The scope of this document is for metallic transmission and distribution piping systems carrying pure hydrogen and hydrogen mixtures as shown in Figure 1 of Appendix A. It is limited to gaseous products with:

• a temperature range between –40 °C and 175 °C (–40 °F and 347 °F);
• total pressures from 1 MPa (150 psi) up to 21 MPa (3000 psi) or for stainless steels only partial hydrogen pressure higher than 0.2 MPa (29 psi); and
• concentration criteria defined in Appendix G.

Specific guidance is provided in Appendix I concerning ultra high purity (UHP) hydrogen.

This document does not apply to the following processes:

• cylinder filling plants;
• producing plants;
• compressor units;
• bulk facilities (liquid or high pressure gas) at the customer’s site up to the point where gas enters the distribution systems; or
• piping on specialized equipment and machines.

The purpose of this publication is to further the understanding of those engaged in the safe design, operation, and maintenance of transmission and distribution systems. It is not intended to be a mandatory standard or code. It contains a summary of the current industrial practices. It is based upon the combined knowledge, experience, and practices of the major producers in Europe and North America.

Some of the practices represent conservative compromises and not all situations are described. The designer is cautioned that this document is not a complete design handbook and does not negate the need for competent engineering judgment and interpretation. The user should review any special problems or concerns with the hydrogen supplier who should be able to provide advice and guidance.