Medical gas cylinders

A brief description of use of cylinders in medical field

1. MEDICAL GAS CYLINDERS By;- ASHISH DHANDARE

2. Different units of pressures :- Psi – pounds per square inch Psig – pounds per square inch gauge. Gauge will read 0 pressure at atm pressure. Measured press – Atm press. Psia – pounds per square absolute Referrence point – 0 press for perfect volume Psia = psig + Atm press. Conversion factors :- 100 kPa =1000 mbar=760 mmHg=1030 cmH2O=14.7 Psi=1 atm

3. Compressed gas :- Defined as;- any mixture in a container with an absolute pressure;- >40 psi at 70 F >104 psi at 130 F or any iquid having a vapour pressure >40 psia at 100 F Compressed gases used in anesthesia :- 1) Liquified gases – N2O, CO2 2) Non- liquified gases – O2, N2, He, Air

4. Critical temp & Critical press :- Critical Temp :- Temp above which a gas cant be liquified , no matter how much pressure is appd . O2 - -119 C N2O - 36.5 C Critical Pressure :- Pressure required to liquify a gas at critical temp.

5. Gas & Vapour :- Gas :- a gaseous substance, which exists in gaseous state @ room temp & atm press, whose critical temp is below the room temp , thus can not be liquified no matter how much press appd , unless its temp is reduced to or below critical temp - Crit. Temp of O2 is -119 C – Thus cant be pressurised to liq unless its temp is reduced to or below -119 C. Thus as a non- liq gas Vapour :- a gaseous substance, which exists in gaseous state @ room temp & atm press, whose critical temp is above the room temp , thus can be liquified under press . - N2O – 36.5 C – Thus can be pressurised to liquid @ room temp. Hence available as a liquified gas.

6. Cylinder material :- Older – Carbon steel Thus Heavier. Modern – Mo + Steel alloy Mo is stronger – thus thinner walls & lighter. In India, Mn + Steel . MRI compatible cylinder – Al

7. Sizes , Internal volumes & Tare weight of cylinders :- Sizes – sized according to alphabets;- A-J A – Smallest size E – M.C.ly used in anaesthesia machine. Int volumes (L) :- D – 3 E – 5 F – 10 H – 46-50 Tare weight :- Weight of an EMPTY cylinder. N2O – 7 Kg.

8. Size & contents of imp cylinders :- O2 Air N2O CO2 Size D Contents (L) 400 375 940 940 Press (Psig) 1900 1900 745 838 Size E Contents (L) 660 625 1590 1590 Press (Psig) 1900 1900 745 838 Size H Contents (L) 6900 6550 15800 NA Press (Psig) 2200 2200 745

9. Service Pressure & Filling limits of cylinders :- Sevice Pressure :- Maximum press to which a cylinder can be filled at 70 F. Always marked on every cylinder. Filling Limits :- Gas in closed container ↓ Rise in temp will cause rise in press ↓ Pressures can rise to dangerous levels ↓ Cylinder may explode. To prevent this, the dept of transportation has regulations regarding the amount of gas a cylinder can hold.

10. Filling limits for liquified & non- liquified gases :- Liquified gases :- Press remains constt till there is liquid. Filling ratio/ Filling density is defined. Filling ratio/ density :- percent ratio of weight of gas in cylinder to weight of water a cylinder can hold. ( Wt of gas : Wt of water ) N2O – 68%, CO2 – 68%, i.e. if a cylinder can hold 100L of H2O, 68L of N2O. Non- liquified gases :- press should not exceed;- a) the service press of that cylinder @ 70 F. b) 1.25 times the service press @ 130 F However for O2, He, CO2-O2 mix, He-O2 mix – Additional 10% is allowed.

11. Identification of contents :- By 2 methods;- 1) Colour coding of the cylinders, 2) Label on the cylinder.

12. Colour coding of cylinders :- Gas International USA Air Black body & shoulders with Black & white quarters Yellow O2 Black body & white shoulders Green N2O Blue Blue CO2 Grey Grey He Brown Brown Entonox Blue body & shoulders with blue & white quarters Cyclopropane Orange Orange Not a full proof method. Hence Label must be checked.

13. Label of the cylinder :- 1) Name of the contents, 2) Chemical symbol, 3) Physical state, 4) Volume (L), 5) Flammable or Non-flammable,, 6) Service pressure of the cylinder, 7) Cylinder size code, 8) Product licence no, 9) Principal hazards of cylinder & its contents, 10) Date of Hydraulic testing – stamped on the cylinder.

14. Cylinder valves :- Attached to cylinders by tapered threads, Brass with Chromium plating, Small cylinder valves – sizes A – E, Large cylinder valves - -sizes F – H.

15. Small cylinder valves :- Parts :- Body, Port, Stem/ shaft, Handle/ hand-wheel, Safety relief device, Conical depression, Pin index.

16. Large cylinder valves :- Bull-nose valves :-

17. Testing of cylinders :- Periodic visual inspection – cracks, dents, leaks, distortion. Tensile Test :- On 1 out of 100 cylinders manufactured. Falttening test │ Bend test ├ on 1/100 cylinders manufactured Impact test │ Hydraulic test :- cylinder tested to 1.66 times the service press helps to determine leaks & stuctural strength. must be done every;- 5 yrs – steel cylinders, 10 yrs – composite cylinders.

18. Safety features :- Label of the cylinder, Symbol of the gas on the cylinder valve, Colour coding, Safety relief device, Pin index safety system. 1, 2, & 3 already explained.

19. Safety relief device :- Present below conical depression. Protects cylinder against high pressures from overfilling or high temp. 3 types :- 1) Frangible disc assembly, 2) Fusible plug, & 3) Safety relief valve.

20. Frangible disc assembly :- Consists of – A frangible disc (made of Cu) & a Safety cap. Disc closes the way out (discharge channel) of the gas & prevents the release of gas. Bursting pressure – Min pressure @ which disc is designed to burst. A non-reclosing device. used in;- O2, Air, N2O, CO2, He, He-O2 mix, CO2-O2 mix. Protects against high pressures d/to;- Overfilling & High temperature.

21. Fusible Plug :- Made up of Woods metal . Woods metal :- fusible alloy – Bi, Pb , Sn & Cd . Depending on the composition, melts at 2 different temp.s (Yield temp) :- either 70-75 C or 95-105 C. Protects against high pressures d/to high temp. but not overfilling.

22. Safety Relief Valve :- Consists of a valve seat. Valve seat occludes the discharge channel. Valve seat – held in place by a spring. Thus, when pressures exceed, valve opens & gas escapes out. Reclosing type – once pressure is reduced, valve closes & prevents further escape. Protects against high pressures d/to Overfilling & high temp. But more susceptible to leaks.

23. Pin Index Safety System :- Used in small cylinders – size E or less which can be connected to anaesthesia machine via yoke 1 st used in 1952. Consists of;- a) pins projecting from yoke & b) 2 holes drilled on the cylinder valve . The pins fit into the corresponding holes when cylinder is attacched .

24. Pins – 4mm diameter & 6mm long. Holes – present below the outlet port of cylinder valve. positioned in an arc – radius of 9/16 inch . 6 holes placed at an angle of 12 degrees from port. 7 th hole at the middle of 6 holes . Port wont seat against the washer of the yoke unles the pins & holes are alligned . Also used while filling the cylinders – wrong gas wont be filled.

25. Pin indices of different gases :- Gas Pin index Air 1,5 O2 2,5 N2O 3,5 CO2 3,6 Cyclopropane 3,6 O2-CO2 (CO2<7.5%) 2,6 O2-CO2 (CO2>7.5%) 1,6 O2-He (He<80.5%) 2,4 O2-He(He>80.5%) 4,6 Entonox 7

26. Fallacies of Pin index system :- Wrong cylinder can still be attached, if;- 1) Pins are broken, 2) 2 washers are placed on the port or yoke.

27. Storage of gas cylinders :- Designated area for storing cylinders. Not be stores in OTs. Storage area – well ventilated, Cool & clean, made of fire resistant material. 4) Separate storage of flammable & oxidizing cylinders – atleast 20 feet distance between them & a fire resistant barrier of atleast 5 feet high. 5) Always secure cylinders with chain or cart. Do not keep them upright unless secured with chain. Do not secure >4 cylinders in a row. 6) Away from Radiators & heat sources, elecctrical circuit. 7) Do not subject to high temp.s , 8) Do not store where water may stagnate,

28. 9) Do not store in front of spill kits, fire extinguishers, eyewash or emergency shower stations. 10) Do not store in hallways, closets or vestibules. 11) Always cap the cylinders when not in use. 12) Keep them properly labelled . 13) Mark cylinders empty. Empty cylinders hsould not be stored with full cylinders. 14) Should not be exposed to continuous dampness or corrosive chemicals or fumes. 15) Should not be covered during storage. All wrappings should be removed during storage.

29. Transport of the cylinders in hosp :- Upright position. Wear protective footwear, gloves & goggles. Do not lift by protective cap or guard. Do not subject to temp extremes. Do not drag or role cylinders. Always use cart. Secure with chain all times when kept upright. Otherwise keep them flat on the ground. Ensure cylinders are properly labelled to its contents.

30. Receiving a cylinder – Things to be checked :- A stamped hydraulic test date within last 5 yrs. Colour coding. Stenciled/ labelled identification of its contents. Presence of valve cap. Cylinder should be rejected, if;- test date, identification markings are not in order. cap is rusted or not properly placed.

31. Cracking the valve :- BEFORE CONNECTING to anaesthesia machine, protective cap should be removed & valve should be opened slowly & briefly & port should point away from user & any other peronnel . This is known as CRACKING the valve. Clears dust & foreign material from the valve outlet before it is connected to the anaesthesia machine. Thus prevents;- a) dust from being blown into the machine & clogging of filters. b) fires & explosions .

32. Always open the valve slowly & before connecting to anesthesia machine:- If valve is opened fast ↓ gas passes quickly between valve & the yoke or regulator ↓ 1) sudden recompression of the gas ↓ large amount of heat generated (Joule-Thompson effect) ↓ if any dust or grease in the space ↓ may be ignited by the heat ↓ fire & explosion,

33. 2) Sudden release of gases ↓ pressure gauge & regulator damage Thus;- 1) Valve should always be opened slowly, 2) Opened before connecting to the anesthesia mach. 3) Never apply grease. Never keep the cylinder valve always open when the cylinder is not being used ↓ Slow depletion of gases from the cylinder when pipeline pressures drop down ↓ no reserve will be available i.c.o pipeline supply failure.