PSFC Cryogenic SOP
Plasma Science and Fusion Center
Office of Environment, Safety, and Health
190 Albany Street, NW21 2nd floor
617-253-8440 (Catherine Fiore)
617-253-8917 (Matt Fulton)
617-253-5982 (Bill Byford)
617-258-5473 (Nancy Masley)
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
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
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.
This procedure covers employee
safety during the handling and use of
cryogenic liquids dispensed from Dewar containers into secondary
Failure to follow this procedure
could result in: tissue burns resulting
from direct contact with cryogenic liquids; pressure build-up in
spaces; physical injury from embrittlement of structural materials,
explosion which can result from the condensation of oxygen on cold
where the cryogenic liquid has a lower boiling point than liquid
fires, explosions, and asphyxiation, which can result from the
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
available from MIT cryogenics, which are much less susceptible to
One liter of liquid nitrogen will
vaporize to approximately 700 liter of
gas, and one liter of liquid helium will vaporize to approximately 900
of gas, with the result that rapid and violent pressure changes can
particularly in confined vessels. Therefore, vent systems must be
above the bath space and in vacuum spaces which surround the fluid.
Experimental equipment lowered into cryogenic liquids may have spaces
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
pressure per square inch may be needed to maintain these gases at
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
Cold nitrogen gas boiling off from
the liquid state stratifies in low areas
and can cause an oxygen deficient atmosphere to develop.
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
gases. The hazard is comparable to that of handling boiling water.
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.
supervisor or responsible person shall be familiar with the hazards
associated with cryogenic liquids and appropriate equipment necessary
proper handling described in this document.
liquids shall only be dispensed or used in appreciable quantities
in well-ventilated areas.
- Hand Protection. Gloves must not be worn where there is
danger of a spill, splash, or overflow getting inside the glove and
the material to the skin. If protection for the hands is required when
containers or cold metal parts, a pot holder-type pad is preferable.
not acceptable, as they have to be layered many times to be effective.
may be used where tha main hazard is skin contact with cold parts if
taken to see that cold liquid cannot enter the top of the glove and be
against the skin.
- Eye Protection. Eye protection must be worn at all times
handling these liquids, because the liquid is almost always boiling and
splash into the eyes. Where stationary dispensing stations have been
or where splashing is a definite hazard, face shields may be required.
- Protective Clothing. Avoid clothing, jewelry, or other
capable of trapping or holding a cryogenic liquid in contact with the
Cuffless trousers, an impervious apron or coat, and high topped shoes
required in certain situations.
- Secondary Containers. ADewar flask is specially sealed to
liquid and gas from penetrating the outer vessel. Only use containers
for use with cryogenic liquids. Never use a thermos bottle for
service since the inner and outer vessels are not adequately sealed at
mouth and the container may explode.
- Transfer Tubes. When transferring liquid from a 150 or 240
nitrogen Dewar or smaller He container, use a fitted transfer tube.
nitrogen transfer tubes shall be covered with insulation to prevent
with the metal surface of the tube and to avoid condensation of oxygen
surface. Liquid helium transfer tubes are insulated by a vacuum space
an inner and outer tube.
- Be familiar with the hazards of the liquid in use.
- Work in an open, well-ventilated location. Even non-toxic
resulting from evaporation of cryogenic liquids can cause asphyxiation
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,
determine whether the room size and ventilation are sufficient to
volume of gas evolved and to prevent hazardous gas concentrations in
of a leak or spill of the amount of material typically used. Continuous
monitoring should be used whenever the potential for an oxygen
- An eyewash and/or sink should be located nearby to flush
with water in the event of contact or splashing. If there is no eyewash
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
protective equipment as required by the operation. Remove clothing or
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
any defect is suspected, notify a supervisor immediately.
- Any exposed glass areas of dewars should be taped with
rubberized tape to prevent spattering of broken glass in the event that
container implodes. Masking tape becomes brittle with age and should
- Keep equipment and systems clean and free of oil, grease, or
materials which may create hazardous conditions upon contact with the
fluids or with condensed oxygen.
- Transfer and Use
- Use only fitted transfer tubes designed for use with the
container. Cracked or damaged insulation on transfer tubes should be
Do not handle transfer tubes with bare hands, as the fitting is not
- When transferring into a secondary container, it is generally
not a good
idea to stand holding the container during filling. It should be placed
stable stand or hung so that the transfer tube rests securely inside
of the secondary container.
- When transferring into a secondary container, do not fill the
container to more than 80% of capacity. If the possibility exists that
temperature of the full cylinder might be increased to above 30oC, do
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
contained in small open surface tanks. Always hang the component on a
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
If a fog develops over the liquid surface, do not blow on it or try to
it off with the hand, because more fog will be created. The fog will
- Immediately re-cap any container to prevent atmospheric
entering and forming an ice plug.
- The vacuum leak detector located in NW21-167 has specific
operating procedures provided by the manufacturer which describe the
use of the
cryogenic trap. These instructions shall be followed when using
- 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,
Dewar, the neck of the container shall be reamed out at least twice
before and after transfer. Perform reaming with a hollow rod,
copper, which has a stop to prevent inserting the rod too far and
bottom of the inner vessel. Copper is best but rods of other materials
used, as long as they are hollow to allow vapors to escape. Note that
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
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
exhaust line may become extremely cold and present a safety hazard.
- Check the liquid helium level in the Dewar prior to
- 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
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
- 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
Insert the rubber fitting into the insertion fitting of the Dewar.
- The tube must be lowered slowly into the Dewar, otherwise
will boil off rapidly, with the result that cold gas will freeze the
break the seal, a potentially hazardous condition. Lower the tube
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
inserted, the pressure should not exceed 5 psi. The rated pressure for
Dewar burst plate may vary with manufacturer.
- Allow the transfer to proceed. As the transfer continues,
pressure drops. Slowly continue to lower the transfer tube into the
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
fully down or shortly thereafter). Dry helium may be used to pressurize
container through a fitting designed for this purpose. It is EXTREMELY
IMPORTANT to purge the line with dry helium prior to pressurizing the
the liquid helium will freeze air and other gases, creating a hazardous
- Handling and Storage
- Handle cryogenic storage containers with extreme caution.
fragile and expensive.
- Do not roll the containers by holding the neck, as it is the
support for the inner vessel of the container and is susceptible to
Always use dollies for moving storage containers.
- Do not allow moisture to come in contact with storage
equipment as it can freeze and plug up the relief devices.
supervisor shall supply this procedure to affected employees and verify
that they understand it. Employees should understand the health and
hazards of cryogenic liquids. Appropriate maintenance of cryogenic
and handling procedures should be emphasized.
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
oxygen. In addition, glass and other materials may shatter upon contact
a small splash or spill will rapidly evaporate into the atmosphere.
In the event of a large spill, the employee should restrict access to
area and dial 100.
Emergency and First Aid Procedures
cases of contact or splashing shall be reported to the
supervisor. In all cases of contact or splashing, immediately flush the
and clothing affected with water. Water is used because of its high
capacity. This is sufficient treatment in most cases where contact is
contact may cause serious burns requiring more sophisticated
medical treatment or blood clots. Any individual with a severe burn, or
liquid held in contact with the skin by gloves or clothing shall be
the MIT Medical Department with a follow-up examination at
all cases of eye contact, the affected individual shall be immediately
examined in the Ophthalmology Department with a follow-up examination
Environmental Medical Services.
individual has lost consciousness due to asphyxiation, dial 100 for
emergency medical assistance.
- MIT Accident Prevention Guide, Chapter 8. Hazardous Materials,
- Prudent Practices for Handling Hazardous Chemicals in
National Research Council, National Academy Press, 2101 Constitution
Washington, D.C., 20418 (1981)
page maintained by Catherine L. Fiore FIORE@PSFC.MIT.EDU