National foreword

This British Standard is the UK implementation of ISO 24499:2026. It supersedes PD ISO/PAS 24499:2024, which is withdrawn.

The UK participation in its preparation was entrusted to Technical Committee RHE/18, Refrigeration safety.

A list of organizations represented on this committee can be obtained on request to its committee manager.

Contractual and legal considerations

This publication has been prepared in good faith, however no representation, warranty, assurance or undertaking (express or implied) is or will be made, and no responsibility or liability is or will be accepted by BSI in relation to the adequacy, accuracy, completeness or reasonableness of this publication. All and any such responsibility and liability is expressly disclaimed to the full extent permitted by the law.

This publication is provided as is, and is to be used at the recipient’s own risk.

The recipient is advised to consider seeking professional guidance with respect to its use of this publication.

This publication is not intended to constitute a contract. Users are responsible for its correct application.

Compliance with a British Standard cannot confer immunity from legal obligations.

This British Standard was published under the authority of the Standards Policy and Strategy Committee on 30 September 2026.

Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of ISO document should be noted. This document was drafted in accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

ISO draws attention to the possibility that the implementation of this document may involve the use of (a) patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a) patent(s) which may be required to implement this document. However, implementers are cautioned that this may not represent the latest information, which may be obtained from the patent database available at www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.

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This document was prepared by Technical Committee ISO/TC 86, Refrigeration and air-conditioning, Subcommittee SC 8, Refrigerants and refrigeration lubricants.

This first edition cancels and replaces ISO/PAS 24499:2024, which has been technically revised.

The main changes are as follows:

the term "blend" was added to Clause 3;
requirements were added to: 6.2 (to measure the concentration of gas mixture in the tube), 8.3 (to train personnel in potential system hazards and have HF safety kits available), 9.5 (to not neglect the effect on flame configuration) and 9.7 (on how to estimate the area of the tipped flame front);
recommendations were added to: 6.3.7 (to compare the glass transmission profiles before selecting the type of glass) and 9.5 (to use a tube of cross-sectional area that is as large as possible).

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www.iso.org/members.html.

Introduction

The safety classification and relative flammability properties of refrigerants are a critical part of ISO 817 [1]. The flammability limits of refrigerant gas in air, as described in ISO 817:2024 [2], Clause B.1, give a partial measure of the relative flammability. Another dimension of flammability is how fast a substance burns, releases energy and spreads a flame. It is possible to measure the rate at which a flame front moves through a cloud of refrigerant gas in air or its burning velocity (BV). This document describes one method that can be useful for BV measurement and thereby better quantify and compare relative flame fronts for some refrigerant classes. In this test, a flame is allowed to propagate upward (vertically) through a well-mixed, quiescent column of a refrigerant-air mixture enclosed in an open-ended glass tube. Optical systems are used to measure the upward velocity of the flame front.

The measurement of BV has been widely used in the past to compare highly energetic fluids, such as motor fuels and rocket propellants. The BV measurement of slow burning fluids, such as ammonia and fluorinated refrigerants, can be more difficult to measure due to the inherent instability of a slow flame. The low rate of energy evolution from a slow flame makes it susceptible to quenching from a variety of sources. For slow burning refrigerants, turbulence and convection currents can break the flame front and hence quench the flame. In addition, the test chamber surface can quench free radical flame intermediates as well as extract some of the heat necessary for flame propagation. Gas-phase thermal radiation is also important for flames with low BV. These effects are important to note as they tend to diminish and sometimes quench a weak flame.

The use of the vertical tube method for BV characterization of slow burning refrigerants was the subject of doctoral research which was used in ISO 817:2024 [2], Annex C and is the basis for this document[3]. In addition, ASHRAE research notes the use and limitations of the vertical tube technique[4][5]. While the basic framework of the method is relatively simple, some sophisticated imaging instrumentation and mathematics are necessary to extract an average local BV separate from the bulk burning speed as the flame progresses up the tube. Since 2004 other laboratories have used the basic principle of vertical tube method and have shown acceptable results for reproducing the measurement of R-32, at (6,7 ±0,7) cm/s. Lower burning velocities (i.e. <4 cm/s) make it more difficult to measure reproducibility, so variability can increase as flame instability increases. The lower BV limit of this method is between 3 cm/s and 4 cm/s, depending on the actual design and geometry of the apparatus being used. The uncertainty of the measurement of flames that burn more slowly than R-32 has not yet been determined in any multi-lab comparative testing. The appealing aspects of this test are the relative simplicity and low cost of its implementation. Therefore, the lower accuracy limit of this BV method is considered to be 4 cm/s; any data generated by this method which yields a lower value should be reported as <4 cm/s.

1 Scope

This document specifies a method of measuring the burning velocity (BV) of lower burning refrigerants (</= 10 cm/s) for use with other standards that utilize the BV for determining the safety classification of refrigerants (e.g. ISO 817 [1]) or that use the BV in establishing requirements on the use of slow burning refrigerants (e.g. ISO 817:2024 [2], 6.1.3.3).

2 Normative references

3 Terms and definitions

4 General test method

4.1 General

4.2 Principle of the test method

5 Measurement parameters

5.1 General

5.2 Flame propagation velocity

5.3 Flame surface area

5.4 Cross-sectional area of the flame base

6 Test method

6.1 General

6.2 Gas handling and mixtures preparation

6.3 Test tube

6.4 Ignition

6.5 Flame front visualization

6.6 Purge, exhaust and gas treatment systems

6.7 Test temperature setting

6.8 Experimental protocol for mixtures prepared using partial pressure technique

7 Evaluation and expression of results

7.1 General

7.2 Uncertainty

8 Safety precautions

8.1

8.2

8.3

8.4

8.5

8.6

9 Overview on flame shape, propagation regimes and stability

9.1 Flame shape

9.2 Flame propagation regimes

9.3 Flame stability in tubes

9.4 Observations of flames in tubes

9.5 Flame quenching in circular tubes

9.6 Flame propagation velocity and tube diameter

9.7 Flame area calculation

Bibliography