: Classification, performance, testing, marking BS EN :1998 High Efficiency air filters (HEPA and ULPA) Part 2: Aerosol production, measuring equipment, particle counting statistics BS EN :1998 High Efficiency air filters (HEPA and ULPA) Part 3: Testing flat sheet filter media BS EN :2000 High Efficiency air filters (HEPA and ULPA) Part 4: Determining leakage of filter elements (Scan method) BS EN :2000 High Efficiency air filters (HEPA and ULPA) Part 5: Determining the efficiency of the filter element Recommended practices of the Contamination Control Division of the Institute of Environmental Sciences and Technology, Illinois, USA. IEST-RP-CC HEPA and ULPA Filters IEST-RP-CC Testing cleanrooms IEST-RP-CC Testing ULPA Filters IEST-RP-CC HEPA and ULPA Filter Leak Tests DOP Solutions Limited July 2008 Page 17 of 17