Medical gas supply

Components

• 1.

  Cylinders are made of thin-walled, seamless molybdenum steel in which gases and vapours are stored under pressure. They are designed to withstand considerable internal pressure.

• 2.

  The top end of the cylinder, the neck, ends in a tapered screw thread into which the valve is fitted. The thread is sealed with a material that melts if the cylinder is exposed to intense heat. This allows the gas to escape, thus reducing the risk of an explosion.

• 3.

  There is a plastic disc around the neck of the cylinder. The year when the cylinder was last examined and tested can be identified from the shape and colour of the disc.

• 4.

  Cylinders are manufactured in different sizes (sizes A–J) and contents ( Table 1.1 ). Sizes A and H are not used for medical gases. Cylinders attached to the anaesthetic machine are usually size E ( Figs. 1.1–1.4 ), while size J cylinders are commonly used for cylinder manifolds (see Fig. 1.19 ).

  Table 1.1

  Contents of the commonly used cylinder sizes for oxygen, nitrous oxide and medical air

  Size	Oxygen	Nitrous oxide	Medical air
  E (4.7 L H 2 O capacity)	680 L	1,800 L	640 L
  J (47.2 L H 2 O capacity)	6,800 L	18,000 L	6,400 L
  CD (2 L H 2 O capacity)	460 L (23,000 kPa)

  
  
  
  

• 5.

  Lightweight cylinders can be made from aluminium alloy with a fibreglass covering in an epoxy resin matrix. These can be used to provide oxygen at home, during transport or in magnetic resonance scanners. They have a flat base for easy storage and handling.

  • Gas exits in the gaseous state at room temperature. Its liquefaction at room temperature is impossible, since the room temperature is above its critical temperature.

  • Vapour is the gaseous state of a substance below its critical temperature. At room temperature and atmospheric pressure, the substance is liquid.

  • Critical temperature is the temperature above which a substance cannot be liquefied no matter how much pressure is applied. The critical temperatures for nitrous oxide and oxygen are 36.5°C and −118°C, respectively.

At room temperature, oxygen is stored as a gas at a pressure of 13,700 kPa (above its critical temperature), whereas nitrous oxide is stored in a liquid phase with its vapour at equilibrium at a pressure of 4400 kPa (and usually below its critical temperature). As the liquid is less compressible than the gas, this means that the cylinder should only be partially filled. The amount of filling is called the filling ratio . Partially filling the cylinders with liquid minimizes the risk of dangerous increases in pressure due to increased vaporization of the liquid with increases in the ambient temperature. This scenario could lead to an explosion. In the United Kingdom, the filling ratio for nitrous oxide and carbon dioxide is 0.75. In hotter climates, the filling ratio is reduced to 0.67.

The filling ratio is the weight of the fluid in the cylinder divided by the weight of water required to fill the cylinder. However, it is important to remember that a filling ratio of 0.75 is not exactly the same as the cylinder being 75% filled. This is due to the differences in the physical properties of the contents, such as its density, and water content.

A full oxygen cylinder at atmospheric pressure can deliver 130 times its capacity of oxygen. A typical size E full oxygen cylinder delivering 4 L/min will last for 2 hours and 50 minutes but will last only 45 minutes when delivering 15 L/min.

At constant temperature, a gas-containing cylinder, e.g. oxygen, shows a linear and proportional reduction in cylinder pressure as it empties, obeying Boyle’s law (first gas law).

For a cylinder that contains liquid and vapour, e.g. nitrous oxide, initially the pressure remains constant as more vapour is produced to replace that which was used. Once all the liquid has been used, the pressure in the cylinder starts to decrease. The temperature in such a cylinder can decrease because of the loss of the latent heat of vaporization leading to the formation of ice on the outside of the cylinder.

Exam tip:

In the exam, make sure you know the difference between gas and vapour. This difference causes the pressure changes in an oxygen cylinder compared to a nitrous oxide cylinder when used.

Cylinders in use are checked and tested by manufacturers at regular intervals, every 5–10 years. Test details are recorded on the plastic disc between the valve and the neck of the cylinder. They are also engraved on the cylinder:

• 1.

  An internal endoscopic examination is carried out.

• 2.

  Flattening, bend and impact tests are carried out on at least one cylinder in every 100.

• 3.

  A pressure test is conducted, whereby the cylinder is subjected to high pressures of about 22,000 kPa, which is more than 50% above their normal working pressure.

• 4.

  A tensile test is carried out on at least one cylinder in every 100, whereby strips of the cylinder are cut out and subjected to impact, stretching, flattening and other tests of strength.

The marks engraved on the cylinders are:

• 1.

  Test pressure.

• 2.

  Dates of test performed.

• 3.

  Chemical formula of the cylinder’s content.

• 4.

  Tare weight (weight of nitrous oxide cylinder when empty).

Problems in practice and safety features

• 1.

  The gases and vapours should be free of water vapour when stored in cylinders. Water vapour freezes and blocks the exit port when the temperature of the cylinder decreases on opening.

• 2.

  The outlet valve uses the pin-index system to make it almost impossible to connect a cylinder to the wrong yoke ( Fig. 1.5 ).

  

• 3.

  Cylinders are colour coded to reduce accidental use of the wrong gas or vapour. In the United Kingdom, the colour coding is a two-part colour, shoulder and body ( Table 1.2 ). The Medicine and Healthcare Products Regulatory Agency in the United Kingdom has decided to bring it in line with the European Standard EN 1089-3 to aid cylinder identification and improve patient safety. All cylinders will have a white body but each gas/vapour will retain its current shoulder colour. It is hoped that these changes will take full effect by 2025. Already some oxygen and Entonox (BOC Healthcare, Manchester, UK) cylinders in the United Kingdom have started using the new scheme.

  Table 1.2

  Colour-coding of medical gas cylinders (old and new system), their pressure when full and their physical state in cylinder

  	Shoulder colour	Body colour (current/old system)	Body colour (new system)	Pressure at room temperature (kPa)	Physical state in cylinder
  Oxygen	White	Black (green in USA)	White	13,700	Gas
  Nitrous oxide	Blue	Blue	White	4400	Liquid/vapour
  Carbon dioxide	Grey	Grey	White	5000	Liquid/vapour
  Air	White/black quarters	Grey (yellow in USA)	White	13,700	Gas
  Entonox	White/blue quarters	Blue	White	13,700	Gas
  Oxygen/helium (Heliox)	White/brown quarters	Black	White	13,700	Gas

• 4.

  Cylinders should be checked regularly while in use to ensure that they have sufficient content and that leaks do not occur.

• 5.

  Cylinders should be stored in a purpose-built, dry, well-ventilated and fireproof room, preferably inside and not subjected to extremes of heat. They should not be stored near flammable materials such as oil or grease or near any source of heat. They should not be exposed to continuous dampness, corrosive chemicals or fumes. This can lead to corrosion of cylinders and their valves.

• 6.

  To avoid accidents, full cylinders should be stored separately from empty ones. Size F, G and J cylinders are 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.

• 7.

  Overpressurized cylinders are hazardous and should be reported to the manufacturer.

Components

• 1.

  The valve is mounted on the top of the cylinder, screwed into the neck via a threaded connection. It is made of brass and is sometimes chromium plated.

• 2.

  An on/off spindle is used to open and close the valve by opposing a plastic facing against the valve seating.

• 3.

  The exit port is for supplying gas to the apparatus (e.g. anaesthetic machine).

• 4.

  A safety relief device allows the discharge of cylinder contents to the atmosphere if the cylinder becomes overpressurized.

• 5.

  The non-interchangeable safety system (pin-index system) is used on cylinders of size E or smaller as well as on size F and G Entonox cylinders. A specific pin configuration exists for each medical gas on the yoke of the anaesthetic machine. The matching configuration of holes on the valve block allows only the correct gas cylinder to be fitted in the yoke ( Figs. 1.8 and 1.9 ). The gas exit port will not seal against the washer of the yoke unless the pins and holes are aligned.

  
  

• 6.

  The external part of the valve in some designs allows manual turning on and off of the cylinder without the need for a key ( Fig. 1.10 ).

  

Mechanism of action

• 1.

  The cylinder valve acts as a mechanism for opening and closing the gas pathway.

• 2.

  A compressible yoke-sealing washer, Bodok seal ( bo nded d is k ), must be placed between the valve outlet and the apparatus to make a gas-tight joint ( Fig. 1.11 ). It is a gasket made of non-combustible material with a metal rim.

  

Problems in practice and safety features

• 1.

  The plastic wrapping of the valve should be removed just before use. The valve should be slightly opened and closed (cracked) before connecting the cylinder to the anaesthetic machine. This clears particles of dust, oil and grease from the exit port, which would otherwise enter the anaesthetic machine.

• 2.

  The valve should be opened slowly when attached to the anaesthetic machine or regulator. This prevents a rapid rise in pressure and an associated rise in temperature of the gas in the machine’s pipelines. The cylinder valve should be fully open when in use (the valve must be turned two full revolutions).

• 3.

  During closure, overtightening of the valve should be avoided. This might lead to damage to the seal between the valve and the cylinder neck.

• 4.

  The Bodok seal should be inspected for damage prior to use. Having a spare seal readily available is advisable. This bonded non-combustible seal must be kept clean and should never become contaminated with oil or grease. If a gas-tight seal cannot be achieved by moderate tightening of the screw clamp, it is recommended that the seal be renewed. Excessive force should never be used.

Components

• 1.

  Central supply points such as cylinder banks and/or liquid oxygen storage tank.

• 2.

  There is a labelled and colour-coded pipeline distribution network throughout the hospital.

• 3.

  Pipework is made of special high-quality copper alloy, which both prevents degradation of the gases it contains and has bacteriostatic properties. The fittings used are made from brass and are brazed rather than soldered.

• 4.

  The size of the pipes differs according to the demand that they carry. Pipes with a 42-mm diameter are usually used for leaving the manifold. Smaller diameter tubes, such as 15 mm, are used after repeated branching.

• 5.

  Outlets are identified by gas colour coding, gas name and shape ( Fig. 1.12 ). They accept matching quick connect/disconnect Schrader probes and sockets ( Fig. 1.13 ), with an indexing collar specific for each gas (or gas mixture).

  
  

• 6.

  Outlets can be installed as flush-fitting units, surface-fitting units, on booms or pendants or suspended on a hose and gang mounted ( Fig. 1.14 ).

  

• 7.

  Flexible colour-coded hoses connect the outlets to the anaesthetic machine ( Fig. 1.15 ). The anaesthetic machine end should be permanently fixed using a nut and liner union in which the thread is gas specific and non-interchangeable (a non-interchangeable screw thread [NIST] is the British Standard).

  

• 8.

  Isolating valves behind break-glass covers are positioned at strategic points throughout the pipeline network. They are also known as area valve service units (AVSUs) ( Fig. 1.16 ). They can be accessed to isolate the supply to an area in cases of fire or other emergency.

  

Problems in practice and safety features

• 1.

  A reserve bank of cylinders is available should the primary supply fail. Low-pressure alarms detect gas supply failure ( Fig. 1.17 ).

  

• 2.

  A single-hose test is performed to detect cross-connection.

• 3.

  A tug test is performed to detect misconnection.

• 4.

  Regulations for PMGV installation, repair and modification are enforced.

• 5.

  Anaesthetists are responsible for the gases supplied from the terminal outlet through to the anaesthetic machine. Pharmacy, supplies and engineering departments share the responsibility for the gas pipelines ‘behind the wall.’

• 6.

  There is a risk of fire from worn or damaged hoses that are designed to carry gases under pressure from a primary source such as a cylinder or wall-mounted terminal to medical devices such as ventilators and anaesthetic machines. Because of heavy wear and tear, the risk of rupture is greatest in oxygen hoses used with transport devices. Regular inspection and replacement, every 2–5 years, of all medical gas hoses is recommended.