Figure 3-1 Valve Sequencing for an LN2 Fill Station
| MN471000, Pressure Safety Manuals Sponsor: Michael W. Hazen, 4000 |
Revision Date: August 4, 2011 Replaces Document Dated: October 31, 2008 |
This document is no longer a CPR. This document implements the requirements of Corporate Procedure ESH100.2.PS.1, Control Pressure Safety Hazards.
IMPORTANT NOTICE: A printed copy of this document may not be the document currently in effect. The official version is the online version located on the Sandia Restricted Network (SRN).
Subject Matter Experts: Shane Page, and David Paoletta
Contributor: Pressure Safety Committee
MN471000, Issue E
Revision Date: August 4, 2011; Replaces Document Dated: October 31, 2008
Review Date: October 16, 2006
Administrative Changes: January 11, 2010, and January 19, 2012
For purposes of this chapter, Members of the Workforce are:
This document applies to all operations that use cryogenic fluids in either open or closed handling operations.
Although this document addresses cryogenic safety issues in general, it primarily focuses on, and gives specific examples of, the inert cryogenic fluids of liquid nitrogen (LN2), liquid helium (LHe), and liquid argon. Liquid nitrogen is the most frequent cryogenic application at SNL.
A thorough evaluation of the safety of a cryogenic application may require a joint effort involving safety engineering, industrial hygiene, and facilities organization(s) for design and maintenance. Division ES&H Teams provide assistance in establishing the appropriate reviews.
The ES&H Teams and the Safety Engineering Department are responsible for the development, approval, revision, and administration of this document. Forward suggestions for improvement to the manager of the ES&H Teams and Safety Engineering Department.Managers shall ensure that Members of the Workforce complete the required training prior to performing the indicated work activity or fulfilling the indicated role. Untrained Members of the Workforce may temporarily work under the direct supervision of an appropriately qualified Member of the Workforce if the conditions/limitations of such work are documented (e.g., specific activities and duration) prior to performing the work.
Members of the Workforce whose activities involve cryogenics shall:Members of the Workforce whose activities involve the use of cryogenic fluids may use SF 2001-PQF, Pressure System Operator Qualification Form (Word file/Acrobat file), to document their qualifications, including training.
See HR100.2.1, Identify and Complete Sandia Required Training for all Members of the Workforce for the corporate Sandia-required training policy.
Cryogenic burns can be serious. Members of the Workforce shall select the appropriate level of protection commensurate with their application. Considerations include:
See Section 5.0 of this chapter for more information on personal protective equipment.
Table 3-1 Properties of Common Cryogens
| Cryogen | Temperature @ 1atm °F (K) |
Liquid/gas expansion |
Pressure generated from trapped liquid, allowed to warm to room temperature |
|---|---|---|---|
| Liquid oxygen LOX | -297 (90.2) | 860 to 1 | Not specified |
| Liquid argon LAr | -302 (87.3) | 847 to 1 | Not specified |
| Liquid nitrogen LN2 | - 320 (77.4) | 696 to 1 | 43,000 psi |
| Liquid hydrogen LH2 | -423 (20.3) | 851 to 1 | 28,000 psi |
| Liquid helium LHe | -452 (4.2) | 757 to 1 | 18,000 psi |
The list of hazards below (Table 3-2) should not be considered all-inclusive. Seek additional guidance from the appropriate division ES&H team for a thorough hazard analysis and safe operation of cryogenic fluid handling systems.
Table 3-2 Hazards Associated with Cryogens
| Hazard | Description |
|---|---|
| Thermal (low temperature) |
|
| Pressurization |
This points out the potential for the rupture of piping or vessels if appropriate venting and pressure relief is not provided. The function of vent lines can be defeated by the formation of ice (from condensed moisture) in the vent line. With LHe, air or other gases can be solidified to form this blockage. If a cryogenic fluid is subjected to a large amount of heat input, a flash vaporization can occur. This will result in a rapid pressure rise that can be described as a BLEVE (boiling liquid expanding vapor explosion). |
| Venting | Required vents and pressure relief devices should be vented to a safe location, determined by taking into consideration:
|
| Oxygen deficiency, asphyxiation |
Cryogenic fluids have large liquid to gas expansion ratios:
With this in mind, it should be noted that any accidental release or overflow of these cryogenic liquids will quickly boil into the gas phase and may create an asphyxiation hazard by displacing the oxygen content of the surrounding area. In the case of liquid nitrogen, the nitrogen gas generated from malfunctioning equipment or spills of LN2 will be cold and denser than ambient air. Even well-ventilated lab spaces that have pits or other low-lying (or recessed) areas could have the oxygen displaced by this cold, dense N2 gas. Argon or carbon dioxide will also present these heavier-than-air hazards. Large volume sources used in small laboratory spaces or in poorly ventilated areas increase the asphyxiation hazard. Oxygen monitors may be advisable in some applications. |
| Ice build-up | The temperatures associated with cryogenic liquids can easily condense moisture from the air and cause the formation of ice. This ice can cause a malfunction from the design intent of components or systems (e.g., plug vent lines and impede valve operation) or can damage piping systems. In the case of liquid helium, air itself can freeze solid and block vent lines. Building exhaust systems accidentally cooled to LN2 temperatures can also be damaged by ice formation (or the weight of the accumulated ice and the weight of the LN2 itself). The resultant run-off water when the ice melts can also present a hazard. |
| Materials concerns | The low temperature of cryogenic liquids will adversely affect the properties of some materials, resulting in system or vessel failure. The selection of the materials of construction for vessels and piping systems for cryogen handling should consider the appropriate behavior of the material at the cryogenic temperatures. Metal: In general, carbon steels and other bcc-structured metals can become brittle and fracture easily at cryogenic temperatures. Commonly accepted materials of construction include fcc-structured metals such as the 300 series stainless steels, some of the aluminum alloys, and copper or brass. Plastic: Plastics, such as Tygon® tubing, become brittle and can easily fail in cryogenic applications. Be sure to consult the appropriate references when selecting materials for cryogenic applications. Applications: Even when the appropriate materials are selected, thermal stresses that can lead to failure can be generated in some applications. Thermal gradients across a material or piping system or the rapid cool-down of a vessel can generate thermal stresses. The joining of materials with dissimilar coefficients of expansion can also generate thermal stresses. |
| Oxygen enrichment | LN2 is cold enough to condense the surrounding air into a liquid form. The concentration of O2 in this condensed air is enhanced. This condensed "liquid air" can be observed dripping from the outer surfaces of uninsulated/nonvacuum jacketed lines carrying LN2. This "liquid air" will be composed of 50% O2, and will amplify any combustion/flammable hazards in the surrounding areas. Open dewars of LN2 can condense O2 from the air into the LN2 and cause an O2 enrichment of the liquid, which can reach levels as high as 80% O2. Air should be prevented from condensing into LN2 by the use of loose-fitting stoppers or covers that still allow for the venting of LN2 boil-off gas. Large quantities of LN2 spilled onto oily surfaces (such as asphalt) could condense enough O2 to present a combustion hazard. In some cases, such as a large-volume LN2 spill onto asphalt, the surface can become saturated with condensed oxygen and can be shock sensitive (can detonate when shocked). LHe can also condense air into the liquid or even the solid phase with an enriched O2 content. |
| Lifting, physical | Studies of accident statistics involving cryogenics will always include back strains or other lifting injuries associated with dewars. Although this hazard is not specifically cryogenic in nature, it is appropriate to note this as a hazard associated with cryogenic applications. Care should be taken in the lifting and movement of cryogenic dewars. The proper use of carts or hand trucks can help prevent these injuries. Alternately, the use of low-pressure liquid transfer equipment and procedures can replace lifting and pouring operations. |
| LN2 in ionizing radiation field | A unique hazard can result from the use of LN2 in high ionizing radiation fields where the generation of ozone or nitrogen oxides may cause a potential explosion hazard when the LN2 has condensed quantities of oxygen from the atmosphere. The applicable control measure is to minimize the accumulation of oxygen into the LN2 and to keep containers free of hydrocarbon contamination. |
| Noise | Transfer or venting of cryogens can generate, in some cases, noise levels that could require hearing protection. Sound levels in excess of 150 dBA have been recorded during routine tank filling. A redesign of the equipment or procedure could also be addressed in these cases. |
| Other, specific | Other cryogenic fluids will present specific hazards in addition to the above concerns. Examples include:
|
SNL personnel should have an awareness of known, common accident scenarios for the SNL environment (Table 3-4).
Table 3-3 Common Accident Scenarios Involving
Cryogens at Sandia National Laboratories
| Injury/Event | |
|---|---|
| Accidental releases or overflows | Accidental releases or overflows of LN2 can present hazards and cause property damage (Table 3-2), most often as a result of inadequate training on the specific hazards and procedures. These releases can come from automated level control systems, but more frequently are the result of manual operations left unattended. The level of concern over these releases increases with the volume of the cryogen source. LN2 house systems represent very large quantities with the potential for release. Separate (stand-alone) supply dewars are inherently safer in this respect because they have smaller volumes. Releases into the building or lab space are the most hazardous, presenting the primary hazards of asphyxiation, personnel exposure, and property damage may result from significant releases of LN2. |
| Releases into building exhaust systems | Releases into building exhaust systems also can present significant hazards. These releases typically occur when the operator opens a bypass valve in an attempt to pre-cool the piping to LN2 temperatures and then mistakenly leaves the bypass valve open. See Figure 3-1 for a typical house LN2 system configuration. These releases can adversely affect the normal operation of the building’s exhaust system or can cause the exhaust system to fail and release significant quantities of LN2 into the building’s air space. Figure 3-1 shows one of the piping configurations installed in some SNL buildings. This configuration presents the potential for human error in that the bypass valve can be left open and the system emptied without the operator knowing it. |
| Pressure buildup (pressure relief valves) | Pressure relief valves (PRVs) are required on cryogenic liquid piping systems to prevent excess pressure build-up when the liquid is trapped between closed valves. These PRVs should be vented to a safe location (not into the lab or the ceiling plenum) in order to prevent a hazardous accidental release upon actuation of the valve or on failure of the valve to re-seal. In addition, the PRV should be rated for that specific application. PRVs rated for LN2 are not appropriate for liquid CO2 applications. |
| Back injuries | Back injuries may result from lifting cryogenic liquid dewars. |
| Tipping of dewars | Storage dewars of LN2 or LHe may be accidentally tipped over when crossing obstructions, such as door thresholds. Handle these dewars with the appropriate care and on the appropriate floor surfaces. |
| Accidents caused by equipment failure (equipment not designed for cryogenic service) | Cryogenic fluids should only be handled in apparatus specifically designed for that purpose. Accidents frequently occur where equipment not designed for cryogenic service is used, such as when a consumer-rated Thermos® bottle is used for LN2 or dry ice. Over pressure and resultant rupture of the container is frequently the result. These types of accidents can also occur when cryogenic-rated equipment is inappropriately modified and the original safe venting design is compromised. |
Figure 3-1 Valve Sequencing for an LN2 Fill Station
Cryogenic system owners shall be responsible for the design, operation, and maintenance of laboratory cryogenic systems and shall verify that:
Laboratory personnel should not authorize or perform maintenance or modification on facilities-owned systems or the facilities-owned portion of a system.
Specific operator aids, such as valve sequencing checklists, are encouraged. Technical work documents may be needed in cases where the level of hazard or operational complexity warrants; see ESH100.2.GEN.3, Develop and Use Technical Work Documents.Engineered controls should be the primary means of worker protection. Where engineered controls may not be complete or feasible, workers shall wear the appropriate personal protective equipment (PPE) to augment any engineered controls in place. Members of the Workforce shall wear:
Cryogenic system owners should be involved in the design of cryogenic fluid handling systems, such as fill stations, to verify that systems are designed to minimize exposure to the liquids, gases, and cold surfaces.
The handling of cryogens within closed systems with the controlled venting of liquids or boil-off gases may not require the use of PPE. Each system should be carefully evaluated for potential exposure to personnel. The system integrity and layout, as well as the frequency of making and breaking connections should be taken into consideration. Remember to vent system pressure before breaking connections. Consider using PPE (eye and hand protection) when breaking connections in order to prevent exposure to residual amounts of liquid or cold gases.
Members of the Workforce may provide pressure relief by opening vent lines, pressure relief valves or burst disks, depending on the application. See Figure 6-1 for an illustration of pressure relief locations.
|
Members of the Workforce should verify that material and equipment that are used in cryogenic applications are constructed only of materials that do not become brittle and hazardous at low temperatures. In general, carbon steels and iron become brittle and fracture easily at cryogenic temperatures and are not suitable for these applications. Common acceptable materials include the 300 series stainless steels, copper, and brass. If brittle materials are used, equipment owners should consider mitigating hazards by using shielding or remote testing.
Members of the Workforce should consider using oxygen monitors in laboratories where the potential to create an oxygen-deficient atmosphere warrants them. Contact the appropriate division ES&H team for assistance in determining the need for oxygen monitors.
At the conclusion of operations, Members of the Workforce shall verify that appropriate valves are shut off.
In the event of an emergency, Members of the Workforce shall:
The following guidance applies to house-supplied liquid nitrogen fill stations located within SNL laboratory spaces. Table 9-1 below can be used to develop an operational procedure for these fill stations. Specific system procedures should always be reviewed by persons who are knowledgeable of the installation.
To create site-specific fill station instructions, include the following:
Table 9-1 Typical Operational Procedure for Valve Sequencing
| Step | Action |
|---|---|
| PREPARATION FOR USE | |
| 1 | Check that the oxygen monitor (if applicable) is operational. |
| 2 | Locate flex lines and fill valve away from point of discharge. |
| 3 | Make any connections for fill or vent that are required. |
| 4 | Position the dewar for filling. |
| 5 | Don personnel protective equipment (PPE). Wearing safety glasses is required for the open handling ofLN2. Additional PPE, such as face shield and gloves, may also be required. Need is dependent on the specific system design and operation. |
| USE | |
| Caution: The safe use of this system requires 100%-manned operation. Operators should not leave the area with either the valve in the open or partially open position. | |
| 6 | When necessary, pre-cool theLN2 line by opening the bypass valve and allowing flow until the supply line is adequately pre-cooled. This may be evidenced by frost appearing on the valve or piping and components. Do not leave the area during line pre-cool. The line is pre-cooled in an effort to minimize the quantity of nitrogen gas that will be vented into the laboratory air space during the filling process.
|
| 7 | Close the bypass valve at this time. The line should be adequately pre-cooled so that when the fill valve is opened, liquid phase nitrogen (or a minimum of gas phase) will flow. |
| 8 | Slowly open the fill valve. You should be able to hear the LN2 flowing through the line and into the dewar. Do not leave the area during the dewar fill operation. |
| SHUTDOWN | |
| 9 | When the dewar is full, close the fill valve and ensure that both valves are in the fully closed position (90 degrees from the line orientation). |
| 10 | Vent off any residual pressure, then disconnect any connections made and remove the dewar from the fill station. |
Even relatively small quantities of liquid cryogens can damage equipment or facilities, by for instance, cracking floor tiles and damaging water pipes and the electrical insulation on wiring. Also, consider the hazard presented by the boil-off gas when any significant quantities of a cryogenic liquid are released.
Contact the appropriate division ES&H Team member for assistance in determining the best way to dispose of cryogenic liquids.
Members of the Workforce shall apply the requirements for pressure safety aspects of a cryogenic fluid handling system as stated in this Pressure Safety Manual.
For SNL designed and assembled systems, the system owner shall compile a data package according to the requirements in this Pressure Safety Manual.Any significant accidental releases should be reported to the appropriate manager and center ES&H coordinator. Notification through an emergency or nonemergency hotline may be appropriate, depending on the severity of the release. Any personnel in the vicinity who could be exposed to the hazards of the release should also be notified. A predetermined point of contact, such as the person responsible for ordering the product, could also be useful because the schedule for re-ordering may be affected by large volume releases.
Incidents that are reported to the nonemergency hotline are useful in tracking and analyzing accident and failure scenarios, determining trends, and changing engineering configuration or procedures.
Commercial (off-the-shelf) vessels may be used as is, but available owner or operator manuals should be retained for reference as part of the system data package.
For assistance on cryogenic fluid applications, including safety engineering, industrial hygiene, and the facilities engineering and maintenance organizations, Members of the Workforce should contact the appropriate division ES&H Team member. The organization's pressure advisor may also provide assistance.
Members of the Workforce should contact facilities organizations, such as the Mechanical and Civil Engineering Department, for assistance with the design, installation, maintenance, and modification of liquid nitrogen house systems.
Hazards and activities related to the use on cryogenic fluids include:
| Hazard/Activity | Reference |
|---|---|
| Asphyxiation | ESH100.2.IH.19, Evaluate and Control Asphyxiant Hazards |
| Confined spaces | ESH100.2.IH.9, Enter Confined Spaces Safely |
10 CFR 851, Worker Safety and Health Program.
29 CFR 1910.101, Compressed Gases
49 CFR 173.316, Cryogenic Liquids in Cylinders
ANSI/ASME B31.3, Process Piping
ASME Section VIII Division 1
CGA Publication P-12, Safe Handling of Cryogenic Liquids
NFPA 55, Standard for the Storage, Use, and Handling of Compressed Gases and Cryogenic Fluids in Portable and Stationary Containers, Cylinders, and Tanks
Public Law 91-596
International Fire Code (IFC)
British Cryogenics Council, Safety Panel, Cryogenics Safety Manual
Edeskuty, F. J., and W. F. Stewart, Safety in the Handling of Cryogenic Fluids
Timmerhaus, K. D., and T. M. Flynn, Cryogenic Process Engineering
Shane Page, srpage@sandia.gov
Al Bendure, aobendu@sandia.gov
