GENERAL CRYOGENIC SAFETY DOCUMENTS · 2011-03-03 · GENERAL CRYOGENIC SAFETY DOCUMENTS. CVjAPRILIO . Department of Physics and Astronomy ... The most common cryogenic fluids in research

GENERAL CRYOGENIC

SAFETY DOCUMENTS

CVjAPRILIO

Department of Physics and Astronomy

University of Sheffield

These notes and instructions form the generic Safe Working Practice for the use of cryogenic fluids.

PROPERTIES OF COMMON CRYOGENIC FLUIDS " "~' :.•.i

SUBSTANCE REMARKS

CARBON DIOXIDE (-79°C) Can cause severe "cold bums" if handled Liquid or Solid (dry ice) without protective gloves .

Solid CO2 (dry ice) is commonly used as Evaporating CO2 gas will not support life a refrigerant in cold boxes. and may cause asphyxiation In poorly

ventilated confined spaces. Liquid CO2 is usually converted into gas for use In gas welding, or In the carbonated drinks industry.

Can cause "cold burns" . LIQUID METHANE (-162°C)

Methane is frequently stored as a liquid Cold evaporating gas is more dense than and then converted to gas for use as a air and can travel large distances through clean fuel in gas engines. drains below ground.

Methane is highly flammable in gas/air mixtures of 5-15%.

Unlike old "Town Gas", Methane derived from liquid has no smell and cannot easily be detected.

Methane is not a breathable gas and can cause asphyxiation,

Will cause severe "cold bums".LIQUID OXYGEN (-183° C)

Commonly stored as a liquid the Supports life and is breathable. con verted to gas for use in a large range of industrial app lications. It also heavily supports combustion very

vigorously.

I Even metals easily melt in an oxygen­enriched flame . i

LIQUID AR GON Will cause severe "cold burns ".

Commonly stored as a liquid then Does not support life and can cause converted to gas for use in industrial inert asphyxiation if allowed to evaporate in gas applications. poorly ventilated confined spaces.

LIQUID NITROGEN (-196oC) Will cause severe "cold burns",

The most common "Cryogen" of all. Does not support life and can cause asphyxiation if allowed to evaporate 10

Used throughout the research world for poorly ventilated confmed spaces. 10 w temperature storage and experimentation.

It is also used extensively in industry for low temperature refrigeration and as an inert gas.

Will cause severe "cold burns". LIQUID HYDROGEN (-253 0 q

Derived mainly by electrolysis of water, Although Hydrogen is one of the basic hydrogen was widely used in Bubble building blocks of life, it IS not a Chamber experiments about 30 years ago, breathable atmosphere. but is not commonly used at present time.

Its biggest hazard is due to it extreme Recent developments as a prospective flammability. clean fuel for vehicle engines.

Will cause severe "cold bums".LIQDID HELIU1'v1 (-269°C)

The lowest temperature of all cryogenic Sometimes used in mixtures with oxygen fluids. for breathing by deep sea divers, but

otherwise is not a breathable atmosphere Commonly' used m many areas of and can cause asphyxiation if allowed to research. evaporate in .poorly ventilated confined

spaces.

The most common cryogenic fluids in research are Liquid Nitrogen and Liquid Helium, and the remainder of this paper will concentrate on these .

The most common hazardous problems related to cryogenic liquids are:

COLD BURNS

ASPHYXIATION

ICE BLOCKAGES

SAFE HANDLING AND GENERAL PRECAUTIONS TO PREVENT ACCIDENTS

COLD BURNS

Occur when body tissue is exposed to cold liquid Nitrogen by touching cold surfaces or splashing due to carele ss handling.

An examination of the "thermometer" shown in Fig.l. will illustrate the extreme of temperature which can be expected.

If we consider that normal body temperature is 370C and a hot bath about SOOC will be considered quite too hot to bear:

A spilled cup of tea or coffee at about ·SOoC can cause scalding and possible sk in blisters.

Accidental spillage of a pan of boiling water would result in serious skin bums most likely requiring urgent medical attention, even possible skin grafts. It is clear that EXPOSURE to liquid Nitrogen temperatures can result in very serious body tissue damage.

See Fig. I.

Cold bum s do not just happen-they are caused by careless handling, poor choice of materials due to lack of knowledge, or ignorance of the properties of materials.

PRECAUTIONS

In all cases when handling liquid Nit rogen, you should take the basic simple precautions to avoid cold burns:

WEAR SUITABLE PROTECTIVE CLOTHIN"G:­

For relatively small amounts of liquid Nitrogen carried in or pour from small dewars , wear: ­

DR Y LEATHER GLOVES Al"lD CLEAR PLASTIC GOGGLES WITH SIDE PROTECTION.

This is quite suffic ient for the small amount usually transferred carefully m most laboratories.

SCALDING COFFEE

BODY TEMPERATURE

LIQUID CARBON DIOXIDE/

DRY ICE

LIQUID OXYGEN

LIQUID NITROGEN

BOILlNG WATER

HOT BATH

WATER-ICE

LIQUID METHANE

LIQUID ARGON

LIQUID HYDROGEN -253°C ----------"""------­LIQUID HELIUM -269°C

ABSOLUTE ZERO

Where larger amounts are handled and risk of spla shing is greater, there are othe r protective garments available, such as:­

LONG ARM LENGTH GAUNLETS, FULL FACE SHIELDS AND WARM PROTECTIVE APRONS.

When carrying or pou ring from dewa rs, don't try to lift, carry or pour from too heavy a container.

For example, small dewars up to 10 litres capacity, only weigh about 10 kg. A modern 25 litre dewar may only weigh about15 kg empty but will be about 35 kg full. This is really too heavy to carry and pour safely and you should use one of the many transport, pouring tilting trolleys which are available.

When pouring into narrow necked openings such as cryostats or cold traps, use a suitable funnel in the cold trap inlet to reduce spillage, and start by pouring only a small amount to let the funnel coo l down before filling properly-POUR SLOWLY!!

PRESSURISED DEWARS

For larger uses in laboratories, vessels over 25 litre capacity are usually mounted on wheels for easy safe movement. These are usually low pressure controlled dewars which are sealed at the top and contain all necessary safety and control valves.

Transfer of liquid from these low pressure dewars is much safer than pouring, since it can take place through transfer tubes and any splashing takes place inside the dewars.

Again , you must wear protective gloves and eye protection when handling cold transfer hoses and especially if filling into small open dewars where some splashing can occur.

To reduce the possibility of splashing when filling into open dewars, a phase separator can be used. This is really a small porous dev ice which is fitted to the end of the transfer tube or hose. It consists of a sintered bronze "p lug" made by compressing a huge number of spherical bronze granules into a shape for attachment to a pipe fitting. The multitude of internal channels inside the "plug" cau ses lots of separate pressure drops and changes of direction of flow as the liquid Nitrogen passes through.

The result is that the gas phase of the liquid/gas mixture is separated internally and escapes from the upper sides of the phase separator whilst the liquid sprays gently from the bottom. This greatly reduces splashing and improves safety.

See Fig.2.

Liquid Nitrogen

Phase Separator

Gas

,.Small

Liquid .Dewar Flask-

Gas ........t--­ -~..

CHOICE OF MATERIALS

Cold burns and other potential damage can easily occur due to the use of wrong materials.

Dewars and cryostats are carefully designed and manufactured from materials which have high ductility even at cryogenic temperatures, such as STAINLESS STEEL, ALUMINIUM, COPPER and COPPER ALLOYS, which are satisfactory even down to lowest liquid Helium temperatures,

9% Nickel Steel is used for large industrial tanks down to liquid Nitrogen temperatures.

'Carbon steels, wrought iron, malleable iron, rubbers, synthetic rubbers and plastics are not suitable because they can all fail. due to brittle fracture at temperatures

. 0 0between -20 C and -60 C.

so

When installing transfer tubes or usmg funnels , you should always use the recommended materials.

See Fig.3.

Rigi d or fl exi b le t ransf er tu be

Pressurised Dewar

Foam Insula t ion ( Armaf lex)

- r ­

- -

Cry ost at

4 litre Polyst yrene Cont ainer

Tra nsfe r Tube

Phase SeDarator Recornrnend ed

V\aterials for Transfer Tubes Jnod .Fun nels : .t ainless Steel/ Copper & Copper \l loys/ Aluminium

II

rl II r :»: , ..z>

Funne l-. ~-1

I I - )e-N8T-u 5e -~---' I I . .ueber. Synthetic Rubber/ b d

lasti c. Glass Typical Vacu u m

Cold T rap

ASPHYXIATION

Most liquefied gases in the cryogenic field expand from liquid to gas at -700 time s their liquid volume when allowed to evaporate to atmosphere.

Consequently, it is important to control the amount of potential spillage m small rooms and to have adequate ventilation.

Cold evaporating Nitrogen gas is more dense than air and will fill a room from floor level upwards as the gas warms up.

Cold Helium vapour is still less dense than air and could fill a room from ceiling level downwards if adequate ventilation is not provided.

Oxygen deficiency monitors are available (both wall mounted and portable types).

In a room with potential Nitrogen hazard, the Monitor should be mounted at low level (about 2-3 feet off the ground).

In a room with potential Helium Hazard, it should be mounted at high level(above head height-approx. 7-8 ft).

The following recommendations for size of vessel vs. room ventilation are extracted from the B.CG.A. code of practice. There may always be the temptation to for convenience or tidiness to store a dewar in a small cupboard where there is no ventilation.

For example»

A small walk in cupboard about 4ft square x 8ft high, with close fitting doors and no ventilation:

Volume 128 cubic feet = 3625 litres

Assume a 200 litre Nitrogen vessel care less ly left with pressure build system open and e ither gas or liquid valves not properly closed, the whole contents could be evaporated in the cupboard.

200 litres of liquid evaporating to 140000 litres of gas

The 'resultant gas concentration in the cupboard would be almost 100% Nitrogen, just waiting to asphyxiate the person who walks into it.

Even a 5 or 10 litre spillage could result in a seriously hazardous atmosphe re.

Storage of anv liquefied gas in small unventilated rooms or cunboards must be prohibited.

ICE BLOCKAGES

If the top cap or gas vent valve of a cr yogenic vessel, especially very efficient low­loss modern dewars, is left open, air can be sucked into the neck of the dewar.

This air contains quite a lot of moisture which will freeze inside the neck at a point where the temperature is O°e.

If left in this condition for any significant length of time, the soft snow ice can build up and solidify to create a complete neck blockage, which then in tum creates a potential explosive situation as the liquid nitrogen in the dewar continues to receive heat and rise in pressure and temperature

Similar ice blockages can also easily occur in liquid Helium dewars, but are much more serious because not only is there a probability of water ice formation, but also the possibility of solid air blockages.

And if blockage does occur, the rate of pressure rise in the dewar may be much more rapid due to its low latent heat of evaporation.

See fig.4.

CRYOGENIC VESSELS SHOULD NOT BE LEFT WIDE OPEN TO ATlVIOSPHERE

Small non-pressure free venting dewars should be fitted with their proper loose fitting cap which controls the vent gas and prevent ingress of air.

Methods of clearing ice blockages vary a lot, but since the pressure inside the dewars is likely to be unknown, some special precautions must be taken to avoid accidents when the blockage is cleared and the pressure released.

For Nitrogen dewars, the blockage is most likely to be more like hard packed snow and it can be possible to break throu gh using a length of heavy walled copper tube.

If the blockage is too hard, then a heated cooper tube can be used to melt the ice by conducting heat down to the ice plug .

For Helium vessels.a flow of hot Helium gas will melt the ice or solid air plug.

See fig.5.

Water {x·

Ice Block

~ o ° C

IlIII I

-200°C

Heat

1 Watt

Water Ice

Block

Solid Air Block

Heat

1 Watt

Will evaporate 0.5 litres/dav Will evaporate 33 litres/day

F, q-. 4-:

/ c;=~ Hot Air II .rn ~

Blow er .. I 1--Ji -

1 Thi ck W al l

-... Copper ---....­Tub in g ~

';!.'d '~:""

V I I'J

Lf\h

Q

E :J

(fiLN 2 ILHe

<:

Soft "Snow" Hard " Ice" So lid Ai r Bloc kage Blo ckage Blockag e

;

FtCTi 5.

r"

.'

J TRAPPED VOLUMES 'I';' .;

,I

/' Cold liquid trapped between two valves in a transfer tube system will expand as it evaporates and warms up, very high pressures, enough to burst pipes can occur.

Thermal pressure relief devices must always be fitted to avoid damage to the system.

A spring loaded relief valve is preferred.

A relief valve together with a manual blow-down valve is even better.

BUT!

A manual valve alone IS not sufficiently safe as it depends on the operator to remember to open it.

See fig.6.

Lev el Contro ller

*

1- - - - - - - ­

I

I

1 I

I

I I

I

I I

I

I

. ;~ .

I

I

Typ ical Automatic Level Control System

* Do Not create "Trapped Vo lumes between valves

Do inst al l pressure re lief devices