Chris Thompson 2003 Gas Supplies 1. GAS MANUFACTURE Oxygen Bulk production usually by fractional distillation of air. Air is compressed to 5 atmospheres and cooled to -181°C using reverse heat exchangers. A 2-stage distillation process yields 99.5% O2 (and 0.4% argon). Oxygen concentrators are used in remote, field or domestic applications. Molecular sieves contain a zeolite matrix that strongly absorbs N2, CO2 and water.. If pressurised air is applied to one side, some gas emerges as 95.4% O2 with 4.5% argon on the other; on reverse depressurisation, the absorbed gases are vented away. Two such units in parallel can provide a continuous supply of O2. Plastic membrane filters exist but can only produce about 30-40% O2. Nitrous Oxide By heating ammonium nitrate: NH4NO3 --> N2O + 2 H2O followed by 'scrubbing' with NaOH then washing with water to remove toxic higher oxides of nitrogen (NO and NO2). Refrigeration and chemical drying removes residual water vapour prior to compression. Compressed Air In larger hospitals dual central electric compressor are used. The air inlet must not be near where trucks park! An air filter/dryer is required. Smaller hospitals use manifolded cylinder banks. 2. BULK SUPPLIES a) Liquid Oxygen : the Vacuum Insulated Evaporator or VIE These devices are designed to store and supply liquid oxygen, and are used in most large hospitals, ie when typical requirements are more than 300 l/sec or 7,000,000 l annually. The inner shell is made of stainless steel and is separated from the outer carbon steel shell by an insulated gap with a vacuum of 0.16 to 0.3KPa. Contents are at 1000 KPa and -150°C (critical temp -119°C). A gauge indicates pressure within and a differential pressure gauge (comparing top to bottom) indicates contents by weight. Low oxygen use / very hot weather: --> increased temperature of liquid oxygen High oxygen use / cold weather --> decreased temperature of liquid oxygen All outgoing gases go through a heat exchanger (often icy) to warm them up. Some sort of cylinder based backup is always required and a system of remote alarms are required to indicate low contents and pressure. Tankers use a non-interchangeable fitting. Never smoke within 6 metres of a VIE in case oxygen is being released into the environment. Hazards: Failure of supply, fire, explosion. b) Manifolded Cyinders Used for N2O in most hospitals and oxygen in smaller ones.
When the bank in use run low on pressure the amber alarm light comes on in the hospital (it is set to 650 KPa), and when the pressure falls below 550 KPa the secondary bank regulator opens automatically and continues to provide gas at 550 KPa until it too is exhausted. The red alarm indicates supply failure and comes on when the line pressure falls below 350 KPa. Usually as soon as the amber light comes on CIG are notified and they bring a new bank of cylinders. The lever is switched over to set the secondary bank to operate at 700 KPa , ie they become the new primary bank, and the fresh cylinders become the new secondary bank.
3. GAS PIPELINES Pipes are degreased, sealed, and steam cleaned seamless copper, tested to 1400 Kpa as per AS 1169. For installation the joints are of silver alloy and all pipes are identified with colour-coded labels every 2m. Colour codes and sleeve indexing parameters are in the following table:
Table 1: pipeline colours and sleeve indexing parameters Non-ferrous isolating valves are strategically located, particularly for oxygen where isolating valves are required in theatres. A terminal block with a 19 tpi 1/4" male British standard pipe thread is attached to the pipe; it protrudes through the wall plate. While the thread diameter is the same for all gases, the wall connector incorporates a surrounding sleeve that is different for each gas (the example on the right is for the sleeve indexed (SIS) system). Other connection systems may be used. In all cases both the terminal block and the wall connector are pin-indexed to each other so that only matching connectors can mate together across the wall panel. Usually a self-sealing O-ring is located in the wall connector, opening when the hose is connected. Testing of new installations must performed twice before use, first by the engineers and then by both the engineers and a medical officer. A white cloth over the end of the pipe should remain clean on purging, there should be no smell, pressure loss over 24 hours should be zero, and finally correct gas composition is confirmed using an oxygen analyser. Status Panels One status / alarm panel is required in the operating room and another in the medical gas supply office (or at switchboard). Indicator lights should follow this convention: green for normal operation, amber the 'on-reserve' state, and red indicating failure. Procedures to act on amber or red alarms should be in place. A gauge or display indicating line pressures is required on the anaesthetic machine. We must know where these panels and indicators are, and what to do if there is a problem. A common Part 2 exam question is to ask you where the gas supply status panel is in your operating room and what it means. Finally, each individual operating room must be able to have it's gas supply disconnected in the event of fire, and it is our responsibility to know where these control valves are Oxygen-fueled fires in pendants, ceilings etc are extremely hazardous; the incoming oxygen supply must be disconnected as soon as possible. 4. CYLINDERS Manufacture AS 2030 1977. Now aluminium. Extruded from bar stock, neck pressed at 400°C, heat slowly to 525°C, quenched in water, and finally aged at 175°C for 8 hours. Typical wall thickness only 3mm. One of each batch (usually 100 in a batch) is selected and the material tested for tensile strength and yield strength; if it fails, 2 more are tested, and if either fail the whole batch is destroyed. Each individual cylinder is connected by its thread to a testing unit. It is filled with water and the water level measured by a gauge. Then the gauge is isolated and the cylinder pressurised to 24,000 Kpa (240 atmospheres) using a hydraulic ram. If it stretches more than 0.02% (ie, the water level drops after the pressure is released), it is rejected. Testing must be repeated every 10 years.
Table 2: cylinder colours and pin indexing codes All screw threads are the same (1:8 taper Whitworth 55° curved ends 14 tpi 18.2mm). A pin indexed brass or bronze valve body screws into this. The valve seat is nylon. Cylinder colours should match contents (AS 1944, 2471). Filling: Gases filled to 13,700 KPa (2/3 of tested pressure). Liquids are filled by weight so that a cylinder at 65°C reaches 85% of rated pressure. For both CO2 and N2O this means, in practice, filling to about 2/3 of the cylinder's water capacity (the filling ratio) - or about 1.87 kg of N20 in the 2.8 kg C size cylinder. A C size nitrous cylinder holds about twice as much nitrous as oxygen (in litres of gas).
Table 3: cylinder size and contents Cylinder Markings:
Problems.
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The usual system is to have two banks each of at least 10 "G" size (7,600 l oxygen, 17,000 l N
Pin indexing: