Medical gas supply

Published on 07/02/2015 by admin

Filed under Anesthesiology

Last modified 07/02/2015

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Medical gas supply

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 1.11.4), 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.

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.

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.

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.

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 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.

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.

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).

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Fig. 1.7 Cylinder valves.

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 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 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 is overpressurized.

5. The non-interchangeable safety system (pin-index system) is used on cylinders of size E or smaller as well as on F- and G-size 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. A more recent modification is where the external part of the valve is designed to allow manual turning on and off of the cylinder without the need for a key (Fig. 1.10).

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 the rapid rise in pressure and the 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.

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).

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).

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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