by Dr. John Weisend II, European Spallation Source, CSA chairman, john.weisend@esss.se

Introduction

The scope of cryogenics is vast. Cryogenics includes the separation of air into its constituent parts and the storage and transport of those parts, the use of liquid oxygen and liquid hydrogen as fuels for space exploration, the creation and transport of liquefied natural gas (LNG), the use of liquid helium for cooling superconducting magnets and radio frequency cavities for use in particle accelerators and fusion reactors and for the cooling of infrared sensors. Cryogenics is also used to preserve cells and other biological specimens and is used in medical treatments. Cryogenic facilities range in size from the Large Hadron Collider at CERN and large air separation plants to small tabletop devices in laboratories.

In all its guises, cryogenics presents unique safety hazards. These include issues associated with

• The extreme cold of cryogenics
• Flammability of some cryogenic fluids (e.g., hydrogen, LNG)
• Enhanced combustion associated with the presence of liquid oxygen
• The displacement of oxygen by gases boiling off from cryogenic liquids (Oxygen Deficiency Hazards)
• The high pressures that can be formed during the volume expansion that occurs when a cryogenic fluid becomes a room temperature gas.

An additional challenge rises from the behavior of materials at cryogenic temperatures. Many materials are inappropriate for use in cryogenics and can fail, resulting in hazardous conditions. Make no mistake, cryogenics can be hazardous and injury and death have occurred in the field of cryogenics. People affected by cryogenic hazards range from experts in the field to those who only use cryogenics peripherally during their day. Engineers, students, technicians, truck drivers, scientists, plant workers and medical professionals are all examples of people who may be exposed to cryogenic safety issues.

Despite these hazards, work at cryogenic temperatures can be performed safely. Cryogenic safety is well understood and based on years of experience. In addition, research on cryogenic safety is on-going as the field of cryogenics advances, and this research is regularly presented at various cryogenic engineering conferences.

Cryogenic safety is a main theme of this issue of Cold Facts. Articles in this issue cover topics on helium cryostat safety (page 10), on safety for space launch vehicles during ground operations (page 12) and on the modeling of LNG vapor dispersion (page 41). The safe use of cryogenics is a vital concern of CSA and its members.

General cryogenic safety guidelines

Detailed information on cryogenic safety is provided in the other articles of this issue and in the resources listed below. General guidelines for cryogenic safety include the following:

Perform a formal hazard analysis on any cryogenic system or process prior to the start of work and design. Identify the hazards and how you will mitigate them. Ask “what if” questions. Keep in mind that equipment breaks, cryogenic fluids can turn rapidly into gas, valves leak or are improperly operated and vacuums can fail. This analysis should be done regardless of the size or complexity of the cryogenic system.

Design safety into equipment and processes from the beginning. Adding safety features at the end of the design process can be expensive and time consuming and may result in hazards being missed. Note that it is always better to remove a hazard via engineering design than to try to mitigate the hazard.

Review, review, review. Everyone, even experts, can miss things or make mistakes. Having the safety of your cryogenic system reviewed by other people, be they other members of your team, external consultants or formal review committees, is key to improving the likelihood of a safe system.

Whenever working with cryogenic liquids or inert gases, no matter how small the amount, always consider the possibility that an Oxygen Deficiency Hazard (ODH) may exist. Either prove by analysis that such a hazard doesn’t exist or apply appropriate design changes or mitigations to eliminate or reduce the hazard. ODH issues are particularly dangerous due to the large volume of gas evolved by even small amounts of cryogenic liquid and by the fact that, in low enough oxygen concentrations, the first physiological symptom can be sudden unconsciousness, followed rapidly by coma and death.

Take advantage of existing regulations and industry codes and standards to develop a safe cryogenic system.

Never disable or remove safety devices or relief valves, and never operate systems outside their operating parameters.

Only use materials at cryogenic temperatures that have been proven to work at those temperatures. Keep in mind during the hazard analysis that materials that nominally operate at room temperature (such as the outer walls of vacuum vessels) could reach cryogenic temperatures in certain failure modes.

Ensure that everyone working with or around cryogenic systems, even casual or periodic users, have the appropriate level of training in cryogenic and ODH safety.

Always use required personal protective equipment and follow required operating procedures. Taking shortcuts frequently leads to accidents.

Be aware that surfaces exposed to air and colder than 77K will start to condense the air and that the condensate will be oxygen rich (due to the higher boiling point of oxygen). This can lead to an increased flammability hazard. Insulate surfaces to prevent air condensation and, if unable to prevent it, manage carefully where the condensate collects and the materials with which it comes into contact. No ignition sources should be present.

Working with hydrogen, oxygen and LNG presents additional flammability hazards not present with inert cryogenic fluids and thus requires additional specialized knowledge. Make sure that such expertise is present in the designing, reviewing and operating of these systems.

Take advantage of existing information and expertise. There are significant resources available on cryogenic safety. These include books, pamphlets, research papers, codes and standards, websites and conferences. Cryogenic safety standards from other institutions, such as Fermilab, may also provide valuable information. The use of experts from other institutions as external reviewers or consultants may be beneficial.

Resources for cryogenic safety

Books and monographs

• Safety in the Handling of Cryogenic Fluids, Edeskuty, F. J. and Stewart, W. F., Springer (1996)
• Safety with Cryogenic Fluids, Zabetakis, M. G., Plenum Press (1967)
• Oxygen Deficient Atmospheres, Compressed Gas Association Bulletin CGA SB-2, Compressed Gas Association (2014)
• “Safety,” Edeskuty, F. J. and Daugherty, M. in The Handbook of Cryogenic Engineering, Weisend II, J. G. (ed), Taylor & Francis (1998)
• “Safety with Cryogenic Systems,” Chapter 10 in Cryogenic Engineering, Flynn, T. M., Marcel Dekker (1997)
• Cryogenic Safety Manual: A Guide to Good Practices, British Cryogenics Council (1991)

Associations and societies

• Compressed Gas Association: a technical society covering all aspects of industrial gas production and use. They produce many useful safety guides including ones on oxygen use, design of pressure relief systems and Oxygen Deficiency Hazards
• American Society of Mechanical Engineers: producer of pressure vessel and other pressure safety codes and standards.
• Cryogenic societies: A number of technical societies exist covering aspects of cryogenics. All are dedicated to supporting the safe use of cryogenics and have a variety of safety references and resources. Examples include:
  • Cryogenic Society of America
  • British Cryogenics Council
  • The Cryogenics Society of Europe
  • The Cryogenics and Superconductivity Society of Japan

Conferences

• Cryogenic Operations Workshop: a biennial meeting (even years) concerning all aspects of cryogenic operations including safety.
• International Technical Safety Forum: a periodic workshop covering safety (including cryogenic safety) at high energy physics and synchrotron light facilities.
• Cryogenic Engineering Conference: a biennial (odd years) meeting concerning all aspects of cryogenics including safety. Papers are published in Advances in Cryogenic Engineering.
• International Cryogenic Engineering Conference: a biennial (even years) meeting concerning all aspects of cryogenics including safety.

Laboratory safety chapters
A number of institutions have developed online safety manuals that describe their safety policies and requirements. Some of these are accessible to the public and may provide useful guidance. There are a few cautions when using this information. First, the policies and processes are subject to change, so one should be careful to use the most recent version. Also, the availability of these to the public is subject to change. Given these issues, the information in the links below is best used as an illustration of an approach rather than used verbatim.

• Cryogenic and ODH safety at the SLAC National Accelerator Laboratory
• Fermilab ESH&Q Manual

Courses

• “Cryogenic Engineering and Safety Course” by Cryoco LLC, info@cryocourses.com
• “USPAS 2015 Cryogenics Course” — online course materials
• The Cryogenic Society of America is in the early stages of developing a webinar on cryogenic safety. Further information may be had by contacting the author of this article.