PSFC Cryogenic SOP

Plasma Science and Fusion Center

Office of Environment, Safety, and Health

Be Safe or Die

Reviewed and Approved By:

PSFC Supervisor--Chris Reddy

PSFC Supervisor--Amanda Hubbard

PSFC Safety Officer--Catherine L. Fiore

Standard Operating Procedures for Handling Cryogenic Liquids

Author: F. Silva/B. Childs/M. Greenwald/A. Eckmann

Version: 1.2

Date: April 8, 1992, Updated 1/7/1999 by C. Fiore

Persons Responsible: Frank Silva - NW21-105

Bob Childs - NW21-109

Paul Thomas - NW17-276

Martin Greenwald - NW17-107

All PSFC Supervisors

Introduction

This document will outline the hazards involved with the handling and use of cryogenic liquids. It will detail procedures to be followed to minimize the risk of exposure of Plasma Science and Fusion Center employees to hazardous concentrations of gases and physical hazards resulting from the evaporation of cryogenic liquids.

Liquid nitrogen is the most frequently used cryogenic liquid at the PSFC. It may be obtained from MIT Cryogenics in 150 or 240 liter metal-clad Dewar containers. It is also available through a transfer station in the Alcator setup lab, Room NW21-156. This comes from the large LN2 storage tank behind NW21 which is maintained for the Alcator project. Liquid helium is obtained from MIT Cryogenics in specially designed Dewar containers. These cryogens are transferred into secondary systems or containers, and may be poured from a secondary container into a vacuum trap or test vessel.

Scope

This procedure covers employee safety during the handling and use of cryogenic liquids dispensed from Dewar containers into secondary containers or systems.

Safety Analysis

Failure to follow this procedure could result in: tissue burns resulting from direct contact with cryogenic liquids; pressure build-up in unvented spaces; physical injury from embrittlement of structural materials, fires and explosion which can result from the condensation of oxygen on cold surfaces where the cryogenic liquid has a lower boiling point than liquid oxygen; and fires, explosions, and asphyxiation, which can result from the evaporation of cryogens.

Oxygen has a higher boiling point (-183°C) than nitrogen (-195°C) or helium (-269°C). Thus, oxygen can be condensed out of the atmosphere during use of these lower-boiling liquids.

Liquid helium has historically been especially prone to forming an ice-plug in the neck of containers. New, wider-mouthed dewars for liquid helium are available from MIT cryogenics, which are much less susceptible to freeze-up.

One liter of liquid nitrogen will vaporize to approximately 700 liter of gas, and one liter of liquid helium will vaporize to approximately 900 liters of gas, with the result that rapid and violent pressure changes can occur, particularly in confined vessels. Therefore, vent systems must be provided above the bath space and in vacuum spaces which surround the fluid. Experimental equipment lowered into cryogenic liquids may have spaces into which the fluid can leak, and when warmed up can rupture because of gas expansion. The forces created are great and tens of thousands of pounds of pressure per square inch may be needed to maintain these gases at liquid density at room temperature.

Structural materials used with cryogenic liquids must be selected with care, because the extreme cold drastically alters the basic properties of many materials.

Cold nitrogen gas boiling off from the liquid state stratifies in low areas and can cause an oxygen deficient atmosphere to develop.

Definitions

Responsibilities

The supervisor or responsible person shall designate and train employees who are required to use cryogenic liquids. The supervisor or responsible person shall ensure that necessary safety equipment is available. The supervisor or responsible person shall ensure that cryogenic liquids are handled in accordance with good work practices in adequately ventilated areas.

Assumptions

The supervisor or responsible person shall be familiar with the hazards associated with cryogenic liquids and appropriate equipment necessary for proper handling described in this document.

Cryogenic liquids shall only be dispensed or used in appreciable quantities in well-ventilated areas.

Required Supplies

• Hand Protection. Gloves must not be worn where there is danger of a spill, splash, or overflow getting inside the glove and freezing the material to the skin. If protection for the hands is required when handling containers or cold metal parts, a pot holder-type pad is preferable. Rags are not acceptable, as they have to be layered many times to be effective. Gloves may be used where tha main hazard is skin contact with cold parts if care is taken to see that cold liquid cannot enter the top of the glove and be held against the skin.

• Eye Protection. Eye protection must be worn at all times while handling these liquids, because the liquid is almost always boiling and can splash into the eyes. Where stationary dispensing stations have been installed or where splashing is a definite hazard, face shields may be required.

• Protective Clothing. Avoid clothing, jewelry, or other items capable of trapping or holding a cryogenic liquid in contact with the body. Cuffless trousers, an impervious apron or coat, and high topped shoes may be required in certain situations.

• Secondary Containers. ADewar flask is specially sealed to prevent liquid and gas from penetrating the outer vessel. Only use containers approved for use with cryogenic liquids. Never use a thermos bottle for cryogenic service since the inner and outer vessels are not adequately sealed at the mouth and the container may explode.

• Transfer Tubes. When transferring liquid from a 150 or 240 liter nitrogen Dewar or smaller He container, use a fitted transfer tube. Liquid nitrogen transfer tubes shall be covered with insulation to prevent contact with the metal surface of the tube and to avoid condensation of oxygen at the surface. Liquid helium transfer tubes are insulated by a vacuum space between an inner and outer tube.

Procedures

1. Preparation

   1. Be familiar with the hazards of the liquid in use.

   2. Work in an open, well-ventilated location. Even non-toxic gases resulting from evaporation of cryogenic liquids can cause asphyxiation by displacement of oxygen. Never work directly over an open vat of liquid nitrogen. Unconsciousness occurs without warning in an oxygen deficient atmosphere. In more confined rooms (e.g., NW17-131, NW17-259, NW21-156B) determine whether the room size and ventilation are sufficient to disperse the volume of gas evolved and to prevent hazardous gas concentrations in the event of a leak or spill of the amount of material typically used. Continuous oxygen monitoring should be used whenever the potential for an oxygen deficiency exists.

   3. An eyewash and/or sink should be located nearby to flush affected areas with water in the event of contact or splashing. If there is no eyewash or sink nearby, another employee who can escort the affected employee to such facilities shall remain in the vicinity of the operation.

   4. Ensure that eye and/or face protection is worn. Wear other personal protective equipment as required by the operation. Remove clothing or jewelry which could trap cryogenic liquids against the skin.

   5. Have pot holder-type pads or the appropriate gloves on-hand; do not use rags to touch uninsulated equipment surfaces.

   6. Examine containers and pressure relief valves for signs of defects. If any defect is suspected, notify a supervisor immediately.

   7. Any exposed glass areas of dewars should be taped with plastic or rubberized tape to prevent spattering of broken glass in the event that the container implodes. Masking tape becomes brittle with age and should not be used.

   8. Keep equipment and systems clean and free of oil, grease, or other materials which may create hazardous conditions upon contact with the cryogenic fluids or with condensed oxygen.

2. Transfer and Use

   1. Use only fitted transfer tubes designed for use with the Dewar container. Cracked or damaged insulation on transfer tubes should be replaced. Do not handle transfer tubes with bare hands, as the fitting is not insulated.

   2. When transferring into a secondary container, it is generally not a good idea to stand holding the container during filling. It should be placed on a stable stand or hung so that the transfer tube rests securely inside the neck of the secondary container.

   3. When transferring into a secondary container, do not fill the secondary container to more than 80% of capacity. If the possibility exists that the temperature of the full cylinder might be increased to above 30oC, do not fill the secondary container to more than 60% capacity.

   4. Do not lower experiments into storage dewars unless provisions have been made to vent the dewar and prevent freezing in the narrow neck.

   5. Components may be tested by submerging them in cryogenic liquids contained in small open surface tanks. Always hang the component on a suitable wire or string to submerge it - never lower a part in by hand. Use a lid on the tank to reduce contact between the cryogenic liquid surface and the atmosphere. If a fog develops over the liquid surface, do not blow on it or try to "sweep" it off with the hand, because more fog will be created. The fog will eventually boil off.

   6. Immediately re-cap any container to prevent atmospheric moisture from entering and forming an ice plug.

   7. The vacuum leak detector located in NW21-167 has specific written operating procedures provided by the manufacturer which describe the use of the cryogenic trap. These instructions shall be followed when using cryogens in this equipment.

   8. Special Procedures for Transferring Liquid Helium:

      Helium should be handled in super-insulated, wide-mouthed dewars as supplied by MIT cryolabs. If it is absolutely necessary to use an older, narrow-necked Dewar, the neck of the container shall be reamed out at least twice daily and before and after transfer. Perform reaming with a hollow rod, preferably copper, which has a stop to prevent inserting the rod too far and damaging the bottom of the inner vessel. Copper is best but rods of other materials may be used, as long as they are hollow to allow vapors to escape. Note that this procedure often results in a splashing liquid or a burst of cold gas.

      Older-style helium storage dewars have an outer container which is filled with liquid nitrogen to act as a heat shield and must be kept filled to be effective.

      The following transfer procedures shall be followed:

      1. If possible, run an exhaust line from the Dewar relief vent to an an open area away from people and equipment. Any uninsulated portions of the exhaust line may become extremely cold and present a safety hazard.

      2. Check the liquid helium level in the Dewar prior to beginning transfer.

      3. Unless you have used the transfer tube very recently, check the tube vacuum. A vacuum of 10 -3 to 10 -4 torr is usually required: the transfer tube manufacturer instructions should be consulted.

      4. The end of the transfer tube which is inserted into the Dewar should be cut at a sharp angle so that tube end does not rest flat against the bottom of the Dewar.

      5. The end of the transfer tube which is inserted into the Dewar has a rubber fitting, through which the tube slides for insertion into the Dewar. Insert the rubber fitting into the insertion fitting of the Dewar.

      6. The tube must be lowered slowly into the Dewar, otherwise the liquid will boil off rapidly, with the result that cold gas will freeze the rubber and break the seal, a potentially hazardous condition. Lower the tube slowly into the Dewar so that it takes about a minute to touch the liquid.

      7. While lowering the tube, watch the pressure gauge on the Dewar. Prior to insertion of the tube, it should indicate 1 atmosphere pressure. As the tube is inserted, the pressure should not exceed 5 psi. The rated pressure for the Dewar burst plate may vary with manufacturer.

      8. Allow the transfer to proceed. As the transfer continues, the Dewar pressure drops. Slowly continue to lower the transfer tube into the Dewar, allowing the pressure to rise to 5 psi.

      9. After most of the liquid has been transferred, the pressure will not be sufficient to complete the transfer (this ususally happens once the tube is fully down or shortly thereafter). Dry helium may be used to pressurize the container through a fitting designed for this purpose. It is EXTREMELY IMPORTANT to purge the line with dry helium prior to pressurizing the Dewar, as the liquid helium will freeze air and other gases, creating a hazardous condition.

3. Handling and Storage

   1. Handle cryogenic storage containers with extreme caution. They are fragile and expensive.

   2. Do not roll the containers by holding the neck, as it is the main support for the inner vessel of the container and is susceptible to damage. Always use dollies for moving storage containers.

   3. Do not allow moisture to come in contact with storage containers and equipment as it can freeze and plug up the relief devices.

Training

The supervisor shall supply this procedure to affected employees and verify that they understand it. Employees should understand the health and physical hazards of cryogenic liquids. Appropriate maintenance of cryogenic equipment and handling procedures should be emphasized.

Spill Procedures

Themajor hazard of a cryogenic liquid spill is the evaporation resulting in displacement of oxygen and asphyxiating atmospheres, or the fire and explosion hazards from evaporation of flammable gases or condensation of oxygen. In addition, glass and other materials may shatter upon contact with cryogenic fluids.

Normally a small splash or spill will rapidly evaporate into the atmosphere. In the event of a large spill, the employee should restrict access to the work area and dial 100.

Emergency and First Aid Procedures

All cases of contact or splashing shall be reported to the supervisor. In all cases of contact or splashing, immediately flush the areas and clothing affected with water. Water is used because of its high heat capacity. This is sufficient treatment in most cases where contact is not prolonged.

Prolonged contact may cause serious burns requiring more sophisticated medical treatment or blood clots. Any individual with a severe burn, or who had liquid held in contact with the skin by gloves or clothing shall be examined at the MIT Medical Department with a follow-up examination at Environmental Medical Services.

In all cases of eye contact, the affected individual shall be immediately examined in the Ophthalmology Department with a follow-up examination at Environmental Medical Services.

Ifan individual has lost consciousness due to asphyxiation, dial 100 for emergency medical assistance.

References

This page maintained by Catherine L. Fiore FIORE@PSFC.MIT.EDU