ISO 15500-21:2023

Road vehicles - Compressed natural gas (CNG) fuel system components - Part 21: Discharge line closures

This document specifies tests and requirements for discharge lines closures, a compressed natural gas (CNG) fuel system component intended for use on the types of motor vehicles defined in ISO 3833. This document is applicable to vehicles (mono-fuel, bi-fuel or dual-fuel applications) using natural gas in accordance with the ISO 15403-1. It is not applicable to the following: a) liquefied natural gas (LNG) fuel system components located upstream of, and including, the vaporizer; b) fuel containers; c) stationary gas engines; d) container-mounting hardware; e) electronic fuel management; f) refuelling receptacles.

Véhicules routiers — Composants des systèmes de combustible gaz naturel comprimé (GNC) — Partie 21: Fermeture des lignes de décharge

General Information

• Status: Published
• Publication Date: 02-Feb-2023
• ICS: 43.060.40 - Fuel systems
• Technical Committee: ISO/TC 22/SC 41 - Specific aspects for gaseous fuels
• Drafting Committee: ISO/TC 22/SC 41/WG 3 - Fuel system components and refuelling connector for vehicles propelled by gaseous fuel
• Current Stage: 9092 - International Standard to be revised
• Start Date: 22-Dec-2024
• Completion Date: 13-Dec-2025

Overview

ISO 15500-21:2023 is an international standard specifying the requirements and test methods for discharge line closures in compressed natural gas (CNG) fuel systems for road vehicles. This component plays a crucial role in preventing contamination while allowing controlled venting of gas from the system. It applies to motor vehicles as defined by ISO 3833, including mono-fuel, bi-fuel, and dual-fuel vehicles running on natural gas compliant with ISO 15403-1. The standard excludes liquefied natural gas (LNG) components upstream of the vaporizer, fuel containers, stationary gas engines, mounting hardware, electronic fuel management, and refueling receptacles.

This document ensures the safety, reliability, and durability of discharge line closures used in CNG fuel systems operating primarily at 20 MPa (200 bar) service pressure, with options for adjustment to other pressures, supporting robust compressed natural gas vehicle (NGV) operation.

Key Topics

• Scope and Application
  Covers discharge line closures in CNG fuel systems for road vehicles using natural gas, vital for venting and safety.

• Marking Requirements
  Components must be clearly marked with manufacturer identification, model designations, serial numbers, and recommended supplementary information like pressure ratings, fuel type, certification marks, and reference to ISO 15500-21.

• Construction and Assembly
  Compliance with ISO 15500-1 and ISO 15500-2 is mandatory, including material tolerances. Discharge line closures may be single-use (e.g., pressure relief devices) or reusable (e.g., pressure relief valves) and must reliably handle low flow rates without compromising performance.

• Testing Procedures
  Essential tests include:

  • Excess torque resistance
  • Bending moment resistance
  • Continued operation under cycling at varied temperatures (from -40 °C to +85 °C)
  • Corrosion, ozone, and heat aging tests
  • Automotive fluid compatibility
  • Vibration resistance
  • Internal leakage vent test - verifies controlled venting without excessive pressure retention at different temperatures
  • Water jet protection - ensures durability against high-pressure water spray simulating harsh environmental conditions

Rejecting or damage under these rigorous tests is not acceptable.

Applications

ISO 15500-21:2023 is critically important for manufacturers, suppliers, and testing bodies involved in the development, certification, and maintenance of CNG fuel systems for road vehicles. It supports applications including:

• CNG Mono-Fuel Vehicles
  Vehicles exclusively powered by compressed natural gas require reliable venting to maintain safety and system integrity.

• Bi-Fuel and Dual-Fuel Vehicles
  Vehicles operating on both natural gas and conventional fuel benefit from standardized discharge line closures for optimized fuel system management.

• Safety and Compliance Assurance
  Ensuring discharge line closures meet this ISO standard helps vehicles comply with international safety regulations and reduce risks related to fuel leakage and contamination.

• Quality Control in Component Manufacturing
  The standard’s marking and test requirements facilitate traceability and consistent quality across production batches.

Related Standards

ISO 15500-21:2023 operates in conjunction with key related standards to provide comprehensive guidance on compressed natural gas vehicle fuel systems:

• ISO 15500-1: General requirements and definitions for CNG fuel system components
• ISO 15500-2: Performance criteria and general test methods for CNG components
• ISO 3833: Road vehicle types and terms, providing vehicle classification definitions
• ISO 15403-1: Natural gas fuel quality for compressed fuel vehicles

Adhering to these interconnected standards ensures full compliance, safety, and interoperability throughout the CNG vehicle fuel system supply chain.

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By implementing ISO 15500-21:2023, stakeholders in the natural gas vehicle industry can enhance component reliability, optimize safety, and support the growing adoption of cleaner alternative fuels in road transportation.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 15500-21
First edition
2023-02
Road vehicles — Compressed natural
gas (CNG) fuel system components —
Part 21:
Discharge line closures
Véhicules routiers — Composants des systèmes de combustible gaz
naturel comprimé (GNC) —
Partie 21: Fermeture des lignes de décharge
Reference number
© ISO 2023
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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ISO copyright office
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Marking . . 2
5 Construction and assembly .2
6 Tests . 2
6.1 Applicability . 2
6.2 Continued operation . 3
6.3 Internal leakage vent test . 3
6.4 Water jet protection . 3
Bibliography . 4
iii
Foreword
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This document was prepared by Technical ISO/TC 22, Road vehicles, Subcommittee SC 41, Specific
aspects for gaseous fuels.
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iv
Introduction
For the purposes of this document, all fuel system components in contact with natural gas have
been considered suitable for natural gas as defined in ISO 15403-1. However, it is recognized that
miscellaneous components not specifically covered herein can be examined to meet the criteria of this
document and tested according to the appropriate functional tests.
All references to pressure in this document are considered to be gauge pressures unless otherwise
specified.
1)
This document is based on a service pressure for natural gas used as fuel of 20 MPa [200 bar ] settled
at 15 °C. Other service pressures can be accommodated by adjusting the pressure by
...