ISO/DIS 24941:2026(en)

ISO TC 8/SC 3/WG 19

Secretariat: ANSI

Date: 2026-12-24

Ships and marine technology–Piping and machinery – Safety guidelines for engine rooms of ammonia fuelled vessels

Copyright notice

This ISO document is a Draft International Standard and is copyright-protected by ISO. Except as permitted under the applicable laws of the user’s country, neither this ISO draft nor any extract from it may be reproduced, stored in a retrieval system or transmitted in any from or by any means, electronic, photocopying, recording or otherwise, without prior written permission being secured.

Requests for permission to reproduce should be addressed to either ISO at the address below or ISO’s member body in the country of the requester.

ISO copyright office

CP 401 • Ch. de Blandonnet 8

CH-1214 Vernier, Geneva

Phone: +41 22 749 01 11

Fax: +41 22 749 09 47

Email: copyright@iso.org

Website: www.iso.org

Contents

Foreword 3

Introduction 4

1 Scope 5

2 Normative references 5

3 Terms and definitions 5

4 General requirements 6

5 Ammonia leak detection 6

5.1 Leakage detectors 6

5.2 Fire detection 7

6 Alarm 7

7 Fire fighting 8

8 Ventilation 8

9 Control and monitoring system 9

10 Other safety considerations 9

10.1 Crew safety 9

10.2 Gas safe engine room 10

Annex A (informative) 11

Annex B (informative) 12

Bibliography 13

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 documents 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).

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any patent rights identified during the development of the document will be in the Introduction and/or on the ISO list of patent declarations received (see www.iso.org/patents).

Any trade name used in this document is information given for the convenience of users and does not constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions related to conformity assessment, as well as information about ISO's adherence to the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT),see www.iso.org/iso/foreword.html.

This document was prepared by Technical Committee ISO/TC8/SC3, [Piping and machinery].

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

Ammonia is considered one of the alternative fuels with the potential to help the shipping industry meet regulations on reducing greenhouse gas (GHG) emissions.

Ammonia has a relatively high boiling point so that liquefaction is easier than other alternative fuels. But ammonia has also characteristics of toxicity and flammability. So, when handling or using ammonia, attention and caution are extremely important.

The International Maritime Organization (IMO) has published interim guidelines for safety of ships using ammonia as fuel, as IMO document (MSC.1/Circ.1687). Furthermore, a variety of manufacturers are trying to develop equipment including ammonia fuelled engines, pumps, and valves for use in an ammonia propulsion system. For these reasons, equipment manufacturers, ship building companies and classification societies are publishing guidance and rules and regarding the use of ammonia onboard ships.

This document is expected to provide ammonia safety guidelines in detail, such that it can be used for system designs by shipyards, ship orders from owners, inspection and survey from ship classification societies, and research and development from equipment manufacturers. This document will contribute to the growth of relevant industries and benefit all stakeholders.

The IMO interim guidelines have been formally approved by the Maritime Safety Committee on its 109^th session (MSC 109). Applicable rules and standards on ammonia fuelled ships are still in the early stages of development. If there is any inconsistency with this document and the IMO guidelines, the guidelines take first priority. The terms and definitions will be developed and harmonized to take into account future guidelines developed by the IMO.

Ships and marine technology —Piping and machinery—Safety guidelines for engine rooms of ammonia fuelled vessels

This document provides safety guidelines for the engine room of ammonia fuelled vessels, in accordance with the “Interim Guidelines for Ships Using Ammonia as Fuel”, published by the International Maritime Organization (IMO). It describes the considerations that should be properly addressed for safety in the engine room when using ammonia. This document is not applicable to systems for ammonia as a cargo and does not address the design of the ammonia fuel tank.

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 60079-10-1:2020, Explosive atmospheres — Part 10-1: Classification of areas — Explosive gas atmospheres

International Maritime Organization (IMO), Fire Safety Systems Code

For the purposes of this document, the following terms and definitions apply.

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

— ISO Online browsing platform: available at https://www.iso.org/obp

— IEC Electropedia: available at https://www.electropedia.org/

3.1

toxic area

area in which ammonia is or may be expected to be present

[SOURCE: MSC,1./Circ.1687 interim guidelines for the safety of ships using ammonia as fuel]

3.2

toxic zone

area on board where the presence of ammonia gas at hazardous concentrations is reasonably foreseeable during normal operation or in the event of a single failure, and which therefore requires ventilation, detection, or restricted access

3.3

fuel

ammonia either in the gaseous state or in the liquefied state

3.4

source of release

a point or location from which a gas, mist, or liquid may be released into the atmosphere such that a toxic or flammable ammonia concentration atmosphere could be formed

3.5

engine room

ship compartments that contain internal combustion machinery used for main propulsion, internal combustion machinery used for purposes other than the main propulsion, such as electrical power generators, where such machinery has an aggregate a total power output of not less than 375 kW, or which contains any oil-fired boiler or oil fuel unit such as inert gas generators, incinerators, etc.

Note 1 to entry: In general, engine rooms are considered category A machinery spaces per IMO regulations (SOLAS).

3.6

PPM

parts per million in mass or volume

3.7

PPE

personal protective equipment

Engine rooms containing ammonia fuel systems and/or ammonia-fuelled machinery should be arranged such that the spaces may be considered gas safe under all conditions, normal as well as abnormal conditions, i.e. inherently gas safe. In a gas-safe engine room, a single failure cannot lead to the release of fuel gas into the engine room. A single failure within the fuel system should not lead to a fuel release into the engine room. All fuel piping within engine room boundaries should be enclosed in a gastight enclosure, taking into account paragraph 9,6 of the IGF Code part A-1. Access to the engine room should not be arranged from toxic areas or toxic zones. To further support optimal safe design, CFD analysis may be performed, taking into account the specific characteristics of each ammonia fuelled vessel under construction. In addition, risks should be minimized through methods risk assessment such as FMEA, HAZID, HAZOP.

Materials which may be exposed to fuel during normal operations should be resistant to the corrosive action of ammonia. Mercury, copper, copper alloys, tin-plated, zinc, zinc-plated and cadmium should not be used for the construction of pipelines, valves, fittings and other items of equipment normally in direct contact with the fuel liquid or vapour.

Conventional type detectors, Light Detection and Ranging (LiDAR), and thermal cameras can be used for leak detection. Strategic placement of sensors near the expected leak points, corners, and outlet ventilation enables effective and comprehensive detection of ammonia dispersion in terms of both distribution and intensity.

A conventional type of detector (including thermal sensors, pressure gauges, and positioning sensors), LiDAR, and thermal cameras can be used to detect ammonia gas or liquid leakage. Leakage detection systems should be designed, installed, and tested according to IEC 60079-29-1 and EN 45544-4 standards, or equivalent. The number of detectors in each location should account for the size, layout, and ventilation of the area. Gas dispersal analysis should be used to determine the best arrangement in engine room. Depending on the engine type, detectors may be installed directly with the fuel consumer. The detection equipment should be placed where leakage may accumulate and at ventilation outlets. Permanently installed fixed leakage detectors should also be fitted at ventilation inlets to manned spaces and locations and spaces if required based on the risk assessment. Additionally, a leakage detector with an alarm set at 110 ppm and which stops the ammonia fuel supply to the engine at 220 ppm should be installed at potential leakage sources.

Permanently installed leakage detectors are to be fitted in:

(1) engine room containing gas/liquid piping, gas/liquid equipment or gas/liquid consumers;

(2) other enclosed spaces containing fuel piping or other fuel equipment without ducting;

There shall be a fixed fire detection and fire alarm system complying with the requirements of the IMO Fire Safety Systems Code. The detection system should be capable of rapidly detecting a fire. The type of detectors and their spacing and location shall consider the effects of ventilation and other relevant factors. After being installed, the system shall be tested under normal ventilation conditions and the response time shall be documented.

For vessels using ammonia as fuel, the fire detection system shall be integrated with the leakage detection system to enable automatic activation of fuel shut-off valves and emergency ventilation upon detection of elevated ammonia concentrations or fire. Heat and/or flame detectors should be used in areas where ammonia may be present, considering that ammonia combustion may not produce significant smoke. Detector placement shall account for the buoyant nature of ammonia gas, favoring installation near the ceiling and ventilation inlets. Upon detection of fire or significant ammonia release, the system shall initiate appropriate safety actions such as nitrogen purging, fuel isolation, and ventilation. Equipment installed in hazardous areas shall be explosion-proof and/or flame-proof as appropriate.

The alarm system shall be activated for the following cases:

In case of ammonia leakage on board, response measures include isolating the source, activating water spray systems to control vapour, and ensuring proper ventilation. In case of smaller ammonia fires, dry chemicals or CO2 can be more suitable. It is crucial, however, to exercise caution and refrain from directing a water jet directly at an ammonia leak or liquid ammonia source, as it may trigger a hazardous reaction.

Engine rooms shall be adequately ventilated to ensure that, when machinery or boilers are operating at full power in all weather conditions including heavy weather, an adequate supply of air is maintained to the spaces for the safety and comfort of personnel and the operation of the machinery. Ventilation must be appropriate to the function of the engine room and sufficient under normal conditions to prevent the accumulation of oil vapour and ammonia vapour.

Means of control shall be provided for opening and closure of skylights, closure of openings in funnels which normally allow exhaust ventilation, and closure of ventilator dampers. Means of stopping ventilating fans shall be provided. These controls should be operable from two locations, one of which shall be outside the engine room. Controls for stopping the power ventilation in engine room shall be entirely separate from those stopping ventilation of other spaces.

The ventilation arrangement fitted in spaces functioning as secondary barriers for ammonia systems should be designed in consideration of:

— Preventing the space from being subjected to pressures above its capabilities by providing a passive pressure relief through the ventilation ducts in cases where liquefied gas is released and vaporises (increasing in volume 850 times).

— Preventing leaked gas from spreading to other spaces by maintaining a negative pressure in the space compared to surrounding areas (extraction ventilation).

— The ability to dilute leakages and transport ammonia vapours from the ship interior to a relatively safe space in open air.

Air inlet openings are to be positioned as low as practicable in the space being ventilated and exhaust openings are to be both the lowest and highest points and at opposite sides to the air inlet openings so that ammonia vapor cannot accumulate in the space.

Emergency Shutdown (ESD)-protected engine rooms are to have ventilation with a capacity of at least 30 air changes per hour. The ventilation system is to circulate air in all spaces and ensure that any formations of gas pockets in the room are detected. Alternatively, arrangements where the engine room is ventilated with at least 15 air changes an hour is acceptable provided that, if gas is detected in the engine room, the number of air changes will automatically be increased to 30 per hour.

Description and plans for control and monitoring systems and fuel changeover arrangements for dual-fuel machinery should include line diagrams of control circuits along with lists of monitoring, control, alarm points, detailed plans of the fixed fuel gas detection, fire detection and alarm/shutdown systems. Also, evidence of software development and conformity shall be provided.

Toxic spaces and hazardous areas shall be clearly distinguished in accordance with IMO guidelines. Toxic spaces shall be minimized to reduce risks to personnel, the ship, and equipment. Only essential equipment shall be installed in hazardous areas or toxic spaces, and such equipment shall be certified for its intended use. The design shall prevent the accumulation of toxic, flammable, or explosive gases, and components shall be protected from external damage. Ignition sources in hazardous areas shall be minimized. Engine rooms containing fuel sources shall be arranged in such a way that, in the event of a single fuel system failure, unacceptable power loss(e.g. not exceeding 30 % of the total power) or failure of essential systems in other compartments does not occur. Liquid ammonia piping shall be designed for a minimum pressure of 18 bar, corresponding to the vapour pressure of ammonia at 45 °C. Operational risks, including those associated with maintenance activities such as purging, shielding, and leak prevention, may be identified and mitigated. Procedures for safe operation, maintenance, and emergency response should be developed and made available to relevant personnel. Adequate training should be provided to all personnel involved in the handling of ammonia fuel. Appropriate marking and signage should be clearly displayed in all toxic and hazardous areas to inform and warn personnel of potential dangers. The technical documentation shall demonstrate compliance with the applicable rules and standards, including those related to safety, availability, maintainability, and reliability.

All crew should not be exposed to harmful gas concentrations. Crew access to toxic areas is to be restricted. Released ammonia gas is not to spread to non-toxic zones.

The ship should have on board Personal Protective Equipment (PPE) for crew members who are engaged in operations, maintenance of ammonia fuel systems. The place where PPE is stored is to be easily accessible and where the toxicity of fuel cannot reach. PPE shall include gas-tight protective clothing, large aprons, long-sleeve special gloves, suitable footwear, chemical-resistant coveralls, and face shields that meet recognized national or international standards. The protective clothing and equipment should cover all skin so that no part of the body is unprotected. Such equipment should not be kept within accommodation spaces, with the exception of new, unused equipment and equipment that has not been used since undergoing a thorough cleaning process. Protective equipment should be used in any operation which may entail danger to personnel. To permit entry and work in a gas-filled space, safety equipment providing adequate personal protection, including gas tight protective clothing and self-contained positive pressure air-breathing apparatus incorporating full face mask, should be available. The protective gas-tight clothing should be designed to a recognize standard. In the event of an ammonia release, there should be provisions for safe refuge on board. Self-contained breathing apparatus should provide at least 15 minutes of service time.

The compressed air equipment is to be inspected at least once a month by a responsible officer and the inspection logged in the ship’s records. Also, this equipment is to be inspected and tested by a competent person at least once a year. A minimum of three complete sets of safety equipment are to be provided in addition to the required firefighter's outfits. One set should include a self-contained positive pressure air-breathing apparatus with a full-face mask, not using stored oxygen, and with a capacity of at least 1 200 litres of free air. Each set must be compatible with the firefighter outfits and include:

i) protective gas tight clothing (without any exposed skin)

ii) boots and gloves manufactured to a recognized standard

iii) steel-cored rescue line with belt

iv) lamp with proper hazardous area rating.

Emergency showers and eye rinsing equipment shall be provided to wash out the toxicity of ammonia. When someone has been exposed to anhydrous ammonia, the best course of action is to move him or her to a safe place and flush the exposed area immediately with water for a minimum of 15 minutes.

The ship should carry at least two sets of portable ammonia gas detectors that meet acceptable national or international standards. Portable ammonia gas detectors typically measure from 0 to 100 ppm. Alarm is set at 25 ppm, with level triggering emergency alert.

A single failure of fuel systems is not to lead to a gas release in the engine room. All fuel piping within engine room shall be in gas and liquid tight secondary enclosures. To prevent the risk of a gas explosion in an engine room with gas-fuelled machinery, a gas-safe arrangement should be applied:

NOTE: Gas-safe engine room means that the arrangements in the engine room are considered gas safe under all conditions including normal and abnormal conditions. I.e., a single failure in a gas safe engine room cannot lead to release of fuel gas into the engine room. The Emergency Shutdown (ESD) engine room arrangement is generally not acceptable for ammonia-fuelled ships because of toxicity risks in the event of a leak.

Gas vents shall be provided to avoid diffusion of gases from the enclosure into non-toxic areas.

Drip trays are to be fitted where leakage may occur that can cause damage to the ship structure or where limiting the area affected from a leak or spill is necessary. Drip trays are to be made of appropriate materials and have adequate capacity to handle the maximum spill volume identified in the risk assessment.

Fuel piping in engine room is to be completely enclosed by a second pipe or duct. Manual and remote isolation valves and fuel line purging arrangements should be fitted to enable safe maintenance of ammonia-related equipment located in engine room.

1. 
   (informative)
   
   Ammonia exposure guideline levels

Table A.1 — Acute exposure guidelines levels

Table A.2 — Exposure limit at workplace

1. 
   (informative)
   
   Ammonia properties

Bibliography

[1] Class ABS, Requirements for Ammonia Fuelled vessels

[2] Class KR, Guidelines for ships using ammonia as fuels

[3] Class KR, Rules and Guidance for the Classification of Ships Using Low-flashpoint Fuels

[4] Class LR, LR-RU-012 Rules and Regulations for the Classification of Ships using Gases or other Low-flashpoint Fuels, July 2024 - Appendices - Appendix LR2 – Requirements for Ships Using Ammonia as Fuel - LR Part A-1. Specific Requirements for Ships Using Ammonia as Fuel

[5] Class BV, Ammonia-fuelled ships Tentative rules(NR671)

[6] Class DNV, Ammonia as a marine fuel Safety Handbook

[7] Class NK, Part C ‘Guidelines for the safety of ships using ammonia as fuel’ of guidelines for ships using alternative fuels

[8] OSHA (Occupational Safety and Health Standards) 1910 Storage and handling of anhydrous ammonia

[9] ISO 14067:2018, Greenhouse gases — Carbon footprint of products — Requirements and guidelines for quantification

[10 IMO SOLAS(International Convention for the Safety of Life at Sea)

[11] IMO Carriage of Cargoes & Containers(CCC) 10th, Amendments to the IGF code and development of guidelines for alternative fuels and related technologies

[12] IMO MSC.370(93), Amendments to the international code for the construction and equipment of ships carrying liquefied gases in bulk (IGC CODE)

[13] IMO MSC.391(95), Adoption of the international code of safety for ships using gases or other low-flashpoint fuels (IGF CODE)

[14] Duong, P. A. A preliminary safety assessment of fuel gas supply system in the engine room of the ammonia fuelled ship. Journal of Marine Engineering & Technology, 2025

[15] MMMCZCS (Maritime Ministries and Maritime Classification Zones and Class Societies). Emerging Ship Design Principles for Ammonia-Fueled Vessels. 2023