Gas cylinders Flashcards
Gas cylinders
Material, info engraved (4), info on label (2 important)
Usually **thin-walled molybdenum steel **to withstand high pressures. New design: aluminium alloy rather than steel (lighter)
Colour coded
Engraved on exterior
* Test pressure: safety test of integrity of cylinder, subjected to pressures of approx 22000 kPa
* Date of tests performed (should be every 5 years).
* Chemical formula of cylinder contents
* Tare weight: the weight of nitrous oxide cylinder when empty
Additionally: Year cylinder was last examined can be identified from shape and colour of plastic disc around neck
Also legal requirement for various labels:
* Name, chemical symbol
* Substant ID and batch number
* Hazard warnings and safety instructions
* Cylinder size code
* Nominal cylinder contents (L) i.e. volume of cylinder contents in L e.g. size E cylinders have volume 5L
* Maximum cylinder pressure (bars) when full
* Filling date, shelf life, expiry date
* Directions for use
* Storage and handling precautions
Oxygen cylinders
Production, pressure, relationship between amount and pressure
- Produced by separation of air using cryogenic distillation
- Stored at pressure of 13700 kPa
- Stored as a gas: there is a linear and proportional reduction in oxygen cylinder pressure as the cylinder empties at a constant temperature
Nitrous oxide cylinder
Pressure, change as empties, relationship between pressure & content
- Stored in liquid phase with its vapour at 4400 kPa
- Therefore cylinder is only partially filled because in liquid form, it is less compressible than as a gas. Amount of filling = filling ratio
- As cylinder empties: initially pressure remains constant as more vapour is produced to replace what has been used. Once all the liquid has evapourated, pressure in the cylinder decreases.
- Therefore cylinder pressure does not accurately indicate cylinder contents
- Note ice may form on outside of cylinder if temperature inside decreases due to the loss of the latent heat of vaporisaition
Nitrous oxide cylinder
Why only partially filled with liquid, filling ratio (UK/abroad)
- Nitrous oxide cylinders are only partially filled with liquid to reduce the risk of a dangerous increase in pressure -> explosion with an increase in the ambient temperature
- Filling ratio = amount of filling
- In the UK, filling ratio for nitrous oxide and carbon dioxide = 0.75
- In hotter climates, filling ratio is 0.67
Cylinder sizes. What sizes are use:
* Attached to anaesthetic machine
* Cylinder manifolds
Cylinder sizes labelled A to J, but A and H not used for medical gases
- Anaesthetic machines: E (cylinder contents have volume 5L, NB can store much greater volumes of gas)
- Cylinder manifolds: J
Colour coding of gas cylinders
Oxygen, Oxygen/helium, nitrous oxide, entonox, air
Oxygen: black with white top
Oxygen/helium: brown/white
Nitrous oxide: teal
Entonox: blue/white
Air: grey/black/white
What is the tare weight
The weight of a nitrous oxide cylinder when empty
What volume of gas/vapour is present in a size E cylinder
Nitrous oxide, oxygen
Volume of contents of size E cylinder is 5L. However gases and vapours are stored in cylinders under high pressure. As gases or vapours are released, their volume is greater than the volume of the cylinder in which they are stored.
Size E nitrous oxide cylinders can release 1800 litres - stored in liquid phase with vapour at 4400 kPa
Size E oxygen cylinders contain 680 litres - stored as a gas at **13700 kPa **
Rules for storage of cylinders
Location, orientation
- Purpose built, fire-proof room, dry and well-ventilated
- No extermes of heat, no flammable materials, not close to any source of fire
- Should not be exposed to continuous dampness, corrosive chemicals or fumes (can lead to corrosion)
- Full cylinders should be stored separately from empty ones to avoid risk of accidentally connecting an empty cylinder
- Size F, G and J cylinders should be stored upright to avoid damage to the valves
- Size **C, D and E **cylinders can be stored horizontally on shelves made of a material that does not damage the surface of the cylinders, usually timber
Cylinder valves
Material, how to open
- Mounted on top of cylinder.
- Made of brass. Chemical formula of particular gas engraved on the side
- Bodok seal is placed between valve outlet and apparatus to make gas-tight joint
Opening valve
* On/off spindle used to open and close the valve. In newer designs: external part of the valve is built to allow the cylinder to be turned on and off manually, without the need for a key.
* Should be opened slowly when attached to anaesthetic machine to prevent rapid rise in pressure + temp of gas in machines pipelines
* Should be fully open when cylinder is in use: requires 2 full anti-clockwise revolutions
Note in case of over-pressure, safety relief device allows discharge of cylinder contents into atmosphere
Bodok seal
Compressible yoke-sealing washer placed between cylinder valve outlet and apparatus to make a gas-tight joint
Should be carefully inspected for damage prior to use
Cylinder valves: pin index system
What is, pin configuration for O2, nitrous, entonox, air, CO2
- Non-interchangeable safety system to minimise likelihood of wrong gas being used due to cylinder identification errors.
- There is a specific pin configuration for each medical gas on the yoke of the anaesthetic machine or regulator.
- The matching configuration of holes on the valve block allows only correct gas cylinder to be fitter in the yoke - the gas exit port will not seal against the washer of the yoke unless the pins and holes are correctly aligned.
Each gas cylinder can only lock onto correct yoke using these ports:
* Oxygen: 2 and 5
* Nitrous oxide: 3 and 5
* **Entonox: single central port 7 **
* Air: 1 and 5
* Carbon dioxide: 1 and 6
Entonox
What is, pressure in cylinder, how delivered
- Entonox is a gas mixture of 50% oxygen and 50% nitrous oxide by volume
- Stored in cylinders at pressure of 13700 kPa
Two-stage pressure demand regulator is attached to the cylinder and used to deliver entonox on demand
* As patient inhales through mask or mouthpiece, gas flow occurs
* Gas flow ceases at end of inspiratory effort
Entonox and the Poynting effect
What is, at what temp, consequences
- Poynting effect: When liquid nitrous oxide and gaseous oxygen are mixed a gaseous mixture (Entonox) is formed, which has different physical properties (including boiling point and critical temp) to the individual gases. Nitrous oxide is normally present in cylinders as a liquid, but is now present as a gas in the mixture with oxygen
- However if cylinder temp is decreased to below -5.5 degrees C, liquefacation and separation of entonox can occur (overriding the Poynting effect)
- -> liquid mixture at bottom of cylinder containing mostly nitrous oxide with ~20% O2, gas mixture above the liquid of high oxygen concentration
- -> when at a constant flow rate, gas with high oxygen concentration is supplied first, followed by gas of decreasing oxygen concentration as liquid evaporates -> may lead to supply of hypoxic mixture with <20% oxygen as cylinder empties.
How to prevent separation and liquifacation of entonox
- Separation and liquefaction can be reversed by re-warming the cylinder and mixing its contents by inverting it repeatedly.
- Cylinders are stored horizontally for about 24 hours at temperatures of at least 5 degreesC before use, to increase the area available for diffusion. If the contents are well mixed by repeated inversion, cylinders can be used sooner than 24 hours.
- Large cylinders have a dip tube with its tip ending in the liquid phase -> liquid used first, preventing the delivery of an oxygen concentration of less than 20%.
Identify the different cylinders
