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It is not unusual to find indoor storage areas for cryogenic fluids that lack proper ventilation.

Low-oxygen alarms may initially indicate oxygen levels that are deficient but within the margin of safety, which is between 16 and 19.5 percent at altitudes less than 3,001 feet above sea level, according to ANSI/ASSE Z88.2-2015, Practices for Respiratory Protection. But levels could continue to drop below IDLH, when acute health effects become probable. As indicated in the Z88 standard, at 16 percent oxygen, workers may experience loss of coordination, impaired attention and thinking, and increased pulmonary ventilation and cardiac output. At 13 percent oxygen, abnormal fatigue, impaired judgement, and emotional upset may occur. At 10 percent oxygen, collapse and unconsciousness become possible. It is important to note that these percentages are based on work sites at sea level; work sites at higher elevations should account for the reduced atmospheric pressure and, accordingly, reduced partial pressure of oxygen (PO2).

NIOSH and ACGIH have defined oxygen deficiency as less than 132 torr PO2. The PO2 at the 5,300-foot elevation of Denver, Colorado, is close to 132 torr PO2, and the equivalent PO2 of 17.5 percent oxygen at sea level. While this low PO2 may seem alarming, workers living in Denver become well-acclimatized to these conditions within four weeks. As with heat illness prevention, due caution and monitoring is warranted for unacclimatized workers arriving at high-elevation work sites, especially when the lower available oxygen could be further reduced by off-gassing cryogens.

VENTILATION As with other atmospheric hazards, ventilation can be an effective engineering control for mitigating asphyxiation hazards when ventilation systems are properly designed. Ventilation design must account for the fact that gases boiling off from cryogenic liquids are extremely cold and denser than the ambient room air, causing them to sink to the floor. NFPA 55-2023, Compressed Gases and Cryogenic Fluids Code, provides specific ventilation guidance. It requires that indoor areas where cryogenic fluids are dispensed must be ventilated in accordance with the guidance in section 6.17 and mechanical code. Section 6.17 requires mechanical exhaust ventilation or fixed natural ventilation of at least 1 cubic foot per minute per square foot of floor space in indoor areas where cryogenic fluids are stored and used. Additionally, mechanical exhaust ventilation systems must account for the density of the gas: the exhaust air openings must be placed within 12 inches of the floor where gases heavier than air may be released. It’s crucial that exhaust air is not recirculated, as typical ventilation systems only filter particulates and would return asphyxiant gases to the occupied space.

Despite these standards, it is not unusual to find indoor storage areas for cryogenic fluids that lack proper ventilation. Without adequate management of change or hazard analysis, problems can arise when spaces intended for general occupancy are converted to areas for the storage or use of cryogens. The typical dilution ventilation systems found in general office occupancies have supply and return air openings in the ceilings and recirculate most of the air, doing little to remove the asphyxiant gases.

Modeling scenarios of incidental off-gassing, spills, leaks, and catastrophic failures can help the facilities design team understand the range of typical and worst-case oxygen displacement. This modeling may find that large-release scenarios could impact areas outside of the designated cryogen storage and usage zones, where ventilation systems may entrain and recirculate the gases. The audio and visual alarms associated with the oxygen monitoring systems may need to be extended to these areas to ensure that potentially impacted employees are alerted to oxygen-deficient conditions.

Proper ventilation is required to manage the asphyxiant hazard and shouldn’t be overlooked. Any administrative controls or PPE alone are likely to be infeasible, impractical, and inadequate to manage the risk when ventilation is lacking. Nonetheless, administrative controls, PPE, and preventive maintenance play important roles in ensuring the safe use of cryogenic fluids, particularly when ventilation failure, spills, or leaks occur.

INCIDENTS Risk assessments, hazard and operability (hazop) studies, or other effective risk evaluation processes are vital in identifying workplace hazards and operational problems that could arise from design and engineering issues in cryogenic systems. Taking a process safety management approach to break down complex process systems into simpler sections is especially helpful for larger cryogenic fluid distribution systems, which are plumbed throughout a facility to process equipment, dispensing stations, phase separators, pressure relief vales, and evaporators. To highlight the importance of careful risk analysis and management, it is helpful to discuss near misses, equipment failures, and accidents that have occurred at work sites, such as the following:

• A dewar containing liquid nitrogen exploded at Texas A&M University in 2006. Someone unaware of the hazards associated with unvented cryogenic fluid containers sealed a malfunctioning pressure-relief device. Approximately 12 hours later, the bottom of the dewar ruptured, and the force of the escaping pressure propelled the tank through the ceiling and pushed the laboratory walls off their foundations. 

• A pipe carrying liquid nitrogen ruptured at a Georgia poultry processing plant in 2021, resulting in six fatalities and 11 injuries. Lack of preventive maintenance and process safety management systems contributed to the accident, which was determined by the U.S. Chemical Safety and Hazard Investigation Board (CSB) to have resulted from the failure of the immersion freezer’s liquid level control system to accurately measure and control the liquid nitrogen level inside the freezer.

Proper training, preventive maintenance, and process safety management could have prevented these accidents.

RISK MANAGEMENT The innate hazards of cryogenic fluids, as well as the complexity of associated storage and distribution systems, serve well to illustrate the importance of effective risk assessment and management strategies familiar to OEHS professionals. Many relevant resources are available from industry associations, NFPA, CSB, and the Compressed Gas Association (CGA). Foregoing process hazard analyses, job safety analyses, and management of change procedures that can ensure appropriate hazard controls and facility procedures for cryogens is unnecessarily risky and can fail organizations and their workers.

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