1 medical gas cylinders and pipeline systems...medical gas cylinders and pipeline systems 3...

Veterinary Anesthetic and Monitoring Equipment, First Edition. Edited by Kristen G. Cooley and Rebecca A. Johnson.© 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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1.1 Medical Gas Cylinders

Gas cylinders are used to store and supply medical gases to clinical areas of the veterinary hospital, including anesthetic machines, mechanical ventilators and surgical instruments. Cylinders are attached, either directly or using a pipeline and distribution system to the anesthetic machine or outlet. Cylinders are available in a number of sizes, described by the letters A–J. Their designated let-ter is dependent on their size; A is the smallest and J the largest (British Oxygen Company 2016). The size used is dependent on the intended use and region of the world. For example, in the United Kingdom (UK) and European Union (EU), the most commonly used sizes are E, F and J, whereas in North America, E and H are more usual. Table 1.1 details the sizes and volumes of the these cylinders.

Cylinders are available containing a number of differ-ent medical gases. The medical gases most used in vet-erinary practice include oxygen, nitrous oxide and medical air, although carbon dioxide may also be found in certain situations. For example, carbon dioxide is used for body cavity insufflation during minimally invasive procedures.

1.1.1 Gas Pressures

A pressure gauge, most commonly of the Bourdon type (Figure 1.1), should be associated with the cylinder con-nection to the system. A Bourdon gauge consists of a coiled tube that changes shape, dependent on gas pres-sure. As the coil changes shape, an attached pointer moves over the scale to display the pressure (Davis and Kenny 2007). The gauge reads the pressure generated within the cylinder or pipeline distribution system, dependent on the amount of gas or vapor supplied. The term “gas” is used to describe the contents of a cylinder

containing a non‐liquefied compressed gas. Examples include oxygen and medical air (Davis and Kenny 2007). The cylinder contents are present in this state when the gas does not change into a liquid at room temperature regardless of pressure applied, since room temperature is above the critical temperature of gas. The critical tem-perature is the temperature above which a substance cannot be liquefied, regardless of the pressure applied. The term “vapor” is the gaseous state of a substance when, at ambient temperature, it is present below its critical temperature (Davis and Kenny 2007). The liquid phase is present in the cylinder with the vapor phase remaining on top; nitrous oxide is an example of a medi-cal gas stored in this state.

The International System of Units (SI) used for pres-sure is the pascal (Pa) which, for convenience, is com-monly expressed as kilopascal (kPa). However, conversion to other commonly used pressure units is frequently per-formed and is as follows:

100 1000 1 7501000 14 5 12

kPa mbar bar mmHgcmH O psi atm= = =

= = =. (1.1)

1.1.2 Medical Gases

Oxygen and medical air are stored as compressed gases (Highley 2009; Westwood and Rieley 2012), whereas nitrous oxide and carbon dioxide are stored as a liquid with a vapor phase above. The gauge pressure will depend on the particular gas. For example, those stored as compressed gases will have a gauge pressure that is directly related to cylinder contents at all times, whereas those stored in their liquid phase will only have a gauge pressure directly related to cylinder contents once all of the liquid has vaporized as the cylinder empties. This concept is discussed further below.

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Medical Gas Cylinders and Pipeline SystemsCarl Bradbrook

European Specialist in Veterinary Anaesthesia & Analgesia, Consultant Veterinary Anaesthetist, ACE Vets Ltd, Rossett, UK

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COPYRIG

HTED M

ATERIAL

Veterinary Anesthetic and Monitoring Equipment2

1.1.3 Cylinder Components

Cylinders are traditionally composed of molybdenum steel, although lighter weight cylinders made from alu-minum with non‐magnetic valves are becoming more common and are magnetic resonance imaging (MRI) compatible (Dorsch and Dorsch 2008a; Highley 2009). Smaller aluminum cylinders with an epoxy resin coating may be used for patient transport (Figure 1.2). Cylinders are tested by the manufacturer every 5 years to ensure they are safe for continued use and a colored disc (Figure 1.3) may be placed around the neck (UK) of the valve to indicate when testing is next due. Information detailing the results of the safety tests is printed onto a sticker placed on the plastic collar surrounding the neck of the cylinder. Tests may be performed to check the strength of the cylinder, including subjecting them to greater than normal working pressures and endoscopi-cally detecting any imperfections that may affect perfor-mance. Approximately 1 in every 100 cylinders is randomly subjected to impact testing and has its struc-tural integrity checked by strip testing. Strip testing involves a piece of the tested cylinder being examined in depth for any damage and imperfections. Information

Table 1.1 Gas cylinder dimensions and capacity.

UK US

E F G J E F G H

Dimensions (inches) 34 × 4 36 × 5½ 54 × 7 56½ × 9 26 × 4½ 51 × 5½ 51 × 8½ 51 × 9¼Oxygen (L) 680 1360 3400 6800 650 2062 5300 6900Nitrous oxide (L) 1800 3600 9000 18 000 1590 5260 13 800 15490

Bourdon (copper) tube

Pointer

Lever

System pressure

Figure 1.1 Left: Interior schematic of a Bourdon type pressure gauge including the expandable copper tubing and pointer. Right: Example of a common Bourdon gauge routinely located on an anesthetic machine.

Figure 1.2 Lightweight CD aluminum oxygen cylinder for use during patient transport.

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Medical Gas Cylinders and Pipeline Systems 3

regarding the cylinder working pressure, serial number and testing may also be permanently etched onto the cyl-inder shoulder.

A valve block (Figure 1.4) is present at the top of the cylinder. The valve allows the cylinder to be connected to

an appropriate outlet and facilitates cylinder opening and closing. The valve type depends on the cylinder size (Dorsch and Dorsch 2008a; Highley 2009; Alibhai 2016). Size E oxygen cylinders have a pin index valve (Figure 1.5), whereas the larger J oxygen and medical air cylinders have a side spindle pin index valve (Figure 1.6). Cylinders

Figure 1.3 Cylinder safety information on the colored disc and printed sticker on the cylinder neck.

Figure 1.4 Side spindle pin index valve on a size J cylinder (UK).

Figure 1.5 Size E (UK) oxygen cylinders in a vertical racking system with pin index valve.

Figure 1.6 Medical grade air cylinder with a side spindle pin index valve.

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fitted with the pin index system require a specific key or handle to open the valve. Size F oxygen cylinders have a bullnose valve (Figure 1.7), size G nitrous cylinders have a hand wheel side outlet (Figure 1.8) and size H cylinders have a bullnose valve, similar to F tanks. The valve allows either direct connection to an anesthetic machine or dis-tribution pipeline supply. Unused, full cylinders fre-quently have a wrapping to ensure dust does not block the valve outlet port. This should always be checked for integrity and removed prior to use. The valve should always be opened slowly prior to use, as this prevents an adiabatic change from occurring (Davis and Kenny 2007). An adiabatic process is defined as one that occurs without an exchange of heat energy with the surround-ings. For example, the pressure in the pipeline and gauge may rise rapidly, subsequently compressing the gas con-tained within the system, which may result in an increase in temperature and pose a potential fire risk. The valve should also not be over tightened when closed to prevent damage to the valve itself. A safety pressure release device is integrated within the valve housing and pre-vents cylinder over‐pressurization and the subsequent risk of an explosion.

1.1.4 Safety

All cylinders are color coded (Dorsch and Dorsch 2011; Alibhai 2016) according to their constituent medical gas

to minimize accidental misconnections. An interna-tional code exists between some countries, but it is not universally adopted. The cylinder color depends on the country where it is used and therefore should always be checked if working outside of the practitioner’s normal environment. The color coding used in the UK, EU and North America is detailed in Table 1.2. The new EU standard cylinders (British Oxygen Company) all have a white body, with different colored shoulders and the constituent medical gas clearly written on the cylinder body to further improve safety. All cylinders are identi-fied either by the color of their body and/or shoulders (Figure 1.9A–C).

Pin indexing (International Standards Organization 2004; Dorsch and Dorsch 2011, Westwood and Rieley 2012; Moseley 2015) is incorporated within the valve block, or stem, of size E and J cylinders, to prevent con-nection to the incorrect gas yoke on an anesthetic machine or yoke on a distribution system. The pin index system is specific to the medical gas contained within the cylinder (Figure 1.10A, B, C), and corresponding holes on the yoke (Figure 1.11) only allow the specific gas cyl-inder to be attached. A compressible washer, “O” ring or Bodok seal (Figure 1.12), is placed between the valve out-let and yoke to ensure a gas‐tight fitting. The pin index

Figure 1.7 Bull nose valve on a size F (UK) oxygen cylinder. This type is similarly seen on size H (US) cylinders.

Figure 1.8 Nitrous oxide size G cylinder with a wheel valve.

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and washer, ring or Bodok seal should be checked prior to cylinder connection and never have grease or oil applied to them.

As previously mentioned, cylinders should always be inspected prior to use, looking for any obvious external damage. The plastic valve wrapping or cap should be checked to ensure that it is intact and removed prior to attachment to the yoke, bull nose valve or distribution sys-tem. The yoke or similar attachment and any seal used should be checked for its integrity. Connections should not be over tightened, and once attached, the cylinder valve should be opened slowly to allow gas to be released. Any leaks should be identified and corrected at this time. The valve should be opened a minimum of two full turns to

ensure complete gas flow and the pressure gauge read to check the cylinder capacity (appropriate for all cylinders to ensure they are not empty, but only reads quantity left for oxygen and medical air cylinders, see above). Cylinders attached to an anesthetic machine should be labeled as “in‐use” or “full” as appropriate, to ensure that a cylinder con-taining a gas in its liquid state (e.g. nitrous oxide) is known to be either full or empty (Dorsch and Dorsch 2011).

Cylinder storage is dependent on size. Size E cylinders may be stored horizontally or vertically in appropriate restraints, so they cannot move or be tipped over (Figure 1.13).

Larger cylinders (size F and greater) should be stored vertically and chained to a wall or attached to the

Table 1.2 Gas cylinder identification.

GasGasSymbol

Color Code(US) Color Code (UK)

New color code (EU standard)

Pin Index System Physical state

Oxygen O2 Green/ Green and white

Black with white shoulders White 2, 5 Gas

Nitrous oxide N2O Blue Blue White with blue shoulders 3, 4 Liquid/vaporMedical air None Yellow Grey with black/white

quarter shouldersWhite with black/white quarter shoulders

1, 5 Gas

Carbon dioxide CO2 Grey Grey White with grey shoulders 1, 6 GasEntonox (50% O2:50% N2O)

O2, N2O Blue Blue with blue/white quarter shoulders

White with blue/white quarter shoulders

7 Gas/liquid/vapor

(A) (B) (C)

Figure 1.9 Examples of larger cylinder color identification. (A) Oxygen cylinders are white (UK) or have white shoulders and a black body (US). (B) Medical air cylinders can be gray with white or black shoulders (UK and EU) or yellow (US). (C) Nitrous oxide cylinders are commonly associated with a blue body or shoulders, in many geographical locations.

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anesthetic machine using a permanent mount. Although not recommended, cylinders may be kept outside if extreme temperatures are not encountered, but should be protected from direct sunlight and rain. When stored outside, they should be kept in a locked area that is tam-per proof. Careful storage is required to prevent cylinder damage and also to prevent injury to personnel. Large cylinders should be moved with the aid of a trolley or dolly, and gloves should be worn when handling them (Highley 2009).

Personal safety should be considered when working with gas cylinders. For example, smoking and flames are pro-hibited in proximity to any cylinders containing flammable

gases or liquid oxygen tanks due to risk of explosion. Cylinders should not be exposed to extreme temperatures, particularly high temperatures, should be handled with care, and should not be dropped due to risk of explosion from high pressures. All areas used for storage of cylinders

O2

5 2

LEFT RIGHT

(C)(B)(A)

123456

Figure 1.10 Pin index system on cylinder valves. The pins are placed at positions 2 and 5 on an E cylinder of oxygen (A) and at positions 3 and 5 on an E cylinder of nitrous oxide (B). The pin index system for an oxygen tank (positions 2, 5) and a complete schematic diagram showing all positions referred to in Table 1.2 are shown (C).

Figure 1.11 Close‐up photo demonstrating the pin index system on the corresponding machine yoke for attachment of an E cylinder of oxygen. Figure 1.12 “O” ring or Bodok seal, with ruler for scale.

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and liquid oxygen tanks should be clearly signposted with safety warnings (Figure 1.14). Appropriate staff training should be undertaken by all personnel involved in using medical gases and attention paid to any local or regional regulations regarding their use (Moseley 2015).

1.1.5 Connections

Cylinders are either connected directly to an anesthetic machine using a yoke, by hosing to an anesthetic machine, or to a pipeline distribution system (Dorsch and Dorsch 2008a,b; Highley 2009; Westwood and Rieley 2012).

Figure 1.13 Cylinder storage rack for small size cylinders (e.g. E and CD).

Figure 1.14 Safety warning signs clearly posted on the wall in a gas cylinder storage area.

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As mentioned above, Size E cylinders are attached directly to an anesthetic machine using a yoke with the integrated pin index safety system (Figure 1.5). Size F cylinders are attached from their bull nose valve (Figure 1.7) to the first stage regulator that has flexible tubing to connect to the anesthetic machine by a non‐interchangeable screw thread (NIST; Highley 2009). Size G cylinders have a hand wheel valve (Figure 1.8) and size J cylinders a side spindle pin index system (Figure 1.6) for connection to a distribution system

1.1.6 Gas Cylinder Volume, Contents and Filling Ratio

The volume of gas (Dorsch and Dorsch 2008a; Moseley 2015) contained within a cylinder is dependent on the cyl-inder size and the physical state of the constituent gas (Table 1.1). For example, oxygen is present as a com-pressed gas (Table 1.2) and therefore its remaining volume within a cylinder is directly proportional to the pressure displayed on the gauge. For example, an E oxygen cylinder (UK) contains approximately 680 L oxygen when full at a pressure of 13 700 kPa. As the cylinder empties, the pres-sure displayed will be directly proportional to the cylinder contents; that is, at a pressure of 6850 kPa (50% full) there will be 340 L oxygen remaining.

Nitrous oxide and carbon dioxide (Table 1.2) are supplied as liquids with vapor above, therefore the pressure gauge does not denote volume remaining until all of the liquid has vaporized. Nitrous oxide cylinders contain a volume calcu-lated according to the filling ratio (Highley 2009; Westwood and Rieley 2012), defined as the mass of liquid (i.e. nitrous oxide) in the cylinder divided by the mass of water required to fill the cylinder. In the EU and North America, this ratio is 0.75; in tropical climates, this is reduced to 0.67 to ensure the pressure generated by the vaporizing liquid does not exceed the cylinder specification. Thus, in contrast to oxy-gen or medical gas, an E nitrous oxide cylinder (UK) con-tains approximately 1800 L when full, but maintains a constant pressure of 4400 kPa until all of the liquid contents vaporize. At this point, the pressure gauge will fall as the remaining gas is delivered to the system. Therefore, there is no correlation between cylinder contents and gauge pres-sure and the cylinders must be weighed to determine the quantity of nitrous oxide remaining in the tank.

Readers are referred to the documents available from the British Oxygen Company (BOC) on cylinder types, valves, pin index, medical gases and capacities in the UK (http://www.bocmedical.co.uk).

1.2 Liquid Oxygen Tanks

In a large hospital with high oxygen demands, liquid oxy-gen stored in tanks may be used to supply the distribution

system. Liquid oxygen is stored at –150 to –170 °C, below its critical temperature (–119°C) and at a pressure of 5–10 atm within a double insulated tank (Howell 1980; Dorsch and Dorsch 2008b; Alibhai 2016). Storage of oxygen in its liquid state allows a larger quantity to be contained within the same volume compared to when in its gaseous state, making it more economical for the practitioner, especially if a nearly constant supply is required. One liter of liquid oxy-gen produces 842 L of gaseous oxygen at room temperature (Howell 1980). If demand is not constant, then wastage will occur due to atmospheric venting, which prevents excessive pressure from building within the tank.

The storage container is a vacuum insulated evapora-tor (VIE: Howell 1980; Dorsch and Dorsch 2008b; Al‐Shaikh and Stacey 2013; Alibhai 2016) (Figure 1.15). Insulation is present between the two container walls and a vacuum is present to maintain its low temperature. The liquid oxygen continually draws heat from its sur-roundings, resulting in its vaporization (this is the latent heat of vaporization). Vaporized oxygen is removed from the VIE along the pipeline at the top of the container, where it passes through a heated coil, expanding as it does so. When demand is high and in excess of the nor-mal vaporization processes, a control valve directs liquid from the bottom of the tank through a vaporizer and then to a super‐heated coil. This gas then combines with the vaporized oxygen to supply the distribution system (Howell 1980). A pressure regulator is used to ensure

Figure 1.15 Vacuum insulated evaporator for liquid oxygen storage.

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that a constant pressure (400 kPa; 3000 cmH2O) is supplied to the pipeline and delivered to the hospital distribution system (Howell 1980).

Safety surrounding the storage and use of liquid oxy-gen is extremely important. Contact of liquid oxygen with skin may cause thermal burns, and pressure build‐up within the system without appropriate venting may lead to explosion. A pressure relief valve is present to allow for excess pressure to be released to the environ-ment due to continuous liquid oxygen vaporization when demand is low. The tank may be covered in ice and there-fore presents a hazard if made contact with, due to con-densing and subsequent freezing of water vapor on its cold surface. However, a liquid oxygen supply allows for remote filling away from the main hospital site and therefore is less disruptive. The VIE sits on a weigh scale to allow calculation of volume, which is displayed on an electronic screen. This may also be remotely connected to the gas supply company to alert them to fill the tank when it drops below a certain volume.

1.3 Oxygen Concentrators

Oxygen concentrators (Figure 1.16) filter and extract room air to produce oxygen for medical use. This may prove to be a cost‐effective method for supplying oxygen, although it requires an electricity supply. Concentrators

consist of two zeolite molecular sieves in parallel. Zeolite retains nitrogen, some of the argon, and other unwanted components of air (Westwood and Rieley 2012; Al‐Shaikh and Stacey 2013). Oxygen is released and pres-surized by a compressor to approximately 137 kPa (1028 cmH2O) for delivery to an anesthetic machine or other patient delivery system. An electronic valve switches between the two columns to ensure a constant oxygen supply. A maximum of approximately 10 L/min may be delivered at up to 92–95% oxygen. If used with a circle breathing system, higher gas flows must be delivered to ensure excessive argon (a small component of room air) concentrations do not accumulate within a low flow sys-tem. Argon accumulates due to its similar molecular size to oxygen and therefore it is able to pass through the molecular sieve (Al‐Shaikh and Stacey 2013, Dorsch and Dorsch 2008c). Different types of zeolite have been uti-lized to reduce the amount of argon that passes through the sieve and increase the amount adsorbed. Appropriate gas monitoring in the anesthetic system should be avail-able to ensure minimum oxygen delivery within the inspired gas mixture.

1.4 Medical Gas Pipeline Systems

Gas pipelines (Howell 1980) allow the supply of gases and vacuum from a central point to sites throughout the veterinary clinic, without the need for large pieces of apparatus at each outlet. Gas is supplied at 400 kPa (4 bar; 3000 cmH2O), although medical air for surgical instru-ments may be delivered at 700 kPa (7 bar; 5250 cmH2O). Special outlets are installed in clinical areas, allowing connections for anesthetic machines or other equipment. Oxygen, nitrous oxide, medical air and medical vacuum/suction may all be supplied through a pipeline system. Active gas scavenging systems (AGSS) may also be pro-vided in combination with gas pipeline systems.

1.4.1 Components

Gas pipeline systems (Howell 1980, Westwood and Rieley 2012) begin with a supply source, consisting of a bank of gas cylinders or liquid oxygen tanks. As previously men-tioned, pipeline systems may be used for multiple gases as well as medical vacuum and suction, if available. A primary pressure regulator that ensures a constant supply to the distribution system controls the gas source. At this central supply point, there is a control panel displaying the status of the pipeline and each medical gas in use (Figure 1.17A,B). If a bank of cylinders is used as the supply source, then a secondary bank should also be available to switch to when the pressure from the primary bank falls below usable quantities. This apparatus is known as a cylinder manifold,

Figure 1.16 Example of an oxygen concentrator (DeVilbiss Healthcare, Tipton, UK).

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and often comprises an automatic changeover between cylinder banks with subsequent activation of an electronic alarm, alerting staff that one bank is depleted and requires changing. Similarly, if liquid oxygen is used, then a reserve bank of cylinders should be available if the primary supply becomes depleted or fails. From the pressure regulator and corresponding pressure gauge, the distribution pipeline supplies the medical gas to the terminal outlet, consisting of a socket which accepts a matching quick connect and disconnect or Schrader probe (Figure 1.18). These compo-nents are detailed below.

1.4.2 Cylinder Manifold

A cylinder manifold (Howell 1980, Westwood and Rieley 2012) consists of two groups of large cylinders (Figure 1.19),

which alternate to supply medical gases to the distribution system. In the UK, size J cylinders are most commonly used for oxygen and medical air and size G for nitrous oxide. In the US, size H cylinders or liquid oxygen tanks are most commonly used. The manifold should be protected from the weather, being kept in a building or other suitable environment. One‐way or non‐return valves (Figure 1.20) using copper alloy pipework, connected to a common pressure regulator and gauge, connect each bank of cylinders. This ensures that the supply to the distribution system is kept constant. When the detected pressure is low from the “in use” cylinder bank, it automatically switches to the second full bank of cylinders. This switching triggers an electronic audible and visual alarm on the display to inform the user. Generally, the user is then required to move the in‐use lever to the full bank (Figure 1.21) and arrange to replace the exhausted cylinders. Some mani-folds may have a third, reserve bank of one or two cylinders to ensure the supply is not lost if both banks fail. If the gas source supply is a liquid oxygen tank, then a reserve cylinder bank should similarly be available.

1.4.3 Distribution System Connections

Gas cylinders connect to the pipeline at a central point, as part of a manifold or directly with connectors (i.e. side spindle pin index valve, wheel valve, etc.), depending on cylinder size and constituent gas. From the cylinder, manifold gases are distributed via the pipeline system to the terminal outlets in the treatment areas of the clinic.

1.4.3.1 Cylinder and Manifold AttachmentsAs described above, if demand is relatively high, an automatic supply system, such as a cylinder manifold, provides a constant supply to the clinic. Gas cylinders are connected to the manifold in two banks by flexible

(A) (B)

Figure 1.17 Pipeline system central control panel. (A) Control panel at the cylinder manifold with associated gauges and valves. (B) Medical gas alarm control panel.

Figure 1.18 Terminal outlet with gas specific indexing collar; Schrader style, quick connect‐disconnect type. The outlet is color coded to match the specific gas being supplied. The oxygen outlet is to the left and nitrous oxide outlet to the right, depicting the UK standard.

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pipework (Figure 1.22) to a common distribution system. Each cylinder supplying the manifold has a separate one‐way or non‐return valve feeding its supply to the central system (Howell 1980). All valves on the in‐use cylinder bank should be fully open to ensure uniform emptying across the bank. These valves are easy to turn and may display a colored red or green line to denote their status. On the secondary bank, at least one cylinder should be fully open to ensure consistency of supply at switchover. At the start of the common distribution system, a pres-sure gauge is fitted (Howell 1980) after the regulator to display and monitor gas supply to the distribution sys-tem (Figure 1.23). The user may manually adjust the

pipeline pressure if required. There is also a pressure relief valve associated with the common part of the system (Figure 1.24). Each bank of cylinders has a vent valve to allow depressurizing when switching over to the secondary bank.

Copper alloy pipework is used to supply the individual medical gases to the required locations within the hospi-tal, ending at self‐sealing terminal outlets. Copper alloy is used as it does not interact with the medical gases and is also bacteriostatic (Howell 1980). Pipework diameter may vary, depending on position within the distribution system; for example, between the central supply point and dividing areas, piping will be of a larger diameter than after branching to supply individual rooms.

Figure 1.19 Cylinder manifold for supply to a pipeline distribution system. This is comprised of a left and right bank, 6‐cylinder oxygen manifold.

Figure 1.20 Non‐return or one‐way valve associated with cylinder connections to a manifold. The red/orange indicates that the valve is closed.

Figure 1.21 Manual switchover lever to discern between left and right cylinder banks in use following automatic changeover.

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An alternative to a manifold is to use a pressure regulator and gauge fitted directly to the cylinder (Figure 1.25A,B). Flexible hosing with a Schrader probe or appropriate screw connection (Highley 2009) is directly attached to the regu-lator and then supplies the distribution system. This is less expensive than a cylinder manifold, but requires manual changeover between cylinders; supply may therefore be interrupted. This may be more useful for the supply of gases other than oxygen, in facilities with a lower gas demand or those places not requiring constant delivery.

1.4.3.2 Terminal Outlets and Machine ConnectionsConnections between the gas cylinders/manifolds and the terminal outlet or the anesthetic machine have a number

Figure 1.22 Cylinder connections to a manifold using the flexible tubing.

Figure 1.23 Pipeline pressure gauge and valve for manual adjustment of pipeline pressure. The manual pressure adjustment valve is labeled as “do not touch” to avoid inadvertent alterations to pipeline pressure.

Figure 1.24 Pressure relief valves to vent one side of the manifold.

(A) (B)

Figure 1.25 (A) Medical air size J cylinder connected to a pipeline directly from a side spindle pin index pressure regulator and gauge. (B) Size G nitrous oxide cylinder connected to a pipeline directly from a side port hand wheel valve.

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of safety features to ensure correct gas supply. The safety system is based on threaded and non‐threaded connec-tions that are gas specific, to minimize risk of incorrect gas supply (Highley 2009; Moseley 2015; Alibhai 2016).

Terminal outlets (Howell 1980; Highley 2009) vary in type but always consist of a socket with an indexing col-lar that is specific to the gas supplied and therefore only a probe with the appropriate collar (Figure 1.26A) or pin system (Figure 1.26B) can be connected. The terminal units are also color coded and matched to the specific gas supplied. This avoids inadvertent connection of the wrong gas supply to the anesthetic machine or other piece of equipment. The terminal outlets are self‐sealing and allow for easy and rapid connection and disconnec-tion of hoses (Highley 2009). Once connected, a “tug test” should be performed to ensure the connection is secure and will not inadvertently detach. Different types of outlet are available, depending on where they are posi-tioned. They may be wall mounted, on a hose dropped from the ceiling (Figure 1.27), or on a ceiling mounted unit. Terminal outlets for AGSS (Figure 1.28) usually have diameter index safety system (DISS) connections, again as a further safety feature to minimize risk of incor-rect connection.

The anesthetic machine is connected to the terminal outlet using static‐free, color‐coded flexible tubing (Figure 1.29). This may have a Schrader probe and collar for attachment to the outlet at one end and a DISS with NIST (Highley 2009, Westwood and Rieley 2012; Moseley 2015) to attach to the anesthetic machine itself (Figure 1.30) at the other end, especially in the UK. The color coding is the same as the color of the appropriate

medical gas cylinder. Each medical gas is assigned a DISS number, which is unique to the gas being supplied, and along with the NIST avoids inadvertent connection to the incorrect outlet. Only single, continuous hoses should be used for connections, as this avoids the risk of misconnection if a junction were present. If any tubing is found to be damaged, it should be replaced with a new unit and in no circumstances should it be attached with any connections to a second hose.

(A) (B)

Figure 1.26 Examples of terminal gas outlets. (A) Schrader probe with gas specific (oxygen) index collar for insertion into the terminal outlet (UK). (B) Standard terminal vacuum (left, white) and oxygen (right, green) wall outlets (US).

Figure 1.27 Available mounting types for terminal outlets. This is a ceiling mounted system with flexible tubing suspended from the ceiling mount.

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1.4.4 Safety Features and Alarms

Pipeline distribution systems have a number of safety alarms (Howell 1980, Dorsch and Dorsch 2008b) to ensure they function correctly and to alert the user to any faults. The gas pressure being supplied to the distri-bution system is regulated from the cylinder or liquid

oxygen tank as it enters the central control point. A pres-sure relief valve is present to prevent excess pipeline pressure, usually set at 50% above the normal working pressure. An electronic central display unit and control panel (Figures 1.17A, B) senses low gas pipeline pres-sures and sounds an audible and visual alarm. It informs the user which medical gas supply is low, which cylinder bank is empty, and if any faults within the system require troubleshooting. This unit should be located in a promi-nent position within the clinic to ensure that appropriate personnel are aware of any changes to the system status. Although the secondary cylinder bank and reserve cylin-ders are responsible for providing adequate oxygen sup-ply back up, each anesthetic machine supplied by a gas pipeline should also be fitted with an oxygen cylinder, usually size E (Figure 1.31), in case of pipeline failure or another emergency situation.

Isolating or shut‐off valves (Figure 1.32) should be strategically placed in the facility to enable the user to cut the supply to a specific area of the hospital, either in an emergency or for planned maintenance (Highley 2009). Emergency shut‐off valves should also be fitted at the manifold to ensure the entire supply can be stopped if required.

Figure 1.28 Active gas scavenging system (AGSS) terminal outlet.

Figure 1.29 Color‐coded, static free, flexible tubing for attachment of an anesthetic machine to the terminal outlet. In the UK, white connects oxygen, blue connects nitrous oxide, black connects medical air, and pale blue connects the AGSS. These colors may differ in other geographical locations (i.e. North America).

Figure 1.30 Diameter index safety system (DISS) with non‐interchangeable screw threads (NIST) for attachment of gas specific tubing to the anesthetic machine.

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A qualified engineer should only carry out testing and maintenance on the gas distribution system to ensure patient and personnel safety. A servicing contract will usually be in place to ensure the safe working of any medical gas pipeline.

References

Alibhai, H.I.K. (2016) The anesthetic machine and vaporizers. In: Duke‐Novakowski. T., de Vries. M., Seymour. C. (eds). BSAVA Manual of Canine and Feline Anaesthesia and Analgesia, 3rd ed. Gloucester, UK: BSAVA, pp. 24–44.

Al‐Shaikh, B. and Stacey, S.G. (2013) Medical gas supply. In: Essentials of Anaesthetic Equipment, 4th ed. Edinburgh, UK: Churchill Livingston Elsevier, pp. 1–18.

British Oxygen Company Group PLC. Cylinder Data Chart. Available from: http://www.bocmedical.co.uk [accessed October 2016].

Davis, P.D. and Kenny, G.N.C. (2007) Pressure. In: Basic Physics and Measurement in Anaesthesia, 5th ed. Edinburgh, UK: Elsevier, pp. 1–11.

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Figure 1.31 Size E oxygen cylinder attached to an anesthetic machine as an emergency backup should the pipeline supply fail.

Figure 1.32 Isolating valves to shut off supply to a specific area of the distribution system.

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