Safety in Cryogenics. Cryogen Handling Hazards. Bernd Petersen DESY MKS1 Quellen (u.a):

Transcription

1 Safety in Cryogenics Cryogen Handling Hazards Quellen (u.a):

2 Objectives: at the end of this lesson you should remember the following hazards cold burns embrittlement asphyxiation thermal stress pressure build-up by evaporation hazards caused by condensation combustion and explosion in the handling of cryogens

3 Outline: Cryogen Handling Hazards (restricted to: He,N2,Ar) Sources of Accidents and Failures -> Means of Protection and Protection Measures Control Quizz

4 Cryogen Handling Hazards In the handling of cryogenic liquids and the operation of low temperature facilities you have to consider two main risks Cold Burns Asphyxiation - caused by the evaporation of cryogenic liquids in closed or badly ventilated areas

5 Cryogen Handling Hazards Hazards Cold Burns Direct contact with cryogenic liquids and gases as well as surfaces at cryogenic temperatures can lead to massive damages of the skin ( cold burns) or even of tissues The danger of cold burns is most prominent when handling open cryogen containers ( LN2!!!!) Eyes and mucuous membranes are most at risk

6 Cold Burns Means of protection and preventive measures: Safety garments which prevent the penetration of cryogenic liquids: - eye protection - gloves of insulating and non combustible material which can be easily be removed -high, tight fitting shoes - trousers without turn-ups which overlap the shoes First Aid: Same procedure as for burns

7 Cryogen Handling Hazards Asphyxiation (oxygen deficiency) The evaporation of cryogenic liquids in closed or badly ventilated areas can lead to oxygen deficiency Due to the fact that most cryogens are odourless and colourless, this hazard cannot be detected without special equipment The victim may not even become aware of the oxygen deficiency!!! Furthermore, argon and cold nitrogen are heavier than air and can therefore collect near the floor or in pits

8 The symptoms of oxygen deficiency are ( oxygen concentration in % ): 19% - 15% pronounced reduction of reaction speed 15% - 12% deep breaths, fast pulse, co-ordination difficulties 12% - 10% vertigo,false judgement, lips slightly blue 10% - 8% nausea, vomiting, unconsciousness 8% - 6% death within 8 minutes, from 4-8 minutes brain damages 4% coma within 40 seconds, no breathing, death Means of protection and preventive measures: sufficient ventilation of the working place feed exhaust from transport dewars and from experiments into a gas recovery system or into the chimney equip working place with oxygen monitor enter confined areas, e.g. pits or tanks, only under supervision and only with portable oxygen monitors (-> HERA Cryo-Control Room)

9 Cryogen Handling Hazards Asphyxiation (oxygen deficiency) Example for Helium: discharge of helium into LHC-tunnel formation of fog!!!

10 Cryogen Handling Hazards Asphyxiation (oxygen deficiency) O2-concentrations after helium discharge into LHC-tunnel similar in HERA-tunnel or TTF-tunnel middle bottom top

11 Cryogen Handling Hazards Asphyxiation (oxygen deficiency) protection measures Evacuation of the area! Use safety garments! Control and measure O2 content of surrounding air! Use independent breathing apparatus if needed! Helium: most dangerous area on top Cold N2 or Argon: most dangerous at the bottom

12 Sources of Accidents and Failures Embrittlement Low temperature embrittlement Causes overloaded components to fracture spontaneously rather than accommodating the stress by plastic deformation Appropriate steels for low temperature use are listed in the Technical Rules for Pressure Vessels AD-Merkblatt W10. (In general, materials with face-centered cubic (fcc) crystal structure as copper, nickel, certain copper nickel alloys, zircon and titanium are suitable for cryogenic applications.) If hydrogen is present during the production of materials or if components come into contact with hydrogen in operation, then hydrogen embrittlement can occur under certain conditions.

13 Sources of Accidents and Failures Thermal Stress The development of thermal stress is due to the contraction of materials when cooled down to cryogenic temperatures. The stress can appear as a transient effect e.g. when cooling down thick walled components or it can appear as permanent load e.g. in piping. In both cases the stress can cause damage. As the expansion coefficient of most materials decreases with temperature, most contraction takes place above LN2 temperature ( 77K).

14 Sources of Accidents and Failures Thermal Stress Thermal Expansion of some solids Plastics > 2% Steel 0,3% 77K

15 Sources of Accidents and Failures Pressure Build-up by Evaporation Cryogen liquids do expand by a factor of 500 to 1500 when evaporated and warmed up to room temperature (300 K) (Helium: 2K l -> 300K d factor= 900) -> significant pressure build-up in a closed container Possible reasons for an elevated heat input are: fast cooldown of components or cryogenic installations, large heat production within the object to be cooled ( e.g. quench), loss of insulating vacuum, thermoacoustic oscillations

16 Comparison (ideal) Gas e.g. Helium T > 30K Pressure x Volume = const x Temperature at Pressure = CONST -> Volume = const/const x Temperature -> double Temperatur ->double Volume

17 Sources of Accidents and Failures Pressure Build-up by Evaporation Large air leak into the wave guide of an insert -> Evaporation of about 2.6 kg/s mass flow through safety valve Heat input caused by the break down of Insulation vacuum: 40kW / m2 without MLI 6kW/ m2 with MLI

18 Sources of Accidents and Failure Wave guide Air Leak : Level??? Temperature Pressure

19 Further mechanisms which can lead to a pressure increase: Boiling retardation In very clean vessels boiling may not start but at temperatures above the boiling point. In this case the boiling can be from violent to explosion like. Boiling retardation can be prevented by introducing porous material as boiling nuclei. Stratifikation If the cryogen in a large tank is not disturbed for some time, a temperature stratification may occur. The stratification causes a larger pessure rise than expected due to the elevated temperature in the liquid surface layer. The release of cryopumped gas

20 Sources of Accidents and Failures Preventive Measures Against Pressure Build-up Redundancy i.e. more safety devices than required double safety devices Diversity i.e. safety devices based on different mechanisms

21 Sources of Accidents and Failures Preventive Measures against PressureBuild-up Release Flap Safety Valve Caution in the Vicinity Of Release devices!

22 Sources of Accidents and Failures Condensation of Gases Purposeful use of condensation in cryo-pumps and cryo-traps Danger: Freeze-up of exhaust pipes of dewars by air and/or air moisture. The plugging causes pressure build-up. Preventive Measures: - Only use dewars with separated eshaust and safety valve lines - Do not leave dewars open to atmosphere -Equip exhaust lines to atmosphere with check valves ( nonreturn) -Perform a leaktest before evacuating cryogen baths with emphasis on leaks by which air might be sucked into the cold part of the apparatus

23 Sources of Accidents and Failures Condensation of Gases LN2 exhaust line: danger of freeze-up

24 Unwanted condensation or freezing can also lead to: - Mechanical damage By ice accretion on valves, turbines etc., By dripping condensate at cryogenic temperatures or By water condensate - Pressure build-up by desorption - Formation of explosive mixtures By condensation of air or oxygen on combustible liquids or materials flammable up to explosive mixtures can be created: in the insulation material of transfer lines, in a liquid hydrogen dewar on the active charchoal of a cryogenic adsorber Important! Air condensate is a strong promoter of combustion as it can have an oxygen concentration of up to 50%. This is due to the higher boiling point of oxygen

25 Phase equilibrium diagram of a N2 / O2 solution 21% O2 79%N2 (air) Boiling temperature O2 Boiling 82K temperature N2 50% O2 100% N2 100% O2

26 Sources of Accidents and failures : Combustion and Explosion Hazard Oxygen Liquid or gaseous oxygen can -strongly promote combustion processes, - reduce the ignition temperature, - accumulate in combustible materials ( especially also in clothes) which will burn from violent to explosion like when ignited. Ozone A safety risk is posed by the production of ozone in systems which contain oxygen and which are exposed to gamma or neutron radiation. An explosive amount of ozone cam already be created from the oxygen impurities in a liquid nitrogen dewar.

27 Preventive measures against condensation and its consequences: Purge and evacuate all equipment thoroughly before operation. Operate installations at slight overpressure in order to avoid impurities from entering through leaks. Whereever possible do use vacuum insulation. Otherways- do use incombustible insulation material. Damage caused by He-Purifier Explosion

28 General safety measures all work in cryogenic areas has to be checked an approved by the MKS cryo-operators in charge inform the MKS cryo-operators about any problems follow the operating instructions ( Betriebsanweisungen )

29 Objectives: at the end of this lesson you should now remember the following hazards cold burns embrittlement asphyxiation thermal stress pressure build-up by evaporation hazards caused by condensation combustion and explosion in the handling of cryogens