CGA P-56

This publication applies to cryogenic liquid supply systems, located either on a customer site or a production site, where cryogenic liquid is vaporized and is then supplied either as the primary or secondary source of gaseous product. This publication is limited to the prevention of brittle fracture in piping and associated equipment installed downstream of the vaporization system.

The secondary source of supply can be a back-up supply to a production plant when the production plant trips or is shut down, a supplementary supply to meet customer demand where it exceeds the capacity of the production plant (peak shaving), or as a back-up supply at a customer site (for example, a healthcare facility).

The safeguard principles presented in this publication apply to any process fluid supply system where the temperature of the cryogenic liquid source is less than the minimum temperature rating of the piping and/or associated equipment downstream of the vaporizer.

Examples include:

• nitrogen;
• oxygen;
• argon;
• helium;
• hydrogen;
• natural gas;
• carbon monoxide;
• methane; and
• ethylene.

Carbon dioxide and nitrous oxide are excluded from the scope of this publication; however, the principles of this publication may be applied.

The supply systems work by vaporizing cryogenic liquid, typically in response to decreasing pipeline pressure.

Supply systems are made up of the following:

• liquid supply from either a low pressure tank and pump arrangement or directly from a high pressure tank;
• vaporization system that could be a natural draft ambient air-heated type or one that uses an external energy source for example, steam, hot water, electricity, direct fired;
• interconnecting liquid and outlet gas piping, valves, and fittings up to the battery limit; and
• low temperature protection system (LTPS) as detailed in this publication.

Although this publication does not cover the following situations, the techniques listed may be considered for cold embrittlement prevention:

• Air separation and other cryogenic processes with columns, separators, or tanks in which a gas stream from a sump is normally supplied through downstream heat exchange equipment, or liquefaction units where a cryogenic fluid can exit through warm-end process piping during a shutdown or upset scenario. Cryogenic processes are assumed to have their own LTPS;
• Piping systems within which a fluid is expanded across a valve or restriction with the resultant temperature being less than the ductile-to-brittle-transition-temperature (DBTT) for the piping system; and
• Vessels that are depressurized rapidly: as work is done in a vessel by a gas expanding as it is discharged out of the valve, the temperature inside the vessel and the vessel wall can be lowered.

This publication recommends safe practice for the design of new cryogenic vaporizer systems. For existing systems, a risk assessment shall be undertaken to establish if any modifications are required. The principles developed in this publication can be used for this purpose.