Author: F. Silva/B. Childs/M. Greenwald/A. Eckmann
Date: April 8, 1992, Updated 1/7/1999 by C. Fiore
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 300 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
Cryogenic Liquid
A liquid which exists at a temperature of -100°F or -60°C to
-460°F or -266°C. At these temperatures, tissue burns may be sustained
after contact with the fluids, surfaces cooled by the fluids, or by evolving
gases. The hazard is comparable to that of handling boiling water.
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
- Preparation
- Be familiar with the hazards of the liquid in use.
- 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.
- 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.
- 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.
- Have pot holder-type pads or the appropriate gloves on-hand; do not use
rags to touch uninsulated equipment surfaces.
- Examine containers and pressure relief valves for signs of defects. If
any defect is suspected, notify a supervisor immediately.
- 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.
- 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.
- Transfer and Use
- 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.
- 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.
- 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.
- Do not lower experiments into storage dewars unless provisions have been
made to vent the dewar and prevent freezing in the narrow neck.
- 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.
- Immediately re-cap any container to prevent atmospheric moisture from
entering and forming an ice plug.
- 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.
- 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:
- 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.
- Check the liquid helium level in the Dewar prior to beginning
transfer.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Handling and Storage
- Handle cryogenic storage containers with extreme caution. They are
fragile and expensive.
- 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.
- 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
