safety with cryogenic systems. safety aspects 1. physiological 2. suitability of materials and...

SAFETY WITH CRYOGENIC SYSTEMS

Safety aspects

1. Physiological2. Suitability of materials and construction3. Explosions and flammability4. Excessive pressure

1. PhysiologicalHazard - Frostbite (cold burns)

Safety - Eye and body protection

Safety glasses, face shields, grease free leather gloves, proper footwear, apron and trouser clothing.Clothing – free from grease and oils.

Clothing for hydrogen service - minimum static electricity build-up.Clothing for oxygen service – minimum flammability.

Hazard Nitrogen Asphyxiation - due to spillage or leakage, in large vessels that have contained gaseous or liquid nitrogen, resulting in dilution of oxygen content

-creates respiratory problem resulting in unconsciousness;may lead to death

Safety

- use oxygen monitors (acceptable 16-25% ) -fresh air purged prior to entry -work in teams, with one team member outside working area. -safety rope and lifeline attached to person entering the tank and lifeline continuously monitored.

Hazard Excess pure oxygen

-lung damage occurs if oxygen concentration in air exceeds 60% -prolonged exposure leads to bronchitis, pneumonia, lung collapse. -oxygen enriched air leads to flammability and explosion hazards.

Safety -place oxygen detectors or monitors -watch for fog or mist

2. Suitability of materials and construction

Brittle fracture :-Failure of materials at low temperature under impact.-occurs very rapidly resulting in instantaneous failure.

- failure causes severe damage if the equipment is under pressure - release of fluid such as oxygen may result in fire or explosion

Tests to determine material suitability

Drop weight tests : NDT (nil ductility transition )temperature of the material - varies with material, thickness,hardness and other properties

Charpy impact test

-low cost test - specifies minimum allowable impact value for various materials - stainless steel, aluminum, copper and brass have high impact resistance at low temperatures

Thermal stress -caused by thermal contraction of materials - thermal contraction leads to spillage of cryogen and possibility of fire or explosion - results in differential contraction when two dissimilar materials are joined

- contraction in long pipes is a serious problem - provision is made for change in length by using bellows, expansion joints, flexible hose

4. Over pressure - due to phase change from liquid to gas - systems containing liquid cryogen reach bursting pressure very rapidly, if not relieved and trapped in enclosures - in uninsulated systems the liquid vapourises more rapidly - more the fluid greater the pressure - liquid and vapour phase exists between TPT and CPT.

- entrapment of cold liqiuds and gases may occur in relief valves and burst discs resulting in overpressure - vent lines without rain traps may collect rainwater and freeze closed

Ullage space – vapour filled space above liquid is provided for safe cryogenic storage. - relief valves to be provided,inspected and

maintained periodically - in event of freeze closing, ice plug is melted by inserting warm copper rod

Conclusion : Safety is ensured in cryogenic systems by following good operating procedures and practices. - to select materials and methods of construction compatible with properties of cryogens - eliminate sources of ignition - provide proper insulation and relief

- eliminate cracks, recesses,pores in the design and assembly of parts - provide adequate ventilation - keep the surrounding clean and free from contamination

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