Introduction
A Superconducting Magnet System can be operated easily and safely, provided the correct procedures are obeyed and certain precautions observed.
This safety section must be read and understood by everyone who comes into contact with a Superconducting Magnet System. They are NOT for the sole information of senior or specialist staff. Proper training procedures must be undertaken to familiarize effectively all persons concerned with such equipment with these requirements. Also since the field from the magnet is 3-dimensional, consideration must be given to floors above and below the magnet as well as the surrounding space on the same level.
The installation and operation of a Superconducting Magnet System presents a number of hazards of which all personnel must be aware. It is essential that:
- Areas in which magnet systems are worked on or used, and their installation generally, are planned with full consideration for safety.
- Such premises and installations are operated in a safe manner and in accordance with proper procedures.
- Adequate training is given to personnel.
- Clear notices are placed and maintained to effectively warn people that they are entering a hazardous area.
- All health and safety procedures are observed.
These notes outline aspects of operation and installation which are of particular importance, however, the recommendations given cannot cover every eventuality and if any doubt arises during the operation of the system, the user is strongly advised to contact the supplier.
It is the obligation of Oxford's customers to communicate effectively to their own customers and to users of the equipment in the information in this manual regarding safety procedures and hazards associated with magnet systems.
FLOOR LOADING
Professional assistance from a civil or structural engineer should be sought when considering any installation.
The Magnetic Field
Certain precautions must be taken to ensure that hazards will not exist due to the effect of a magnetic field on magnetic materials or on surgical implants. Typical of such effects are the following:
Large attractive forces may be exerted on equipment brought near to the magnet. The force may become large enough to move the equipment uncontrollably towards the magnet. Small pieces of equipment may therefore become projectiles, large equipment (e.g. gas bottles, power supplies) could cause bodies or limbs to become trapped between the equipment and the magnet. Either type of object may cause injury or death. The closer to the magnet, the larger the force. The larger the equipment mass, the larger the force.
The operation of medical implants, such as cardiac pacemakers, may be affected either by static or changing magnetic fields. Pacemakers do not all respond in the same way or at the same field level is exposed to fields above 5 Gauss.
Other medical implants, such as aneurysm clips, surgical slips, or prostheses, may contain ferromagnetic materials and therefore would be subject to strong forces near to the magnet. This could result in injury or death. Additionally, in the vicinity of rapidly changing fields (e.g. pulsed gradient fields), eddy currents may be induced in the implant resulting in heat generation.
The operation of equipment may be directly affected by the presence of large magnetic fields. Items such as watches, tape recorders, and cameras may be magnetized and irreparably damaged if exposed to fields above 10 Gauss. Information encoded magnetically on credit cards and magnetic tape including computer floppy disks, may be irreversibly corrupted. Electrical transformers may become magnetically saturated in fields above 50 Gauss. The safety characteristics of equipment may also be affected.
To prevent situations as described above from occurring, the following general precautions are provided as guidelines. These should be regarded as minimum requirements. Every magnet site location should be reviewed individually to determine precautions to be taken against the above hazards. Also, since the field from the magnet is 3-dimensional, consideration must be given to the floors above and below the magnet as well as the surrounding space on the same level.
Before ramping the magnet to field
The following precautions must be taken,
- Ensure all loose ferromagnetic objects are moved from within 2 meters of the OVC, or 3 meters for high field magnets ( > 11 Tesla).
- At all points of access to the magnet room display warning signs that the magnet is operating.
- Display warning signs giving notice of the possible presence of magnet fields and of the potential hazards in all areas where the field may exceed 5 Gauss.
- Ensure all electronics and interfacing equipment supplied by Oxford instruments are placed in areas where the field is less than 10 Gauss.
- The safe working field level of other equipment must be individually assessed by the system manufacturer.
After ramping the magnet to field
- Do not bring ferromagnetic objects into the magnet room.
- Use only non-magnetic cylinders and dewars for storage/transfer of compressed gas or cryogenic liquids. Equipment for transportation of cylinder/ dewars must be non-magnetic.
Fire and Explosion Hazards
In the case of fire evacuate personnel from the area and sound the fire alarm.
Water must not be used on electrical equipment and when sprayed on cryogenic liquids will rapidly freeze. The magnet ventilation may become blocked by ice with subsequent risk of explosion and the release of cryogens from the system.
The surface temperature of containers for liquid nitrogen and helium, if not vacuum insulated, may be sufficiently low to condense oxygen or oxygen enriched air. This liquid in contact with flammable substances can become explosive.
Portable fire fighting equipment must be non-magnetic and should be installed by agreement with the local fire authority.
Local emergency services must be informed of the presence of a magnet operating in their area as this may affect their procedures in dealing with fires or other accidents.
In case of a large cryogen spillage avoid direct contact with the liquid; sound fire alarm and evacuate the area.
Oil mist filters should be fitted to pumps to reduce the emission of toxic oil vapors which pose both health and explosion hazards.
The Safe Handling of Cryogenic Substances
Cryogenic liquids can be handled easily and safely provided certain precautions are obeyed. The recommendations in this section are by no means exhaustive and when in doubt, the user is advised to consult the supplier.
They safe handling of cryogenic liquids requires a knowledge of the properties of these liquids, common sense, and sufficient understanding to predict the future behavior of such liquids under certain physical conditions.
The substances referred to in these recommendations are nitrogen, air, and helium.
General Safety Rules
Cryogenic liquids, even when kept in insulated storage vessels (dewars), remain at a constant temperature at their respective boiling points and will gradually evaporate. The very large increase in volume accompanying this vaporization is approximately 700:1 for helium and nitrogen and therefore:
CONTAINERS OF CRYOGENIC LIQUIDS MUST NOT BE COMPLETELY CLOSED AS THIS WOULD RESULT IN A LARGE BUILD UP IN PRESSURE AND THUS PRESENT AN EXPLOSION HAZARD. The nitrogen jackets is always fitted with a non-removable overpressure valve, the OVC is fitted with a combined overpressure / evacuation valve, and the helium bath must have the large black and red quench valve fitted. (This is normally supplied in the spares kit or left attached to the helium exit port).
In the event of a large spillage operate the fire alarm and evacuate the area.
Health Hazards
Asphyxia of varying severity will occur if the magnet room is not properly ventilated. (Helium can displace air from the top of a room and cold nitrogen can displace air from lower levels).
Burns. Cryogenic substances in liquid or vapor form or as low temperature gases produce effects on the skin similar to burns (cold burns).
Exposed or insufficiently protected parts of the body coming into contact with uninsulated venting pipes or vessels (see ventilation section) will stick fast and the flesh will be torn if removed.
First Aid
If any of the cryogenic liquids come into contact with eyes or skin, immediately flood the affected area with large quantities of cold or lukewarm water and then apply cold compresses. NEVER use hot water or dry heat. MEDICAL ADVICE SHOULD BE SOUGHT IMMEDIATELY.
Protective Clothing
Protective clothing must be worn mainly to avoid cold burns and dry leather or PVC gloves must be worn when handling or working with cryogenic liquids. Gloves must be loose fitting so that they can be removed easily in case of liquid spillage. Eyes must be protected by goggles. Do not wear any metallic objects (e.g. jewelry) on those parts of the body where they may come into contact with the liquid.
Handling
Cryogenic liquids must be handled and stored in well ventilated areas.
Do not allow cryogens to come into contact with the body.
Always handle the liquids carefully - boiling and splashing will always occur when filling a warm container or when inserting warm objects into the liquid. When inserting open ended pipes into the liquid, block off the warm end until the cold end has cooled down (otherwise cold liquid may spurt out of the open end under self-generated pressure). Never direct pipe/piping towards any person.
Beware of liquid splashing and rapid flash-off of helium when lowering equipment at ambient temperature into liquid. This operation must be carried out very slowly.
Use only metal tubing connected by flexible metal hose for transferring liquid nitrogen. For the coupling DO NOT use rubber tube, silicon rubber tube (including hospital grade tube - this explodes!), or plastic tubing e.g. garden hose and including reinforced tubes e.g. for air lines - this shatters unexpectedly and may cause injury to personnel. It should be noted that polythene and nylon lines are sometimes used, however, this should not be taken as an implied recommendation, all lines should be tested in safe circumstances or used only after the manufacturer's recommendation.
Equipment
Only use containers specifically designed for use with particular cryogens and constructed of non-magnetic materials.
The weight of the system is approximately 1,000 kg. Appropriate lifting gear must be used to move the cryostat.
This system, when energized to full field has a considerable stray field, ex-tending over many meters, and the system stored energy is approximately 0.5 MJ. It is therefore VITAL that the safety section is read by ALL personnel coming near the system.
Liquid Nitrogen
Good ventilation is essential.
Store and use in a well ventilated place. If enough gas evaporates from the liquid in an unventilated place (e.g. overnight in a closed room) the oxygen concentration in the air may become dangerously low. Unconsciousness may result suddenly without previous warning symptoms and may be fatal. For example, the evaporation of 25 liters of liquid nitrogen produces 17,000 liters of nitrogen gas (600 cu. ft.). If this vaporization takes place in a room of 54 m3 (2,000 cu. ft.), i.e. 3 x 6 x 3 meters high (10 x 20 x 10 feet high) it can produce a very dangerous situation if the room is not ventilated. Appropriate multiplication of these parameters will indicate actual site conditions.
Minimize contact with air
Since liquid nitrogen is colder than liquid oxygen, the oxygen of the air will condense into the nitrogen and if allowed to continue for some time, the oxygen concentration may become so high that the liquid may become as dangerous to handle as liquid oxygen. This applies particularly to wide-necked dewars. Therefore ensure that contact with air is kept to a minimum.
Do not smoke (not just in the context of this manual. It's really bad for you.)
Rooms in which cryogenic liquids are being handled should be designated no smoking areas. While nitrogen and helium do not support combustion, their extreme cold can cause oxygen from the air to condense on cold surfaces and may increase the oxygen concentration locally. There is a particular fire danger if the cold surfaces are covered with oil or grease which is itself combustible.
Liquid Helium
Liquid helium is the coldest of all cryogenic liquids. It will therefore condense and solidify any other gas (air) coming into contact with it, with the consequent danger that pipes and vents may become blocked.
Liquid helium must be kept in specially designed, storage or transport dewars. Dewars should have a non-return valve fitted in the helium neck at all times in order to avoid air entering the neck and plugging it with ice. Vacuum insulated pipes should be used for liquid transfer; breakdown of the insulation may give rise to condensation of oxygen.
Ventilation of Exhaust Gases
Gaseous nitrogen and helium exhausted from the cryostat will displace oxygen and if not properly ventilated, the possibility of asphyxiation exists.
Cryogenic substances in liquid, or vapor form, or as low temperature gases, produce effects on the skin similar to burns (cold burns).
Exposed or insufficiently protected parts of the body coming into contact with uninsulated venting pipes or vessels will stick fast and the flesh may be torn if removed.
Exhaust systems are required in order to vent to atmosphere any discharge from the system cryostat as described below.
The static helium evaporation exits from the turret via a non-return valve. The valve prevents ambient air leaking back into the cryostat. The outlet from this valve should be vented out of the room to atmosphere or, if required, to a helium recovery system. In the event of a quench, the evaporated helium will be exhausted from the manifold via the pressure relief valve(s). The amount of gas is dependent on the type of system, but for a 500MHz NMR magnet quenching with 100% helium, the volume of gas at room temperature will be approximately 50,000 liters. If the system is located in a small room then a system should be provided that is capable of exhausting this gas to the atmosphere or to a recovery system.
The static nitrogen evaporation will exit from one (or two) of the nitrogen ports. This gas should be vented out of the room to atmosphere.
Environmental Safety
It is the responsibility of the user to ensure that all equipment, services, data links, or personnel passing through the affected space are adequately protected and that access to the area is controlled. Access doors leading into the affected areas must be capable of being secured against unauthorized entry and fitted with warning signs. It is also recommended that local barriers be erected around the magnet and be fitted with warning signs. Care must be taken to advise personnel who have access (in particular security or cleaning staff who often have their own keys) of all the risks associated with magnetic fields and systems operating with cryogens.