Safety in Cryogenics Cryogen Handling Hazards Bernd Petersen DESY –MKS1 Quellen (u.a): [email protected]

Safety in Cryogenics

Cryogen Handling Hazards

Bernd Petersen DESY –MKS1

Quellen (u.a):

[email protected]

Objectives:at the end of this lesson you should remember the

following hazards

cold burns

embrittlement

asphyxiationthermal stress

pressure build-up by evaporation

hazards caused by condensation

combustion and explosion

in the handling of cryogens

Outline:

Cryogen Handling Hazards(restricted to: He,N2,Ar)


[email protected]

Sources of Accidents and Failures

-> Means of Protection and

Protection Measures

Control Quizz



[email protected]

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

http://gperinic.home.cern.ch/gperinic/kryokurs/de/kryok_11.htm

Cryogen Handling Hazards Hazards

Cold Burns


[email protected]

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

Cold Burns

Means of protection and preventive measures:

Safety garments which prevent the penetration of cryogenic liquids:


[email protected]

- eye protection

- gloves of insulating and non

combustible material

which can be easily be removed

First Aid:

Same procedure as for burns

-high, tight fitting shoes

- trousers without turn-ups which

overlap the shoes


Asphyxiation (oxygen deficiency)


[email protected]

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

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) Bernd Petersen DESY –MKS1

[email protected]

Cryogen Handling HazardsAsphyxiation (oxygen deficiency)

Example for Helium: discharge of helium into LHC-tunnel

formation of fog !!!


O2-concentrations after helium discharge into LHC-tunnel

similar in HERA-tunnel or TTF-tunnel

bottom

top

middle


protection measures

Evacuation of the area !

Helium: most dangerous area ‚on top‘

Use safety garments !

Control and measure O2 content of surrounding air !

Use independent breathing apparatus if needed !

Cold N2 or Argon: most dangerous ‚at the bottom‘

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. Bernd Petersen DESY –MKS1

[email protected]


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).


[email protected]


Thermal Stress

Thermal

Expansion of some solids

77K

Steel 0,3%

Plastics > 2%


[email protected]


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 oscillationsBernd Petersen DESY –MKS1

[email protected]

Comparison (ideal) Gase.g. Helium T > 30K

Pressure x Volume = const x Temperature

at Pressure = CONST

-> Volume = const/CONST x Temperature

-> double Temperatur ->double Volume


[email protected]


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 bythe break down ofInsulation vacuum:

40kW / m2 without MLI 6kW/ m2 with MLI


[email protected]

Sources of Accidents and Failure

Wave guide Air Leak :13.06.2002 Level ???

Temperature

Pressure


[email protected]

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


[email protected]

Redundancy

i.e. more safety devices than

required

double safety devices


[email protected]


Preventive Measures

Against Pressure Build-up

Diversity

i.e. safety devices based on different

mechanisms


Preventive Measures against PressureBuild-up

Release Flap

Caution in the

Vicinity

Of

Release devices !


[email protected]

Safety Valve


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 Bernd Petersen DESY –MKS1

[email protected]


Condensation of Gases

LN2 exhaust line: danger of freeze-up


[email protected]

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


[email protected]

82K

21% O2 79%N2 (air)

50% O2

Phase equilibrium diagram of a N2 / O2 solution

100% N2 100% O2

Boiling temperature O2

Boiling

temperature N2


[email protected]

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.


[email protected]

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.


[email protected]

Damage caused by He-Purifier

Explosion

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‘)

Objectives:at the end of this lesson you should now remember the

following hazards

cold burns

embrittlement

asphyxiationthermal stress

pressure build-up by evaporation

hazards caused by condensation

combustion and explosion

in the handling of cryogens

Documents

Safety in Cryogenics Cryogen Handling Hazards Bernd Petersen DESY –MKS1 Quellen (u.a): [email protected]