A new standard aired

Following the publication of BS EN ISO 1644-3: 2005: Test Methods, PD 6609 has been up-dated and will be published by the British Standards Institute (BSi) in the early part of 2007. As in previous editions, it will contain information that is supplementary to the most recent standards. In addition it will contain suggested templates for the specification of filters and the reporting of tests. Because the in situ leak test is not the same as the filter manufacturers’ test, it is recommended that the in situ test is included in the specification when filters are ordered.

The scope of PD 6609: 2007 is specific to filters that can be face scanned, i.e. cleanroom filters and filters in unidirectional airflow cabinets. Therefore, there is a need for an additional standard to cover the leak testing of filters in configurations that cannot be readily scanned, such as are found in separative devices (BS EN ISO 14644-7: 2004) and microbiological safety cabinets (BS 12469: 2000). A proposal for such a standard has been put to BSi.

Background

The first PD 6609 was published in 1996 as an explanatory supplement to BS 5295-1: 1989 and provided comprehensive supplementary information on filter testing. When BS EN ISO 14644-1: 1999: Classification of air cleanliness was published, BS 5295-1: 1989 was withdrawn. As a result, PD 6609: 2000 was rewritten to carry forward not only the comprehensive supplementary information on filter testing, but also the filter test itself from BS 5295-1 as this had been “lost” without being replaced in BS EN ISO 14644-1. There were additional sections on the measurement of pressure differentials and the measurement of particle contamination by sampling.

The publication of BS EN ISO 14644-3: 2005 Test methods, meant that all these areas were now covered by the ISO standard. However LBI/30, the BSi committee responsible for cleanroom standards in the UK was not happy with this new ISO standard and especially with section B.6: Installed filter system leakage test. As a result, a proposal was prepared and approved for a new issue of PD 6609.

Scope

It is worth dwelling on the Scope as this is a section that is often skipped over. The scope should specify what is covered by the standard and what is not. It is therefore a yardstick against which the content of the standard can be judged. Sometimes the scope of a standard can be over-ambitious and cover areas that are outside the expertise of the technical committee drafting it. One example of this is ISO 10648: Containment enclosures which is written by nuclear experts and yet claims to cover sterile applications! With PD 6609: 2007, the opposite is the case and the scope is both specific and narrow.

The document provides recommendations and explanatory guidance for in situ leak testing of high efficiency filters using an oil aerosol challenge and photometer. The guidance is only for filters that can be face scanned. Interestingly, the equivalent wording in BS EN ISO 14644-3 B.6 Installed filter system leakage test, states that the test is of the complete filter installation comprising the filter media, frame, gasket and grid system. This might have been more appropriate wording to include in the scope of PD 6609.

Test requirements

A number of guidance points are given under five headings. Rather than list all of these under each heading, a selection of the more practical points under each heading is given here: -

• Aerosol injection

– Oil aerosol injection pumps may be required in positive ducts.

– In negative ducts the photometer may be used in differential sampling mode, i.e. the photometer exhaust air is returned to the negative duct.

• Aerosol challenge

– The upstream concentration should be checked and recorded both at the start and upon completion of each filter scan.

• System scanning

– The sealing device, housing and filter are all subject to the test.

– Slow scanning may improve sensitivity for finding leaks and fast scanning may miss leaks.

• Airflow through filters

– If the flow rate through the installed filter is higher than when tested by the filter manufacturer, the penetration may increase to a point where it gives an in situ test failure.

• Repair of filters

– In a non-unidirectional cleanroom the air supply will quickly mix with room air and a less than perfect repair can be tolerated.

– In a unidirectional cleanroom, especially if the air is supplied directly to a critical area, the filter would normally be renewed.

– Where temporary repairs are made to allow vital work to continue, a replacement filter needs to be installed as soon as possible.

Annex A

Annex A explains the leak tests carried out by the filter manufacturer, the difference between these and the in situ leak test (as described in BS EN ISO 14644-3: B.6 and PD 6609) that is carried out on installed filters, and why this in situ test must be specified at the time the filters are ordered. Table 1, found in Annex A, is a template for a detailed specification (see table 1).

Annex B

Annex B. lists the information to be included in the in situ leak test report. The table given in Annex B is a detailed template for a full test report.

Other filter configurations

The scope of PD 6609: 2007 is restricted to the leak testing of high efficiency filters that can be faced scanned, and there is a further requirement for access to measure the upstream aerosol challenge concentration. There are a large number of clean air and containment devices built to BS EN ISO 14644-7: 2004 (separative devices or isolators) and BS EN 12469: 2000 (microbiological safety cabinets with ‘difficult’ filter configurations) that do not allow one or other of these requirements to be met. Such devices therefore fall outside the scope of PD 6609: 2007, and indeed the earlier editions. For this reason a proposal has been submitted to BSi for a new standard to cover these situations which include:

• Ducted exhaust filters

• Dual in line filters

• Filters where there is no upstream access for measuring the challenge

• Filters where the aerosol challenge cannot be applied with system running

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• Filters, such as cartridge filters and multi-V filters, where a downstream scan in accordance with PD 6609 is not possible

BSi has invited the author of this article to be project leader for this new standard if it goes ahead, therefore comments and suggestions from readers would be very welcome.