Medical Gases Supply (Part One) PDF

Wondershare 1 PDFelement Medical Gases Supply (Part One) ‫الطبيب االستشاري‬ ‫الدكتور‬ ‫معتز فؤاد األغا‬  Supply of Medical Gases...

Wondershare 1 PDFelement Medical Gases Supply (Part One) ‫الطبيب االستشاري‬ ‫الدكتور‬ ‫معتز فؤاد األغا‬  Supply of Medical Gases Wondershare 2 PDFelement Most of medical gases, used in anaesthetic practice, are stored in cylinders or, in the case of oxygen, in a large container outside the hospital called a vacuum-insulated evaporator (VIE). From these sources, the gas is piped to where we need it in the clinical areas. A working knowledge about the gases we use every day and the safety features that prevent harm to patients and the hospital itself is vital. Cylinders Medical gas cylinders are made of thin-walled seamless molybdenum ( & or chromium ) steel to 1 increase strength and to 2 minimize weight and wall thickness. Light weight aluminium alloy or carbon composite varieties are increasingly common though & can be used to provide oxygen at home, during transport or in (MRI) scanners.  Wondershare 3 PDFelement As the cylinders have to withstand a high gas pressure and rough handling during transport, they are subjected to mechanical testing before marketing for use. The tensile test, flattening test, impact test, and bend test are performed on one cylinder from a batch of every 100 finished cylinders. Hydraulic test or pressure test is performed on every finished cylinder from the batch before filling and marketing. This test confirms that the cylinder is leak proof. An internal endoscopic examination is carried out also. Cylinders must be tested by manufacturers every 5 – 10 years. The top of cylinder is called the neck. 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. They are also engraved on the cylinder.  Wondershare 4 PDFelement  Wondershare 5 PDFelement The neck ends in a tapered screw thread into which the valve is fitted. The thread is sealed with a fusible plug made from a metallurgic alloy known as Wood’s metal,( Barnabas Wood, American dental surgeon). It is a eutectic mixture (one having a lower melting point than that of its constituents), consisting of bismuth, lead, tin and cadmium. It has a melting point of around 70 °C , allowing dissipation of pressure in a fire that might heat the cylinder to the point of explosion. This pressure relief valve is designed to rupture at 3300 psi. Cylinders are manufactured in different sizes, labeled alphabetically from A (smallest) to J (largest). Size J cylinders are commonly used for cylinder manifolds. Cylinders used on anaesthesia machine are C, D & E size.  Wondershare 6 PDFelement  Wondershare 7 PDFelement  Wondershare 8 Each cylinder contains a gas under specified pressure, PDFelement which is known as service pressure. Service pressure is the maximum pressure at 70°F (20 °C) to which the cylinder is ordinarily filled. Pressure in a filled cylinder at 70°F may not exceed the service pressure marked on the cylinder. O2, He, He-O2, CO2-O2 are allowed additional 10%. For safety, the cylinders are designed to withstand pressures 65–70% above their working range pressure (23,000 kPa or 5000 psi). 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. Parts of Cylinder Body, Valve, Port, Stem, & Pressure relief valve  Wondershare 9 PDFelement  Wondershare 10 Body PDFelement 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 i.e. weight of the cylinder when empty Bodies of the cylinders have flat or concave bases. Valve Cylinder outlet valves are made of bronze or brass, which is heavily plated with nickel and chromium, as to allow a rapid dissipation of heat of compression. Cylinder valves are of various types. Those to be used on anesthesia machines are “flush” type which fits with the pin index system on the machine , and for the medium and large capacity cylinders the valves are “bullnose” type.  Wondershare 11 PDFelement  Wondershare 12 Port PDFelement The port is the point of exit for the gas. It fits into the nipple (on the hanger yoke of the anaesthesia machine). It should be protected in transit by a covering. When installing a cylinder on anaesthesia machine, it is important for the user not to mistake the port for the conical depression. The Conical depression, is situated on the opposite side of the port on the cylinder valve above the safety relief device. Stem Each valve contains a stem (spindle/screw-pin) or shaft that is rotated to open or close the cylinder valve. It is made up of very hard steel. Gas cylinder identification The gas contained within the cylinders can be identified in a number of ways : (1) colour-coding system: this is an international standard to which most countries adhere. However USA, still have non-standard colours in use.  Wondershare 13 PDFelement  Wondershare 14 PDFelement (2) Chemical symbol engraved onto the cylinder valve (3) Name and chemical symbol on the cylinder label (4) Pin index system. The use of pin-index system began in 1952. Cylinders have multiple safety features to ensure that they deliver the correct gas and minimise the risk of explosion. The valve, which is engraved with the chemical symbol for the gas within, has a single exit port for the gas. Before attaching a full cylinder to the machine ,it should be briefly opened and closed to blow out any dust or oil that is sitting in the exit port, to prevent this entering the anaesthetic machine. This is called cracking the cylinder. The port sits close to the holes for the pin index system, an arrangement of pins and corresponding holes located in one of seven possible positions to form a pattern unique to each gas.  Wondershare 15 PDFelement  Wondershare 16 PDFelement The valve features the holes and the cylinder yoke on the anaesthetic machine carries the pins. This ensures that only the correct gas can be attached. Between the exit port of the cylinder and the anaesthetic machine there is a Bodok seal (bonded disk). This is a neoprene washer, within a brass or aluminium ring, which ensures a gas-tight fit between the cylinder and anesthetic machine yoke and prevents any leakage of the cylinder contents into the atmosphere. Under no circumstances, may oil or grease be used as a seal; the pressurised gases give off heat as they are released from the cylinder and may cause EXPLOSION. Machine cylinders should initially be opened slowly to prevent a rapid pressure rise within the machine pipework. A rapid increase in pressure within a fixed volume (the pipework) causes an increase in temperature. this is called (adiabatic temperature rise).  Wondershare 17 PDFelement  Wondershare 18 PDFelement 19 Adiabatic process is any change in a system that does not PDFelement Wondershare involve transfer of energy between the system and its surroundings. In this example, the change in pressure is so rapid that the thermal energy doesn’t have time to be conducted into the pipework of the machine. Consequently, as no energy has left the gas, its temperature increases and, in the presence of a small amount of grease or other contaminant, a fire or explosion result. The cylinder valve should be fully open when in use (the valve must be turned two full revolutions). During closure, over-tightening of the valve should be avoided. This might lead to damage to the seal between the valve and the cylinder neck. The Bodok seal should be inspected for damage prior to use. Having a spare seal readily available is advisable.  Wondershare 20 This non-combustible seal must be kept clean and should PDFelement 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 Bodok seal be renewed. Excessive force should never be used. 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. Volume of Gas within a Cylinder Cylinders have a standardised internal volume ( capacity) ,independent of what they contain.  Wondershare 21 PDFelement  Wondershare 22 PDFelement This is quantified as the volume of water the cylinder could hold if full. The volume of pressurised gas or vapour that the cylinder holds when full is dependent on the individual substance and also the region of the world in which it will be used. Oxygen, N2, air, and He are in the form of compressed gases in the cylinder at service pressure, but N2O, CO2, are in the liquid form in the cylinders. The cylinders are not fully filled with liquids otherwise a rise of temperature may cause a rise of pressure leading to bursting. They are filled up to the specified filling ratio (the weight of gas with which the cylinder is filled, divided by the weight of the water the cylinder could hold). In temperate climates, such as the UK, cylinders are filled up to filling ratio of 0.75. In hotter climates, this is reduced to 0.67. For oxygen, the volume of gas available can be worked out using Boyle’s law. As long as the temperature stays constant: P cylinder x V cylinder = P atmosphere x V atmosphere  Wondershare 23 PDFelement  Wondershare 24 PDFelement This can be rearranged : V atmosphere = P cylinder x V cylinder/ P atmosphere Full oxygen cylinder is pressurized to 13,700 kPa For a size E cylinder( the capacity = 4.7 litre) as found attached to the back of an anaesthetic machine: Volume available = 13, 700 x 4.7 / 101.325 = 635.5 litre of oxygen As oxygen is stored above its critical temperature of –118 °C, no matter how much pressure is applied it will never turn to liquid. As it always exists as a gas in the cylinder, the pressure on the gauge will always be directly proportional to the volume of gas remaining. This rule does not apply for nitrous oxide. As the critical temperature for N2O is 36.5 °C, at room temperature it exists in the cylinder as a liquid in equilibrium with a vapour above it. A vapour is any substance in the gaseous state but at a temperature below the critical temperature for that gas.  Wondershare 25 PDFelement  Wondershare 26 What distinguishes a vapour from a true gas is that a vapour PDFelement can be liquefied by application of pressure whereas a gas cannot. A full nitrous oxide cylinder is pressurized to 4,400 kPa but, due to the storage as a liquid and vapour, this can change significantly with changes in ambient temperature. Because of this, cylinders are only partially filled to allow some space for more liquid to vaporize if the temperature increases. The amount by which the cylinder is filled depends on the climate. As the nitrous oxide vapour is used, the pressure within the cylinder falls, which results in more of the liquid turning to vapour to maintain the equilibrium. Consequently, the vapour pressure is maintained as a constant. This pressure is the only quantity a standard Bourdon gauge can measure. This means that the pressure in the cylinder does not drop until all of the liquid has been used and the cylinder is nearly empty.  Wondershare 27 A small drop in pressure is sometimes seen during high- PDFelement consumption use of the cylinder. In this circumstance, the constant vaporization of fresh liquid consumes (latent heat of vaporization) and thus causes the temperature to drop. As the volume in the cylinder is constant, Gay-Lussac’s law states that if the temperature drops, the pressure will also drop. To determine the volume of vapour within the nitrous oxide cylinder we must therefore use the mass of chemical within the cylinder. All cylinders have their tare weight (empty weight) printed on the label and engraved into the neck. By weighing the cylinder, we can then work out the mass of nitrous oxide within. This can be converted into the volume of gas using Avogadro’s law (1 mol of gas will occupy 22.4 litre at atmospheric pressure). For example, if a cylinder contained 2 kg (2,000 g) of nitrous oxide (which has a molecular mass of 44):  Wondershare 28 PDFelement 2,000 / 44 = 45.45 mol 45.45 x 22.4 = 1,018 litre of nitrous oxide vapour. Nitrous oxide can exist as a liquid, vapour and gas in anaesthetic practice. In the cylinders, vapour and liquid co-exist as described. In the anaesthetic machine, it is a vapour and, in the circle, which operates at around 38 °C to 40 °C on account of the heat from the reaction in soda lime, it can exist as a true gas. The interface between all these states can be described by the pressure–volume isotherm diagram. To make sense of it, read the isotherms from right to left. Entonox is a 50:50 mixture of oxygen and nitrous oxide, which is commonly used as an analgesic within emergency departments and maternity wards. This mixture is presented in cylinders, or as a part of a cylinder manifold connected by pipeline to the outlets in the hospital.  Wondershare 29 PDFelement  Wondershare 30 PDFelement A full cylinder is pressurized to 13,700 kPa. Entonox is created by bubbling oxygen through nitrous oxide. The production of a mixture of these two gases is an example of the Poynting effect. There is a change in vapour pressure and other physical properties observed when gaseous oxygen is combined with the nitrous oxide. The mixture of Entonox exhibits a pseudocritical temperature of (–5.5 °C). If the mixture is cooled below this point, it liquefies in an uneven manner called lamination, where more nitrous oxide liquefies, leaving an oxygen-rich mixture above. As this is used up, the composition of the mixture changes, becoming increasingly rich in nitrous oxide content and progressively more hypoxic. To prevent this from happening, Entonox cylinders should be stored at room temperature in a horizontal position to increase the surface area of the liquid for re-vaporization. In cold conditions, the cylinder should be up-ended to mix the contents before use.