Requirements:
Procedures and Specific Requirements

ODH Risk Assessments

Before establishing a cryogenic work area, an ODH risk assessment must be performed. The assessment must include calculations to determine whether the proposed storage and use of the cryogen will create an ODH in the event of the worst possible accident. The guidelines to be followed during these risk assessments are shown in Tools, Cryogenic and Oxygen Deficiency Hazard Safety: ODH Risk Assessment Procedures. Each ODH risk assessment must be approved by HEEC.

Special care must be taken to examine the areas at elevations below (pits, trenches, tunnels) and above (such as service buildings, crane cabs, and roof maintenance areas) the elevation at which cryogens are being stored and used, depending on the physical properties of the cryogen.

Engineering Controls

Since cryogenic fluids exist as liquids only at temperatures considerably below room temperature, full containment of the fluid at room temperature is usually not feasible. Normal storage facilities and piping systems must allow for unavoidable heat input from the environment. For ordinary operations this means appropriate insulation, adequate pressure-relief devices, and proper disposal or recycling of the gases. (See Tools-ODH Control Requirements, for details on environmental, engineering, and personal controls of cryogenic hazards.)

Insulation

Certain cryogens, such as helium and hydrogen, are cold enough to solidify atmospheric air. Entry of air into cryostats containing these cryogens can be prevented by pressurizing the system. If openings to the atmosphere exist, they are likely to become plugged by solidified air, leading to overpressure and vessel failure. Such conditions will also result in hazardous contamination of the fluid. Again, adequate pressure-relief devices must be provided to vent all gas that could be produced during a theoretical maximum heat flux into the system. The system configuration must ensure that any air that enters the cryostat and freezes cannot prevent proper functioning of pressure relief devices.

Unless these fluids are handled in vacuum-jacketed vessels and piping, air will condense on the exterior of the system. This condensate will be rich in oxygen. Hazards are created by the exposed cold surfaces, dripping liquid air, and the potential for exploding insulation. The latter can happen when air condenses between the metal surface and the insulating layer. On warming, the air vaporizes and can rip off the insulation with explosive force. Such insulation systems must be specially engineered to prevent air penetration, or the insulation must have sufficiently low strength that it will yield at low gas pressure.

Pressure Relief

Heat flux into the cryogen is unavoidable regardless of the quality of the insulation installed. Pressure relief must be provided to permit routine release of gas vapors generated by this heat input. Typically such relief is best provided by rated spring-loaded relief devices or an open passage to the atmosphere with a check valve.

Additional relief devices should be provided as backup to the operational relief when the capacity of the operational relief device is not adequate to take care of unusual or accidental conditions. This may be the case if the insulation is dependent on the maintenance of a vacuum in any part of the system (this includes permanently sealed dewars), if the system may be subject to an external fire, or if rapid exothermic (heat releasing) reactions are possible in the cryogen or a container cooled by the cryogen. In each case, relief devices capable of handling the maximum volume of gas that could be produced under the most adverse conditions must be provided.

Each and every portion of the cryogenic system must have uninterruptible pressure relief. Any part of the system that can be valved off from the remainder must have separate and adequate provisions for pressure relief.

Examples of parts that usually require separate relief systems include

• Pressurized supply dewars
• Piping (except ambient) and manifolds
• Tubing and hoses used to transfer a cryogen, unless an air gap is provided
• Bath space surrounding experimental volume
• Any volume cooled externally by a cryogen
• Vacuum spaces in contact with cryogen

Pressure-relief devices must not be set higher than the containment's maximum allowable working pressure. It is further required that this pressure rating can be met with the usual safety at the temperature of minimum strength, and that embrittlement under cryogenic conditions be within acceptable limits.

Specific requirements exist for room temperature systems (oil-removal systems and gas-storage yards, located at the central helium facility and research yard), and the BaBar helium liquefier. These systems operate at medium pressure (15-20 atmospheres), and only ASME code-stamped vessels are used. Only ASME code-rated reliefs or rupture disks can be used to relieve pressure for these vessels.

Material Specifications

All parts in contact with the cryogens must be appropriate for cryogenic service. This is applicable for all operational modes of the cryogenic system. Assistance in selecting the appropriate materials will be provided by the Cryogenic Operations Group upon request. Appropriate material use will be checked as part of HEEC reviews.

In certain cases, HEEC reviews can provide a design basis for an exception from this requirement, for example niobium accelerator cavities that operate in direct contact with cryogens; experience with this material combined with careful design of containment systems renders niobium an acceptable material choice in this case even though Niobium metal is not rated for this use.

System Interlocks

Several systems at SLAC make use of interlocks to either prevent entry or shut off gas flow when undesired operating conditions exist. Examples of such systems include the "short" NLCTA clean room in End Station B and a Group E laboratory in the Central Annex. In one such type of interlock, the absence of makeup air (say, for instance, due to a failure of the local HVAC system) disable a nitrogen boil-off purge gas and thus serves as an engineering control on the hazard. In another, a fixed alarm would be interlocked in a feedback loop to the supply valve and shut off the flow of gas in the system. Each cryogenic system is unique and the use of interlocks should be considered whenever feasible.

ODH Area Classification and Inventory

Following conduct of the ODH risk assessment and review of the proposed engineering controls, HEEC will issue a pre-operational approval and/or an operational approval (subject to certain conditions) to the project sponsors. (See the HEEC website for approval examples.) The cryogenic and ODH safety program manager, who serves as the HEEC secretary, will make a determination as to which of the ODH hazard classifications (ODH 0 through ODH 4) will be applied to the cryogenic work area. These classifications are subject to review by HEEC and will be approved in writing. The classification decision in turn will result in specification of which hazard mitigation procedures are necessary to be followed for the subject work area. (See Tools-ODH Control Requirements, for which controls are applicable to which of the different area classifications.)

Following classification determination, the specified controls will be implemented and the cryogenic work area will be posted by the area/facility manager with signs corresponding to the degree of ODH hazard. Signs may be obtained from either the Cryogenic and Oxygen Deficiency Hazard Safety program manager or the confined space program manager; these individuals will be available to assist with signage as necessary. (See Tools-ODH Signs.)

The cryogenic and ODH safety program manager will maintain an inventory of ODH areas and their classifications and make it available through the web. This inventory will be updated whenever an official HEEC record of decision is entered. (For a snapshot of the ODH area inventory on the date of chapter publication, see Tools-ODH Locations.) The confined space program manager will assist the cryogenic and ODH safety program manager with mapping the ODH areas and all permanently installed atmospheric monitoring equipment and making these maps available for use by all parties.

Cryogen Dispensing and Usage

Cryogen storage facilities of all sizes and scale exist at SLAC. Many of these facilities have dispensing capabilities. As examples, liquid nitrogen from a bulk storage tank may be transferred to a trailer-mounted tank or from a dewar to a small research sample preparation container.

Eye, hand, and body protection are necessary to prevent potential cold burns when handling cryogens.

It is the responsibility of line managers and supervisors to ensure that PPE appropriate to the size and scale of the dispensing system is readily known to all operators. A solution in use at several facilities simply posts in picture format the PPE required for dispensing operations at that location. The industrial hygiene program manager is available to assist line managers and supervisors in making such determinations.

General Cryogen Hazard Prevention and Response Guidelines

See Tools-General Guidelines.