Sources of central gas supply Description Gases in

Sources of central gas supply Description: Gases in piped medical gas and vacuum systems are supplied from central supply sources in the hospital. This session provides an overview of cylinder manifolds, liquid oxygen stores and the oxygen concentrator.

Session introduction learning objectives: By the end of this session you will be able to Define the key components, functionality and safety features of cylinder manifolds. Recognize the important aspects of liquid oxygen stores (vacuum insulated evaporators) and their associated safety features. Recognize the key components, functionality and safety features of oxygen concentrators.

Cylinder manifold one source of central medical gas supply is central manifolds. These are used to supply nitrous oxide, entonox and oxygen at a constant pressure. This is normally achieved via a control panel from tow equal banks of large cylinders e. g. large J known as: Duty banks Standby banks The cylinders in each bank are connected through non return valves to a common pipe. All cylinders in each bank are turned on and interconnected. The duty and the standby banks alternate in supplying the pipe lines. The changeover from one to the other is an automatic process. Oxygen can also be supplied from vacuum insulated evaporators were it is stored as liquid oxygen or from large oxygen concentrators.

Automatic changeover automatic changeover from the duty to standby bank should occur at a cylinder pressure that will ensure the maximum usage of the contents of the duty bank. Each bank of cylinders has separate pressure regulator valves. Although the pipelines are fed from pressure regulators and work at about 400 KPa, changes in demand can lead to small fluctuations in pipeline pressure. Pressure gauges are also use to indicate cylinder bank pressures and distribution pressure. The number of cylinders in each bank is determined by expected demand.

Automatic changeover The total storage capacity: It should be one week supply. Each bank of cylinders should contain not less than two days supply, with a three days supply of spare cylinders kept in the manifold room. The nitrous oxide manifold is often larger than oxygen manifold. This is because nitrous oxide is present in cylinders only, whereas liquid oxygen is normally used to supply piped oxygen in hospitals, so the oxygen manifold is a back up.

The changeover units In the normal operation of the changeover control depends on an electrical supply, the design ensures that in the event of an electrical supply failure, there is no disruption to the flow of gas into the piped medical gas and vacuum system (PMGV). On restoration of the supply, the bank running when it failed started up again. Cylinders can be changed, or the pressure regulating valves removed for overhaul without a loss of continuity in the gas supply. The changeover unit is provided with monitoring to detect and display information regarding the function of the two banks and the pipeline pressure in cases of fault. The unit is housed in a lockable, steel enclosure with a glass fronted door to keep the control cleans and free from contamination.

manifolds For reasons of safety, the manifold is housed in a well ventilated room, built of fireproof material, away from the main buildings of the hospital. The room should facilitate the easy and quick changing of the cylinders. The manifold room is not used as a general cylinder store. All empty cylinders are removed immediately and any separate emergency cylinders are full and available for use. They are checked and the stock is rotated on a regular bases. Audio and visual warning systems should be constantly monitored, and if required, be placed in multiple locations. Responsibility for ensuring supplies to the manifold should be clearly established, defined, documented and implemented within the hospital.

Liquid oxygen Oxygen can also be stored as liquid, in a vacuum insulated evaporator made of steel ( the inner wall of stainless steel and the outer wall of carbon steel). This is the most economical way to store and supply oxygen, and allows for easy maintenance and access. Liquid oxygen is stored at a temperature of – 150 to – 170 ˚C ( below its critical temperature of - 118˚C ) and at a pressure of 5 – 10 atmospheres. The temperature of the VIE is maintained by the high vacuum shell ( effectively a vacuum flask). For a liquid oxygen to evaporate, it requires heat ( the latent heat of vaporization). This heat is taken from the liquid oxygen , helping to maintain its low temperature. The out side surface of VIE is painted white to reduce absorption of ambient heat. The VIE should be capable of delivering up to a maximum of 3000 L/min of oxygen. At a temperature of 15˚C and atmospheric pressure, liquid oxygen can give 842 times its volume as gas. VIEs can be supplied in up to 50 different sizes depending on the oxygen use in the hospital.

The liquid oxygen supply system Oxygen pass through a pressure regulator that allows it to enter the pipelines, and maintains the pressure through the pipelines at about 400 Kpa. If there is an under demand for oxygen, the pressure in the VIE starts to builds up, a safety valve opens at 1700 Kpa, allowing the gas to escape and there by reducing the pressure.

Hazards of central gas supply Due to the potential fire hazard, any liquid oxygen storage vessel should be housed away from main hospital buildings. The storage vessel is protected by a caged enclosure which also houses two banks of reserve cylinders. These reserve banks take over automatically as the hospital supply if the VIE out put fails. The risk of fire is increased in cases of liquid spillage. Spillage of cryogenic liquid can cause cold burns, frostbite and hypothermia.

Oxygen concentrators can be used to supply oxygen by extracting it from air by differential adsorption. Ambient compressed air is filtered and pressurized to about 137 k. Pa and enters one of two parallel, alternating absorber towers located on either side of the central mix tank, strongly attract nitrogen molecules, while allowing oxygen molecules to pass through. By the time all of the nitrogen and most other impurities have been removed. All that remains is the oxygen and trace amounts of inert argon. This oxygen passes into the mix tank, completing the one cycle of the concentrator.

Portable oxygen concentrators Also available portable oxygen concentrators that produce less than one liter per minute of oxygen. They use a demand valve to deliver oxygen only when the patient is inhaling. These portable concentrators plug into the wall out let using either AC or DC power.

Session key points • Cylinder manifolds can be used to supply piped gases at a constant pressure. Large size cylinders are used in two banks, duty and standby. There is automatic changeover between the two banks. • Liquid oxygen is usually used to supply piped oxygen in hospitals. It is stored in vacuum insulated evaporators at a temperature of – 150 to – 170 ˚C ( below its critical temperature of - 118˚C ) and at a pressure of 5 to 10 atmospheres. • Oxygen concentrators are used to extract oxygen from air using differential adsorption. Oxygen concentration of 93 – 95 % can be achieved.

Session summary • Learning objectives: • Define the key components, functionality and safety features of cylinder manifolds. • Recognize the important aspects of liquid oxygen stores ( vacuum insulated evaporators) and their associated safety features. • Recognize the key components, functionality and safety features of oxygen concenterators.