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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

Contents:

Page

Annex A Examples of test methods for filter configurations that

cannot be conventionally tested

6

Annex C Calculations of the effect of volumetric testing compared

with a scan test

13 Annex D Theoretical calculation of the size of hole that can be

detected by means of the aerosol leak test

14

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 14644-3:2005 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 14644-7:2004 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 14644-3:2005 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 14644-3:2005 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

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 14644-3:2005, 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

3.2.1 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 14644-3:2005 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

3.2.2 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: -

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 non-critical 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

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 14644-3:2005 and PD 6609:2007

Annex A: Examples of test methods for filter configurations that cannot be

conventionally tested

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

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

be defined by one coordinate

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

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

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

3 Filters where there is

no provision for

measuring the

upstream challenge

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

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 scan-tested in the normal way A penetration of one or two times the media penetration would be considered to be a leak

4 Filters where there is

no provision for

providing an upstream

challenge

This may apply in older or poorly designed

installations

An upstream challenge point should be installed by the test engineer

5 Filters where it not

possible to apply the

aerosol challenge with

the system running

In these situations, it is not possible to inject the aerosol challenge into the airflow Therefore, the upstream challenge cannot be measured

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

6 Filters, such as

cartridge filters where it

is not possible to carry

out a downstream scan

in accordance with BS

EN ISO 14644-3 - B

The construction of cartridge filters is such that a scan-test is simply not possible

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 14644-3 – B.6.4 Note: This test cannot detect individual local leaks

7 Filters, such as v-form

filters where the actual

downstream faces of

the filter elements are

inaccessible for

conventional scanning

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

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

8 Filters where there is

easy access for the

aerosol challenge on

the upstream side but

no access for

scan-testing on the

downstream side

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

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 non-quantitative indication of leaks

9 Filters in safe change

filter boxes

Systems that are designed to allow bagging

of contaminated filters for safe disposal are often prone to leaks at filter seals

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

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 open-faced 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

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

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 14644-3:2005 (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 of customer

contact:

Job title:

Name and address of testing organisation:

Name of test engineer:

Testing qualification:

Date of test::

Clear identification of

facility to be tested, i.e

name of department or

location on site:

ISO classification in

operational occupancy

state:

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:

State if by reference to sketch/plan:

State if sketch/plan is attached:

Details of the test method that has been agreed between the customer and the supplier:

Method Statement reference number:

State if Method Statement

is attached:

Specific

Standards/Guidelines on

which the Method

Statement is based (in

order of relevance):

Special conditions: Departures from the

Method Statement:

Details of DOP

Photometer:

Serial Number:

Calibration status (date of calibration certificate):

Calibration certificate attached (tick):

Details of probe:

Serial Number:

Details of aerosol

generator:

Serial Number:

Service and test status (date of certificate):

Certificate attached (tick):

Details of sparge pipe or

any other equipment

provided to facilitate

aerosol mixing:

Details of aerosol injection

pump:

Serial Number:

Service and test status (date of certificate):

Certificate attached (tick):

Details of any other test

equipment:

Service and test status if applicable (date of certificate):

Certificate attached if applicable (tick):

Rated air volume flow

through filter m3/s:

Rated air volume flow through filter m3/s:

Rated air volume flow through filter m3/s:

Actual air volume flow

through filter m3/s:

Actual air volume flow through filter m3/s:

Actual air volume flow through filter m3/s:

Upstream challenge

concentration at start of

test mg/m3:

Upstream challenge concentration at start of test mg/m3:

Upstream challenge concentration at start of test mg/m3:

Acceptance filter

penetration %:

Acceptance filter penetration %:

Acceptance filter penetration %: