LIQUID HELIUM SAFETY DOCUMENTS CV/APRILIO

Transcription

1 LIQUID HELIUM SAFETY DOCUMENTS CV/APRILIO

2 Department of Physics and Astronomy University of Sheffield These notes and instructions form the generic Safe Working Practice for the use of liquid helium Liquid Helium User's Only experienced and properly instructed persons should handle liquid helium. Before using Liquid Helium a) Read the following guidelines b) Know and understand properties and hazards associated with it c) Establish plans to cover any emergency situations d) Understand your cryostat and correct operation If in doubt - ask your supervisor Liquid Helium - Special Precautions To know what precautions to take is important to recognise that at 4 kelvin all other gases solidify. Therefore, systems and helium dewars must prevent backflow of air, as this constitutes a major safety hazard. Small volumes of liquid evaporate into large volumes of gas and must be allowed to vent safely, therefore: 1) Always connect to helium recovery line and open appropriate vent valves 2) Never lea ve a dewar open to atmosphere Dewars open to atmosphere for prolong periods can cause "ice plugs" causing pressure build up which can lead to disastrous consequences. Liquid Helium Supplies Requirements for supplies of liquid helium should be arranged with a member of the liquefier staff. Removal to Laboratorv 1. Note how much liquid the dewar contains 2. Before removal: (i) Check bladder valve is open and in good condition (ii) Close both recovery line valves and dewar return valve. (iii) Proceed immediately to the laboratory and re-connect dewar to helium recovery line. (iv) Note gas meter reading and liquid helium in the dewar before commencing the experiment.

3 Liquid Helium - Safe Handling and Usage a) Minimum protection recommended is ( i) Cryogenic gloves (ii) Face visor or safety glasses b) Always In spect the Dewar High Pressure in the dewar wilj be indicated by: (i) An inflated rubber bladde r when the conne ction valve is opened or (ii) Pressure gauge on dewar (if fitted) Vent any gas slowly through the helium rec overy line if found in this condition. c) Never Drop Objects into the Liquid Beware of cold gas and rapid boiling of helium when lowering equipment at ambient temperature into the Dewar. THIS OPERATI ON MUST BE CARRIED OUT SLGWLY TO MINIMISE BOlL OFF AND POTENTIAL COLD VAPOUR BURNS! d) Never Accompany a Dewar in a Lift A sudden vapour release in a co nfined space could be fatal! e) Always Use Correct Syphon and Fittings A vacuum insulated syphon is the only method of transferring helium from a dewar. It consists of a vacuum shielded tube which dips below the liquid level in the de war. The dewar is then lightly pressurised which forces the liquid up and out thr ough the syphon tube. It is important that the correct order of assembly of a dewar head syp hon is foll owed to prevent leaks. Remove brass plu g and lock nut used to seal dewar whe n not in use. Insert syphon 'slowly ) and tighten brass flange with the lock nut to seal the ' 0 ' ring. ENSURE ALL PRESSURE HAS BEEN REMOVED 'BEFORE PERFORlVIING THIS OPERATION f) Always Transfer Liquid Slowly SEE FIG. 1 To prevent thermal shock, avoid high pressure build up (back pressure) and EFFICIENT USE OF THE LI QUID HELTIJM. g) Never Pressurise with Gas other than Heli um Pressuri sation with a bladder is suffic ient for most purp oses and this is ob tained by sgueezing the bladder to create a slight over-pressure. Do not use extern al regula ted supplies unless compe tent to do so!

4 HELIUM SYPHOt~ SEILS for COMMONLY USED TRANSFER LI ES Frer.I. TYPICAL CONTINUOU S FLOW SYPHON SEAL TYPICAL HELIUM TRAN SFER LINE SEAL ~ 'c51 STEM SEAL/ STEM S EAL~.. 5] 0"" RE OUCER/~ I R:==::::;---:".--'O' RIN GS HELIUM DEW AR HELIUM DE WAR

5 h) Pre-cooling Equipment Any liquid nitrogen used to pre-cool liquid helium space in cryostats must be fully removed prior to adding liquid helium. i) Purging Purging of syphons and cryogenic equipment for liquid helium service should only be done with dry helium gas j) Always Thaw Equipment with Hot Air This is by far the quickest and safest method. Liauid Helium - Transfer Efficiency a) Syphon and Cryostat Ensure good vacuum is maintained in both. b) Syphon and Cryostat cooldown Complete this operation slowly to prevent thermal sho ck or high back pressures. c) Boil off due to pressurisation gas (external supplies only ) Slowly apply the helium pressurisation gas as it is hot compared to the liquid d) Depressurisation Loss Do not pressurise the dewar more than is necessary to perform the transfer and always try to fiii in one go. Ice Plugs in Dewar Necks On rare occasions an ice plug may form in the neck of the dewar. This must be dealt with quickly since pressure build up is potentially dangerous. If you discover a dewar which has been left open to atmosphere for a period of time e.g. via syphon entry port, helium recovery valve or bladder pressurisation valve. Initially, probe inside dewar with helium level dipstick to establish it is clear and able to vent. Report occurrence to a member of the liquefier staff. If the Dewar is BlockedJPartialIv Blocked 1) Clear laboratory of all personnel 2) Report immediately to the liquefier staff and/or your supervisor

6 Emergency Procedure for Liquid Helium Low Temperature Research Laboratories Forword Liquid helium is delivered in special dewars which unlike liquid nitrogen cannot be dispensed into a container via a liquid withdrawal valve.all the evaporation gas is returned via pipework connected to the recovery system and therefore should not poise an asphyxiation hazard. It has been calculated that under normal operation of a helium experiment and the rooms natural ventilation that it is not possible to lower the oxygen content below 18% even if the gas was not being returned to recovery. The exceptions are as follows: a) Catastrophic failure ofthe dewars vacuum insulation. b) Multiple dewars and experiments within a single laboratory c) Large volumes in bath cryostat i.e. D.S8 large magnet d) More than one type of cryogen It has been determined than c) and d) only applies to D.S8(or the large magnet should it ever be moved) Oxygen Deficiency alarm (if fitted) I.Never enter a laboratory if the Oxygen Deficiency alarm is sounding/flashing.report immediately to a member of the liquefier staff 2. If the alarm sounds whilst in the room. Immediately, tum off the flow pump, open windows and exit the room. Report immediately to a member of the liquefier staff. 3. Uncontrollable releases of gas i.e. catastrophic dewar failure. Exit immediately and sound the fire alarm Air/ice blockages in the dewar 1. If you are unable to remove a transfer tube from a dewar OR insert a transfer tube, clear the laboratory and adjacent laboratories of personnel. Report immediately to a member of the liquefier staff. 2. If the dewar top is missing or the dewar has been left with the return valve open to atmosphere. Initially, probe with a dip stick to confirm that it is not totally blocked. Connect to recovery and/or replace dewar plug. Report immediately to a member of the liquefier staff 3. On insertion or removal of transfer tubes you experience any restrictions or "Sticking" please report to the liquefier staff

7 Department of Physics and Astronomy Safe Working Procedure This form should be completed for each procedure that may be undertaken in the designated area, which has a hazard or potential hazard associated with it. Each procedure an undergraduate, postgraduate, research associate, staff member, technician or secretary is expected to perform, must be adequately explained, (with the aid ofthis form) at the time of their induction into the working area, by a suitably qualified person. Date: October 2005 Location; Experimental Physics & Astronomy Research laboratories Name of supervisor Details of the procedure: Use of Continuous Flow Helium Cryostats. 1. User 's must have appropriate PPE and experimental training prior to using these systems. 2. It is assumed that the sample is fixed inside the cryostat and evacuation of the system. 3. Connect the dewar to the helium recovery line. Open both the dewar and recovery line li4 swing valves. 4. Remove the dewar blanking plug and keep in a safe place 5. Open the transfer tube needle valve and slowly insert the syphon into the dewar. It is preferred that you get assistance whilst attempting this procedure.insert to a depth of approximately 30 ems and lightly screw the gland seal on the syphon inlet. Gently ease down the tube as it enters the liquid, taking care to minimise the frost on the flexible dewar return line 6. De-frost the syphon gland seal and check, ensuring a gas tight connection. 7. Connect to cryostat ensuring that the white PTFE seal is in position. 8. Connect the helium pump/gas controller to the syphon return connector. Open the valve in the laboratory for the helium pump return line. 9. Switch on the helium pump. 10. When the base temperatures have been reached adjust the flow via the syphon needle valve and gas control panel for efficient use of the helium. End of experiment If the experiment is to continue the next day, simply switch off the helium pump and close the helium pump recovery valve.for re-start open the helium pump return valve and then switch on the helium pump. Remove transfer tubes before the weekend or end of the experiment. Always replace the dewar plug and check both recovery valves are open to the dewar!!! Current safety precautions for this procedure 1.Guidance notes in the Laboratory Safety folder 2. Trained operators only 3. Use PPE 4. Know the Emergency Procedure

8 Recommended procedure for transportation of Cryogenic Lig uids using the Goods lift Liquid Nitrogen Open top dewars - typically the 4L Polystyrene containers should NOT be transported in the lift. In the event of spillage the liquid expansion volume would cause an asphyxiation environment. All such movements should be made using the staircase and no more than two at a time. Low Pressure sealed dewars (160L volume) - rarely, if ever should these vessels need to be transported in the lift. Ifrequired to do so, it is important that the gas over pressure is reduced to 50% ofthe reliefvalve pressure in a well ventilated area. This would be typically 10psi/ 0.7 barg. In the event of breakdown, use the emergency alarm for assistance, do not panic since the relief valve setting will take more than 12 hours to be reached. It is recommended that such a procedure is undertaken by a member ofthe liquefier staff. Liquid Helium lool sealed dewars - commonly transported between C&D floors. Only vessels that have been previously connected to the helium recovery system may be transported. Under these circumstances the dewar overpressure is ~ atmospheric. Upon removal, proceed immediately to your destination, reconnecting the dewar back to the recovery system. Visually check the pressure gauge prior to entering the lift or open bladder inflation valve (if fitted). Please ensure all valves remain closed during transportation and the dewar plug is fitted. In the event of breakdown, use the emergency alarm for assistance, do not panic since the relief valve setting will take more than 12 hours to be reached. Under no circumstances transport dewars that were not correctly connected to the recovery line or exhibit any frosting on the top flange. Seek advice from a member ofthe liquefier staff. General 1. Do not to use the passenger lifts unless the goods lift is out of order 2. Always travel alone 3. Try to avoid transportation during Out of Hours April 2008