Medical gas supply

Published on 07/02/2015 by admin

Filed under Anesthesiology

Last modified 07/02/2015

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Chapter 1

Medical gas supply

Gas supply

Medical gas supply takes the form of either cylinders or a piped gas system, depending on the requirements of the hospital.

Cylinders

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 is called the neck, and this 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 so 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 can be identified from the shape and colour of the disc.

4. Cylinders are manufactured in different sizes (A to J). 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. Size E oxygen cylinders contain 680 L, whereas size E nitrous oxide cylinders can release 1800 L. The smallest sized cylinder, size C, can hold 1.2 L of water, and size E can hold 4.7 L while the larger size J can hold 47.2 L of water.

Fig. 1.1 Nitrous oxide cylinder with its wrapping and labels.

Fig. 1.2 Oxygen cylinder valve and pin index.

Fig. 1.3 Nitrous oxide cylinder valve and pin index.

Fig. 1.4 Carbon dioxide cylinder valve and pin index.

5. Lightweight cylinders can be made from aluminium alloy with a fibreglass covering in epoxy resin matrix. These can be used to provide oxygen at home, during transport or in magnetic resonance scanners. They have a flat base to help in 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 and −118°C respectively.

Oxygen is stored as a gas at a pressure of 13 700 kPa whereas nitrous oxide is stored in a liquid phase with its vapour on top at a pressure of 4400 kPa. 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 with any increase in the ambient temperature that can lead to an explosion. In the UK, 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.

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 per minute will last for 2 hours and 50 minutes but will last only 45 minutes when delivering 15 L/min.

A full oxygen cylinder at atmospheric pressure can deliver 130 times its capacity of oxygen.

At constant temperature, a gas-containing cylinder shows a linear and proportional reduction in cylinder pressure as it empties. For a cylinder that contains liquid and vapour, initially the pressure remains constant as more vapour is produced to replace that used. Once all the liquid has been evaporated, the pressure in the cylinder decreases. 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.

Cylinders in use are checked and tested by manufacturers at regular intervals, usually 5 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. Internal endoscopic examination.

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

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

4. Tensile test where strips of the cylinder are cut and stretched. This test is carried out on at least one cylinder in every 100.

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).

Fig. 1.5 Anaesthetic machine cylinder yokes. For the sake of comparison, the Bodok seal is absent from the nitrous oxide yoke (left).

3. Cylinders are colour-coded to reduce accidental use of the wrong gas or vapour. In the UK, the colour-coding is a two-part colour, shoulder and body (Table 1.1). To improve safety, there are plans to change the colours of the bodies of cylinders using medical gas to white while keeping the colours of the shoulders according to the European Standard EN 1089-3.

Table 1.1

Colour coding of medical gas cylinders, their pressure when full and their physical state in the cylinder

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. F, G and J size cylinders are stored upright to avoid damage to the valves. C, D and E size 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.

Cylinders

• Cylinders are made of thin-walled molybdenum steel to withstand high pressures, e.g. 13 700 kPa and 4400 kPa for oxygen and nitrous oxide respectively. Lightweight aluminium is also available.

• They are made in different sizes: size E cylinders are used on the anaesthetic machine; size J cylinders are used in cylinder banks.

• Oxygen cylinders contain gas whereas nitrous oxide cylinders contain a mixture of liquid and vapour. In the UK, nitrous oxide cylinders are 75% filled with liquid nitrous oxide (filling ratio); this is 67% in hotter climates.

• At a constant temperature, the pressure in a gas cylinder decreases linearly and proportionally as it empties. This is not true in cylinders containing liquid/vapour.

• They are colour-coded (shoulder and body).

Cylinder valves

These valves seal the cylinder contents. The chemical formula of the particular gas is engraved on the valve (Fig. 1.6). Other types of valves, the bull nose, the hand wheel and the star, are used under special circumstances (Fig. 1.7).

Fig. 1.6 Chemical formula (N₂O) engraved on a nitrous oxide cylinder valve.

Fig. 1.7 Cylinder valves.

Cylinder valves

• They are mounted on the neck of the cylinder.

• Act as an on/off device for the discharge of cylinder contents.

• Pin-index system prevents cylinder identification errors.

• Bodok sealing washer must be placed between the valve and the yoke of the anaesthetic machine.

• A newly designed valve allows keyless manual turning on and off.

Piped gas supply (piped medical gas and vacuum – PMGV)

PMGV is a system where gases are delivered from central supply points to different sites in the hospital at a pressure of about 400 kPa. Special outlet valves supply the various needs throughout the hospital.

Oxygen, nitrous oxide, Entonox, compressed air and medical vacuum are commonly supplied through the pipeline system.

Components

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

2. Pipework 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.

3. 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.

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

Fig. 1.12 Inserting a remote probe into its matching wall-mounted outlet socket.

Fig. 1.13 Gas probes for oxygen (top), nitrous oxide (middle) and air (bottom). Note the locking groove on the probe to ensure connectivity.

5. 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).

Fig. 1.14 Outlet sockets mounted in a retractable ceiling unit. (Courtesy of Penlon Ltd, Abingdon, UK (www.penlon.com).)

6. 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 where the thread is gas specific and non-interchangeable (non-interchangeable screw thread, NIST, is the British Standard).

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