ISO/PRF 29461-1

INTERNATIONAL ISO
STANDARD 29461-1
Second edition
Air intake filter systems for rotary
machinery — Test methods —
Part 1:
Static filter elements
Systèmes de filtration d'air d'admission pour machines tournantes —
Méthodes d'essai —
Partie 1: Éléments filtrants pour filtres statiques
PROOF/ÉPREUVE
Reference number
ISO 29461-1:2021(E)
ISO 2021
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ISO 29461-1:2021(E)
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© ISO 2021

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Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Symbols and abbreviated terms ........................................................................................................................................................... 2

5 Testing and classification of filter efficiencies ...................................................................................................................... 2

6 Determination of the air flow resistance versus the mass of test dust captured ..............................4

7 Conditioning method to determine the minimum fractional test efficiency .........................................5

8 Reporting ...................................................................................................................................................................................................................... 5

Annex A (normative) Net area calculation ..................................................................................................................................................... 7

Bibliography .............................................................................................................................................................................................................................15

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This document was prepared by Technical Committee ISO/TC 142, Cleaning equipment for air and other

gases, in collaboration with the European Committee for Standardization (CEN) Technical Committee

CEN/TC 195, Cleaning equipment for air and other gases, in accordance with the Agreement on technical

This second edition cancels and replaces the first edition (ISO 29461-1:2013), which has been technically

— a new test method, referring to ISO 16890 (all parts) and ISO 29463 (all parts), has been added;

— previous Annexes A, B, C and D have been deleted; previous Annex E has become Annex A.

Any feedback or questions on this document should be directed to the user’s national standards body. A

In rotating machinery applications, the filtering systems, typically a set of filter elements arranged in

a suitable manner, are an important part of the whole turbine/compressor system. The development

of turbine machinery used for energy production or others has led to more sophisticated equipment;

and therefore the importance of good protection of these systems has become more important in the

recent years. It is known that particulate contamination can deteriorate a turbine power system quite

This event is often described as “erosion”, “fouling” and “hot corrosion” where salt and other corrosive

particles are known as potential problems. Other particulate matters can also cause significant

reduction of efficiency of the systems. It is important to understand that air filter devices in such systems

are located in various environmental conditions. The range of climate and particulate contamination is

very wide, ranging from deserts to humid rain forests to arctic environments. The requirements on

these filter systems are obviously different depending on where they will be operating.

ISO 29461 (all parts) has based the performance of the air intake filter systems not only upon heavy

dust collection but also particulate efficiency in a size range that is considered to be the problematic

area for these applications. Both ultra-fine and fine particles, as well as larger particles, should be

considered when evaluating turbine fouling. In typical outdoor air, ultra-fine and fine particles in the

size range from 0,01 μm to 1 μm contribute to > 99 % of the number concentration and to > 90 % of the

surface contamination. The majority of the mass normally comes from larger particles (>1,0 μm).

Turbo-machinery filters comprise a wide range of products from filters for very coarse particles to

filters for very fine, sub-micron particles. The range of products varies from depth to surface loading

systems, which can be regenerated e.g. by pulse cleaning. The filters and the systems have to withstand

a wide temperature and humidity range, very low to very high dust concentration and mechanical

stress. The shape of products existing today can be of many different types and have different functions

such as droplet separators, coalescing products, filter pads, metal filters, inertial filters, filter cells,

bag filters, panel-type, cleanable and depth loading filter cartridges and pleated media surface filter

ISO 29461 (all parts) provides a way to compare these products in a similar way and define what

criteria are important for air filter intake systems for rotary machinery performance protection.

The aim is to compare different filters and filter types with respect to the operating conditions they

finally will be used in. For instance, if a filter or a filter system is meant to operate in an extreme, very

dusty environment, the real particulate efficiency of such a filter cannot be predicted because the dust

loading of the filter plays an important role. A further part of ISO 29461 will address the performance

of cleanable and surface loading filters. Filters in turbo-machinery applications can also face very harsh

operating conditions such as high air flow rates or water and salt ingress. Further parts of ISO 29461

This document specifies methods and procedures for determining the static performance of particulate

air filters used in air intake filter systems for rotary machinery such as stationary gas turbines,

compressors and other stationary internal combustion engines. It applies to air filters with an efficiency

of 85 % or more for the MPPS (EPA and HEPA filters) which are tested according to ISO 29463 (all parts)

and filters with a lower efficiency which are tested according to ISO 16890 (all parts). The procedures

described in both ISO 16890 (all parts) and ISO 29463 (all parts) are applied and extended by this

document to air filters which operate at flow rates within the range 0,24 m /s (850 m /h) up to

Static filter systems normally use multiple stages of coarse, fine and optional EPA or HEPA filter elements

to protect the machinery. The scope of this document includes methods for performance testing of

individual filter elements. It does not include methods for the direct measurement of the performance

of entire systems as installed in service except in cases where they can meet the qualification criteria

for the test assembly. Nevertheless, cumulative filter efficiencies of multistage systems of fine filters

This document refers to static (barrier) filter systems but can also be applied to other filter types and

systems in appropriate circumstances, for example to evaluate the initial efficiency of cleanable and

The performance results obtained in accordance with this document cannot be quantitatively applied

(by themselves) to predict performance in service with regard to efficiency and lifetime.

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 29463 (all parts), — High efficiency filters and filter media for removing particles from air

For the purposes of this document, the terms and definitions given in ISO 29464 and the following

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

filters with performance complying with requirements of filter class ISO T10 to ISO T12 as per this

ratio of the mass of a standard test dust retained by the filter to the mass of dust fed after the first 100 g

For the application of this document, the following symbols and abbreviated terms apply:

ePM Minimum efficiency value with x = 1 µm, 2,5 µm or 10 µm of the conditioned filter element,

Filters with an efficiency of 85 % or more for the MPPS (EPA and HEPA filters) shall be tested

according to ISO 29463 (all parts), while filters with a lower efficiency shall be tested according to

ISO 16890 (all parts). Filters are classified in groups and classes based on their efficiency as defined in

NOTE For the classification of ISO ePM and ISO ePM filters only the ePM values are used.

HEPA filters (class T13) shall be individually tested and their efficiency determined at MPPS according

to ISO 29463-5. Filters shall be individually leak tested according to ISO 29463-4 where, in addition to

the reference leak scan method, four alternate methods for leak testing are allowed. For HEPA filters

with geometries which do not allow a scan testing, like e.g. cartridges or V-bank filters, the oil thread

test method or one of the other suitable (non-scanning) methods described in ISO 29463-4 can be

applied. Alternate norms used for leak testing should be clearly identified on the filter and certifications.

In order to extent the volume flow rate and the range of filter geometries (e.g. cylindrical filters),

deviations and extensions to the test rig defined by ISO 16890-2 and ISO 29463-5, respectively, are

The test rig consists of several square duct sections with typical 610 mm × 610 mm nominal inner

dimensions except for the section where the filter is installed. This section has nominal inner

dimensions between 616 mm and 622 mm. The length of this duct section shall be at least 1,1 times

the length of the filter, with a minimum length of 1 m as shown in Figure 1 (for more details on the test

rig see ISO 16890-2). The filter shall be within the section and shall not protrude out of this section,

either upstream or downstream. The test duct may need to have larger dimensions in cases when very

large filters or integrated filter-system-element are to be tested. In those cases, other dimensions are

allowed as long as the qualification procedures described in ISO 16890-2 are fulfilled. An example of a

In case of circular cartridges, the test setup (mounting of the filters in the test duct) shall be as close

to the real application as possible. In cases of large cylinders, a mounting plate with an additional hole

for the air inlet/outlet can be sufficient (see Figure 4). In terms of much smaller cylinders an additional