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1 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice Liquid Nitrgen (N 2 ) is nitrgen gas held at an extremely lw temperature (arund -195 C) where it frms a liquid. Dry Ice (CO 2 ) is a slid frm f carbn dixide gas held at arund C. Bth are part f a grup f lw temperature liquefied gases and slids knwn as crygenic materials. The purpse f this dcument is t prvide users f these materials with guidance n hw t use them safely. Rev 2. University SIRC Office

2 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice Cntents 1 What are the hazards frm liquid nitrgen 3 2 What are the hazards frm dry ice 6 3 Hw D I Prtect Myself Frm The Hazards Of N 2 & Dry Ice? 8 4. What T D If Smething Ges Wrng 14 Appendix Online Oxygen Depletin / Carbn Dixide Enrichment Assessment Spreadsheet Appendix Liquid Nitrgen Spillage Oxygen Depletin Assessment Appendix Liquid Nitrgen Strage - Oxygen Depletin Assessment Revisin Histry Rev 0 Issued January 2010 Rev 1 Issued January 2011: Utilise Safe Decanting Methds sectin inserted. Other minr amendments made. Rev 2 Issued Nvember 2104: Sectin n dry ice intrduced. Sample incidents inserted. Online assessment tl added. Other minr changes made. Rev. 2 2

3 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice 1. What Are The Hazards Frm Liquid Nitrgen? Frstbite / Cld Burns Liquid nitrgen can cause significant damage t living tissue n extended cntact. Whilst a single brief cntact may nt cause skin damage prlnged cntact certainly will. Cld burns and frstbite can lead t serius permanent disfigurement including amputatin f the digits. Liquid nitrgen burns can be a particular prblem if the material is spilled nt clthing withut the wearer being aware r if prus glves are wrn and becme cntaminated. Als if liquid nitrgen is inhaled then it can cause damage t the lungs and respiratry tract. Again if splashed int the eyes irreparable eye damage can ccur. Asphyxiatin Liquid nitrgen is held in a cled state. As it begins t warm (abve arund -195 C) it starts t change frm a liquid int a gas. One litre f liquid nitrgen if allwed t warm will rapidly generate 682 litres f nitrgen gas. Nitrgen gas is a clurless nntxic durless gas. It is what is knwn as a simple asphyxiant. This means that if present in a high enugh cncentratin the nitrgen gas can suffcate a persn by virtue f the fact that its presence has driven dwn the relative percentage f xygen in the air being inhaled by the individual. This is a significant risk in areas where liquid nitrgen is being stred and used. Table 1. Asphyxia Effect f Reducing O 2 Cncentratin % O 2 In Atmsphere Effects and Symptms N discernible symptms can be detected by the individual Reductin f physical and intellectual perfrmance withut the sufferer being aware Pssibility f fainting within a few minutes withut prir warning. Risk f death belw 11 vl%. 6-8 Fainting ccurs after a shrt time. Resuscitatin pssible if carried ut immediately. 0-6 Fainting almst immediate. Brain damage may ccur, even if rescued. A study identified 85 nitrgen asphyxiatin incidents in the USA between 1992 and 2002, which lead t 80 fatalities and 50 injuries The failure t detect a nitrgen enriched / xygen depleted atmsphere was cnsidered t be a key factr in several incidents (Hazards Of Nitrgen Asphyxiatin, US Chemical Safety Hazard Investigatin Bard, N B, June 2003). Rev. 2 3

4 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice Evaprated Liquid Nitrgen-Induced Asphyxia: A Case Reprt (J Krean Med Sci February; 23(1): ) A 27-yr-ld pstgraduate student was fund lying n the flr adjacent t an empty cylinder f liquid nitrgen (150 L) which was cnnected t an empty Dewarflask (10 L) via a cpper infusin tube. N injury was fund externally r internally. Death was attributed t nitrgen asphyxiatin. Hw the nitrgen escaped was unclear. The enclsed nature f the area within which the liquid nitrgen was held was deemed t be a cntributing factr (see pht). Safety Prblems Lead T Lab Death (BBC News Online, June 20 th 2000) A lab wrked died after liquid nitrgen leaked at a research unit in a Scttish Hspital. During the incident apprx. 700 litres f liquid nitrgen leaked int a freezer rm asphyxiating the victim. Anther staff member was rendered uncnscius. A curt case heard that the victim was engaged in an unapprved prcedure during which he had switched ff the xygen depletin alarm. In additin ventilatin in the area was pr and unable t cpe with a majr leak. System Pressurisatin Liquid nitrgen as utlined abut will expand rapidly as it begins t warm generating nitrgen gas. This can cause the build-up f extreme pressure if the material is held in a sealed r prly vented system. This can give rise t vessel explsin / rupture. Oxygen Cndensatin In sme scenaris it is pssible fr cntainers hlding liquid nitrgen t becme sufficiently cled s that the xygen in the atmsphere cndenses and frms liquid xygen n the cled surfaces. This can ccur when vessels that are pen t the atmsphere are cled n the utside by liquid nitrgen thus allwing liquid xygen t frm n the inside f the vessel. Similarly pipe wrk cled internally by liquid nitrgen can allw liquid xygen t cndense n the utside. It is als a risk where Rev. 2 4

5 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice cntinuusly cled liquid nitrgen flasks have been sitting fr a lng time. Liquid xygen is an explsive material, especially in the presence f rganics. It has a light blue clur and ften appears as a light blue hue within cled vessels. Rev. 2 5

6 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice 2. What Are The Hazards Frm Dry Ice? Frstbite / Cld Burns Dry ice can cause damage t living tissue n extended cntact. Whilst a single brief cntact may nt cause skin damage prlnged cntact certainly will. Cld burns and frstbite can lead t serius permanent skin damage. If the material cmes int cntact with the eyes then serius damage can be dne. Teacher Fund Guilty Of Dry Ice Assault (Australian Bradcasting Crpratin Online, April 22 nd 2013) A high schl teacher was fund guilty f assault after daring a grup f year seven students n the New Suth Wales central cast t hld dry ice in their bare hands. The grup f students sustained burns t their hands (see pht). Asphyxiatin Dry ice passes readily frm a slid state t a gaseus state. One kg f dry ice if allwed t warm will rapidly generate litres f CO 2 gas. CO 2 is a chemical asphyxiant and if present in a high enugh cncentratin it can pisn an individual. This is a significant risk in areas where dry ice is being stred and used in bulk. It shuld be nted that the rate f carbn dixide evapratin increases rapidly if the dry ice is wetted. Table 2. Asphyxia Effect f CO 2 Cncentratin % CO 2 In Effects and Symptms Atmsphere 1 Drwsiness 3 Mild narcsis, reduced hearing and increased shrtness f breath 5 Dizziness, cnfusin, headache and shrtness f breath 8 Dimmed sight, sweating, tremrs and uncnsciusness >8 Uncnsciusness and death Rev. 2 6

7 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice Fatality Due T Dry Ice Asphyxiatin (Jurnal Frensic Sci. July 2009, Vl. 54, N. 4)! The paper reprts a rare fatality due t carbn dixide (dry ice) intxicatin.! A yung healthy male hid in a cntainer 1.5m x 1m x 1m in size! He was fund 5 minutes later suffering frm cnvulsins.! He subsequently died.! Dry ice was knwn t have been stred previusly in the cntainer and was attributed as the cause f death. System Pressurisatin Dry ice as utlined abut will expand rapidly as it begins t warm generating CO 2 gas. This can cause the build-up f extreme pressure if the material is held in a sealed r prly vented system. This can give rise t vessel explsin / rupture. This is a risk where dry ice is used in sealed r prly cnstructed reactin set ups. Rev. 2 7

8 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice 3. Hw D I Prtect Myself Frm The Hazards Of N 2 & Dry Ice? Use Persnal Prtective Equipment Users f liquid nitrgen and dry ice must always wear persnal prtective equipment apprpriate t the task in hand s that in s far as is pssible they are physically prtected frm the material. Users shuld always ensure that they keep crygenic material ff their bdies / clthing and ut f their lungs and eyes. The types f persnal prtective equipment required will depend n the type f usage. The larger the vlume f material handled and the greater the ptential fr bdily cntact then the greater the degree f prtectin that is required. Glves Prtective glves shuld be wrn if there is a risk f material spillage nt the hands. Prtective glves must cnfrm t BS EN 511 (Cld Prtectin). The glves shuld be specifically designed fr crygenics, with ribbed cuffs t prevent splashing int the glve r be lse fitting gauntlets that can easily be remved. The glve material shuld be rugh t give gd grip while handling cled vessels and nt increase the chance f spillage. Glve material shuld be nn-prus t prevent liquid nitrgen entering the space between the glve material and the skin. Prtective Clthing Standard lab safety clthing is suitable fr handling small vlumes f liquid nitrgen and dry ice in a university / hspital type envirnment. This cnsists f a clsed lab cat and if necessary an aprn. The wearing f clthing that leaves the legs uncvered shuld be avided when wrking with large vlumes f crygenic material. Eye Prtectin When wrking with small vlumes f liquid nitrgen r dry ice safety glasses with integral side prtectin shuld be wrn. When handling larger vlumes f crygenics r decanting material, cnsideratin shuld always be given t the wearing f gggles r a face shield. Ftwear Feet shuld be cvered when wrking with larger vlumes r when decanting material. The wearing f sandals r pen ted shes and similar shuld be avided. Rev. 2 8

9 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice Utilise Safe Decanting Methds Liquid Nitrgen Decanting f liquid nitrgen shuld be designed such that the risk f cld burns is minimised. As well as the wearing f PPE the fllwing shuld be cnsidered where necessary: The minimisatin f decanting peratins The use f tipping devices, trlleys and wheeled cntainers fr the safe manipulatin f larger Dewars The use f liquid withdrawal devices t minimise the need fr Dewar manipulatin The use f phase separatrs in cnjunctin with liquid withdrawal devices t reduce the risk f splashing In additin asphyxiatin shuld als be cnsidered as a ptential hazard when decanting liquid nitrgen. When penly puring liquid nitrgen assume a 10% lss rate. Reduce The Risk Of Asphyxiatin Liquid nitrgen and dry ice must be used and stred safely s as t minimise the risk f user asphyxiatin. Asphyxiatin can ccur if nitrgen r carbn dixide is allwed t reach an unsafe level in the atmsphere. Remember that ne litre f liquid nitrgen can generate 682 litres f nitrgen gas whilst 1kg f dry ice can generate litres f carbn dixide. Excess nitrgen can drive dwn the relative cncentratin f xygen in the atmsphere (especially in cnfined and prly vented areas) leading t ptential health affects see Table 1 abve. Excess carbn dixide is txic t the bdy and can give rise t significant health effects at relatively lw atmspheric cncentratin see Table 2 abve. Liquid Nitrgen Strage Strage vessels fr liquid nitrgen are designed s that nitrgen gas can bil ff. In larger systems this vented nitrgen gas is ften cllected and piped ff safely. Hwever in mst strage vessels the gas vents t atmsphere. This can be a significant safety issue in bulk strage area. When string and using liquid nitrgen there are a number f rules must be fllwed t minimise the risk f asphyxiatin: Rev. 2 9

10 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice Only thse vessels cmpsed f suitable material may be used fr string liquid nitrgen. Sme materials e.g. glass and sme plastics, may fracture at lw temperatures releasing nitrgen gas. Liquid nitrgen shuld nt be transprted thrugh heavily ppulated areas f buildings in case f spillages. Stairs shuld be avided unless the vlumes being transprted are small. Persns invlved in the transprt f liquid nitrgen shuld wear apprpriate persnal prtective equipment. When using lifts t transprt liquid nitrgen the fllwing shuld be adhered t: Dewars must nt be accmpanied in lift. A lift is a cnfined space and shuld leakages ccur asphyxiatin is pssible. One persn shuld place the Dewar in the lift whilst anther waits t receive the Dewar frm the lift nce the jurney is cmplete There shuld be a clearly visible sign n the Dewar warning thers nt t enter the lift with the Dewar Where pssible a gds lifts shuld be use If transprting Dewars in vehicles then they shuld be held in a well ventilated area in a separate cmpartment t the driver. Lne wrking with liquid nitrgen shuld be avided wherever pssible. If required a Lne Wrking Risk assessment shuld be carried ut. Bulk strage (>25 litres) areas fr liquid nitrgen must: a. Display hazard warning signage b. Be restricted t authrised persnnel nly c. Be cntinuusly ventilated if pssible Strage areas and areas in which liquid nitrgen is in use shuld be subjected t an assessment f ptential xygen levels during spillages, strage and tpping up / refilling activities. If deemed necessary by this exercise then atmspheric xygen depletin sensrs must be installed in the relevant areas r the ventilatin must be imprved. When dispsing f liquid nitrgen d nt pur it dwn the sink r allw it t vaprise int enclsed areas such as labratries, fridges, freezers, cld rms, etc. Liquid nitrgen t be dispsed f thrugh vaprisatin must be left in wellventilated area e.g. a fume hd. D nt stre liquid nitrgen in walk in freezers / cld rms. Rev. 2 10

11 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice Dry Ice Strage When string and using dry ice there are a number f rules must be fllwed t minimise the risk f asphyxiatin: Large vlumes f dry ice shuld nt be transprted thrugh heavily ppulated areas f buildings in case f spillages. Stairs shuld be avided unless the vlumes being transprted are small. Persns invlved in the transprt f dry ice shuld wear apprpriate persnal prtective equipment. When using lifts t transprt dry ice the fllwing shuld be adhered t: Cntainers must nt be accmpanied in lift. A lift is a cnfined space and shuld leakages ccur asphyxiatin is pssible. One persn shuld place the cntainer in the lift whilst anther waits t receive same frm the lift nce the jurney is cmplete There shuld be a clearly visible sign n the cntainer warning thers nt t enter the lift with same. Where pssible a gds lifts shuld be use If transprting dry ice in vehicles then they shuld be held in a wellventilated area in a separate cmpartment t the driver. Lne wrking with dry ice shuld be avided wherever pssible. If required a Lne Wrking Risk assessment shuld be carried ut. Bulk strage (>10kgs) areas fr dry ice must: a. Display hazard warning signage b. Be restricted t authrised persnnel nly c. Be cntinuusly ventilated if pssible d. Have mre than ne escape rute if pssible Strage areas and areas in which dry ice is in use shuld be subjected t an assessment f ptential carbn dixide levels during spillages r strage. If deemed necessary by this exercise then atmspheric carbn dixide sensrs must be installed in the relevant areas r the ventilatin must be imprved. When dispsing f dry ice d nt pur it dwn the sink r allw it t vaprise int enclsed areas such as labratries, fridges, freezers, cld rms, etc. Dry ice t be dispsed f thrugh vaprisatin must be left in well-ventilated area e.g. a fume hd. D nt stre dry ice in walk in freezers / cld rms. Always stre dry ice in cntainers with lse fitting lids; this will slw dwn the rate f sublimatin. Rev. 2 11

12 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice Reduce The Risk Of Pressurisatin Liquid Nitrgen As utline abve nce liquid nitrgen begins t warm it generates relatively large quantities f nitrgen gas. In sealed r prly vented systems this can be prblem. T vercme this the fllwing must be adhered t: Purpse designed nn pressurised vacuum flasks must be used t stre vlumes f liquid nitrgen f between 1-50 litres, i.e. Dewar Flasks. Dewar Flasks must have nn sealed stppers t allw biling material t vent. Any glass cmpnent f a Dewar flask must be cvered in tape t prevent shattering in the event f an explsin. Always make sure that cntainers f liquid nitrgen are suitably vented and unlikely t blck due t ice frmatin. D nt use liquid nitrgen within sealed systems unless precautins have been taken t prevent warming. In such cases the use f a pressure release value shuld be cnsidered. Be aware that if liquid nitrgen leaks int a stred test tube r similar then upn remval frm the Dewar the test tube can explde due t the rapid expansin f the liquid nitrgen. As a precautin when remving such tubes frm a Dewar they can be placed int a cntainer and a lid placed ver them until they have warmed up. Prevent Oxygen Cndensatin Liquid Nitrgen Measures shuld be taken t prevent the generatin f the explsive liquid xygen. These include: Use liquid nitrgen t cl sealed r evacuated systems nly in rder t prevent xygen cndensatin n the inside f cled vessels. The ability f liquid nitrgen t cause cndensatin f liquid xygen frm the air nt cled pipe wrk must be cnsidered when designing prcesses invlving liquid nitrgen. Strage vessels shuld be emptied and allwed t dry n a regular basis wherever pssible. Users shuld always be vigilant fr the presence f liquid xygen which emits a blue hue. Additinal Safety Precautins D nt allw untrained persns t use r handle liquid nitrgen r dry ice. A Material Safety Data Sheet must be readily available. Rev. 2 12

13 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice All metallic jewellery shuld be remved when handling liquid nitrgen as metal items will quickly spread the cld frm any cntact with the crygenic material. When puring liquid nitrgen d s slwly and carefully t minimise splashing and rapid cling f the receiving cntainer. Always use tngs when placing r remving items frm liquid nitrgen r dry ice. Never verfill Dewars. Use dip sticks t check liquid depth in Dewars. D nt use fingers. Pregnant females and asthmatic wrkers must seek medical apprval prir t wrking with liquid nitrgen r dry ice. Lw temperature damage t the insulatin n electrical cables can lead t electrcutin and equipment damage. Crygenic users must ensure that cables are nt placed where they can be affected by spillages. Users shuld carry ut a risk assessment fr the use f liquid nitrgen and dry ice (refer t Only use purpse built trlleys and cantilevered units fr the mvement and decanting f large nitrgen cylinders. Avid the manhandling f large cylinders fr decanting. Rev. 2 13

14 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice 4. What T D If Smething Ges Wrng In the event f a cld burn frm liquid nitrgen r dry ice: a. Remve any restrictive clthing - but nt any that is frzen t the tissue b. Flush the affected area with tepid water (nt abve 40 C) t return tissue t nrmal bdy temperature c. D nt apply any direct heat r rub affected area d. Cver with a lse, sterile dressing and keep patient warm e. Obtain medical assistance immediately All users f liquid nitrgen shuld be aware f the symptms f anxia (physilgical xygen depletin). These include dizziness, a narctic type affect; nausea, cnfusin, etc. Persns experiencing such symptms shuld remve themselves t fresh air. Persns bserving such symptms in c-wrkers shuld remve them t fresh air. In the event that breathing stps infrm the lcal first aider and give artificial respiratin. Users f dry ice shuld be aware f the symptms f carbn dixide expsure. These include drwsiness, narctic effects, headache, shrtness f breath, sweating and bdy tremrs. Persns experiencing such symptms shuld remve themselves t fresh air. Persns bserving such symptms in c-wrkers shuld remve them t fresh air. In the event that breathing stps infrm the lcal first aider and give artificial respiratin. D nt attempt t rescue anyne frm a cnfined space if they were wrking with liquid nitrgen r dry ice and they have lst cnsciusness - pen the dr if pssible and raise the alarm n internal telephne number 7999 (UCD 24HR Emergency Line).. Fr minr spillages (<1 litre) f liquid nitrgen the fllwing prtcl shuld be fllwed: a. Evacuate the immediate area. b. Allw liquid t evaprate, ensuring adequate ventilatin c. If present ensure that the xygen depletin sensr has been reset befre re-entering the rm. d. Fllwing return t rm temperature inspect area where spillage has ccurred Rev. 2 14

15 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice e. If there is any damage t the flrs, benches r walls infrm the buildings ffice n internal telephne 1111 f. If any labratry equipment has been damaged fllwing the spillage infrm the labratry manager / supervisr Fr minr spillages (<500g) f dry ice the fllwing prtcl shuld be fllwed: a. Evacuate the immediate area. b. Allw slid t evaprate, ensuring adequate ventilatin c. If present ensure that the carbn dixide sensr has been reset befre reentering the rm. d. Fllwing return t rm temperature inspect area where spillage has ccurred e. If there is any damage t the flrs, benches r walls infrm the buildings ffice n internal telephne 1111 f. If any labratry equipment has been damaged fllwing the spillage infrm the labratry manager / supervisr Fr majr spillages (>1 litre) f liquid nitrgen r dry ice (>500g) the fllwing prtcl shuld be fllwed: a. Evacuate the immediate area b. Infrm emergency services n internal telephne 7999 c. If present ensure that the xygen depletin sensr r carbn dixide has been reset befre re-entering the rm. d. In the event that an xygen depletin r carbn dixide sensr is nt present d nt return t the area until it has been declared safe Rev. 2 15

16 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice Appendix 1. Online Oxygen Depletin / Carbn Dixide Enrichment Assessment Spreadsheet An nline calculatr is available which allws persns t calculate the expected nitrgen r carbn dixide enrichment f a space fllwing the release f same. The spreadsheet can be fund at: Alternately a versin f the spreadsheet can be btained frm safety@ucd.ie Rev. 2 16

17 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice Appendix 2. Liquid Nitrgen Spillage Oxygen Depletin Assessment The mst significant risk frm liquid nitrgen is death by asphyxiatin where a spill r leakage depletes the atmspheric xygen lcally. If the xygen cncentratin falls belw 18% adverse effects will ccur resulting in lss f mental alertness and perfrmance cmbined with distrtin f judgement. In atmspheres cntaining less than 10% xygen death by asphyxiatin is rapid: just tw breaths f xygen-free air kills. The resulting xygen cncentratin fllwing a spill can be determined using the equatin: Where: V 0 = 0.209(V r - V g ) V r is the vlume f the rm in m 3 V g is the maximum gas release upn the expansin f the crygenic liquid (vlume f liquid in m 3 x expansin cefficient - 682) T calculate the xygen cncentratin when xygen is the crygenic liquid, V in the frmula becmes 0.209(V r V g ) + V g. An example f the xygen depletin resulting frm a spill f 50 litres f liquid nitrgen in a rm 5m x 10m x 3m is calculated belw: 100 X 0.209(150 m 3 - (0.05 m 3 X 682))/150 m 3 = 100 x X 115.9/150 = 16.2% Oxygen levels will deplete t 16.2% in the immediate area fllwing a spill f 50l f liquid nitrgen in a rm f this size. This is a serius drp in xygen levels and culd lead t serius injury in the event f a spill. In this scenari frced ventilatin r xygen depletin sensrs wuld be required. Rev. 2 17

18 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice Appendix 3. Liquid Nitrgen Strage - Oxygen Depletin Assessment Liquid nitrgen evaprates frm Dewar flasks at knwn rates. Material is als released t the atmsphere during cntainer puring and filling peratins. These factrs may lead t ptential depletin f xygen levels in strage and puring areas. If the calculated xygen levels are likely t be 19% at any time then cntrl measures are required. Other factrs impact n xygen depletin rates, including the rate f air exchange within the strage areas. The principles f assessing the level f xygen depletin during strage and puring peratins are utlined belw, alng with a wrked example fr the strage and daily refilling f tw 200 litre Dewars f liquid nitrgen held in a rm 4m x 5m x 3m in size with n frced ventilatin. THEORY 1. Assume strage f n Dewars f w litres capacity. 2. Static lsses frm the flasks can be taken as x litres/hr. These data are available frm the supplier f the Dewar flasks and vary frm mdel t mdel. T take accunt f flask wear and tear the rate f lss is dubled fr flasks in regular use i.e. 2(x). 3. The expansin cefficient f liquid nitrgen is 682 WORKED EXAMPLE 1. 2 x 200 litre flasks f liquid nitrgen. n = 2; w = Frm infrmatin supplied by the manufacturer static lss rate frm each flask (x) = 0.25 litres/hr. Duble t take accunt f flask wear and tear (2x) =0.5 litres/hr per flask 3. Liquid nitrgen expansin cefficient = n times 2(x) times expansin cefficient = litres f gas lss per hur 4. 2 x 0.5 x 682 = 682 litres/hr 5. Cvert lss t m 3 per hur by multiplying by x 10-3 = lss rate f 0.682m 3 per hur. 6. Estimate strage rm size in m m x 5m x 3m strage rm = 60m 3 Rev. 2 18

19 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice 7. Estimate the number f air exchanges per hur in the strage rm. Fr rms with n frced ventilatin a figure f 0.4 is recmmended. 8. (Rate f release) / (Number f air exchanges per hur x rm vlume) = % change in ambient xygen levels. Nrmal ambient xygen levels are taken as 20.9%. 7. N frced ventilatin, air exchange value f 0.4 used. 8. (0.682) / (0.4 x 60) = ~0.028 r 2.8%. This is equivalent t a change in ambient xygen cntent frm 20.9% t 20.3%. This wuld give n cause fr cncern during strage peratins. 9. When tpping up flasks can assume a lss rate f 10% fr each vlume f liquid, y, decanted. S rate f lss during tpping up in m 3 /hr equals (y)(0.1)(expansin cefficient)(10-3 ). 9. Bth Dewars are tpped up daily using 24 litres f liquid nitrgen in rder t take accunt f abve static lsses. Tpping up lss rate therefre = (24)(0.1)(682)(10-3 ) = m 3 f nitrgen gas lst. Rev. 2 19

20 University Cllege Dublin Guide T The Safe Use Of Liquid Nitrgen & Dry Ice 10. Add lss due t tpping up activities t static lsses frm flasks t get verall level f crygen release. If ttal rate f change is 5% then frced ventilatin r nging xygen depletin mnitring may be required. A relative change in ambient xygen level f 5% applied t the standard ambient xygen level f 20.9% equates t a drp f apprximately 1% ambient xygen level. 10. Change in % ambient xygen levels equals lss during decanting plus static lsses (refer t step 8 abve) = (1.637 / 0 +60)+(0.682) / (0.4 x 60) = r ~5.5%. This equates t a change in ambient xygen cntent frm 20.9% t 19.8%. This is getting clse t a sub ptimal xygen level s during tpping up activities increased ventilatin and / r nging ambient xygen level mnitring culd be required. We assume that the decanting prcedures takes significantly less than ne hur and n air exchanges take place during the decanting prcess, therefre the value f this parameter (number f air exchanges per hur) is taken as zer and the full rm vlume is used as the divisr here. Rev. 2 20