SAFETY WITH CRYOGENIC SYSTEMS. Safety aspects 1. Physiological 2. Suitability of materials and construction 3. Explosions and flammability 4. Excessive pressure.
1. PhysiologicalHazard - Frostbite (cold burns)Safety - Eye and body protectionSafety glasses, face shields, grease free leather gloves, proper footwear, apron and trouser clothing.Clothing – free from grease and oils.
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.
2. Suitability of materials and constructionBrittle 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 suitabilityDrop 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