A gas cylinder is a containment apparatus that will store a gaseous compound under pressure for use in medical settings. The physical form of the stored compound can be gas and/or liquid, with the ultimate output from the apparatus being gaseous. Gas cylinders allow for portable and safe storage of compounds needed in a medical setting. The sizing of gas cylinders has been attributed to a nationally recognized letter, with the more commonly used medical sizes listed below. Gas cylinders are labeled from A to M, with increasing volume as the letters of the alphabet proceeds. E sized cylinders are the most commonly used size in medical settings. E cylinders have a service pressure of 1900 psi, but may be filled up to 10% more to 2200 psi. Higher filling pressure allows for expansion at temperatures greater than 70 degrees Fahrenheit.
The following are standard container sizes and the volume of oxygen contained at 2200 psig (maximum).
- B: 200 L
- D: 425 L
- E: 660 L
- F: 1360 L
- G: 3400 L
- M: 3450 L
Color System for Compounds
A standardized color system identifies the compound in the container. The United States color system for oxygen and air differs from the colors used internationally:
- Oxygen - Green (*White)
- Carbon Dioxide - Gray
- Nitrous Oxide - Blue
- Nitrogen - Black
- Helium - Brown
- Air - Yellow (*White & Black)
In order to prevent connecting the wrong gas cylinders, a safety system is in place. This system, called the Pin Index Safety System, provides a standardized, unique pin configuration system that acts as a lock and key system to prevent the mismatching of gas cylinders with their corresponding connections in medical settings.
Issues of Concern
Pressure Release Device
The release of pressurized gas can be hazardous, and extreme caution should be exercised. Gas pressure levels should be reduced from stored high pressure to a workable, usable level. A pressure regulator should be used in this situation and any time contents are being removed or used from the cylinder. Do not tamper with pressure release devices. Do not use any cylinders with visibly defective pressure release devices. Pressure release devices and gas outlets should never be pointed in any direction that could cause harm.
While attached to an anesthesia machine, the cylinder valves should be in the off position while not in use. This is to prevent leakage and so that the provider will be notified via an alarm of failure of the pipeline supply of gas. If the cylinder were to be left open and a pipeline gas supply failure occurred, the anesthesia machine would consume the oxygen in the cylinder, and the provider would not be notified until the cylinder was depleted.
A cylinder should be inspected for malfunctions and defects before use. Full cylinders are usually placed with a tamper-evident seal. This is generally a tear-off seal on the outlet of the valve and is removed before use. Proper inspection of a cylinder includes the outlet, pin index safety system, and especially the pressure relief device. The valve outlet should be clean before use. Only use cylinders marked with DOT (Department of Transportation) or ICC (Interstate Commerce Commission). In Canada, cylinders may be marked with BTC (Board of Transportation Commissioners) or CTC (Canadian Transport Commission). A cylinder should be connected to a regulator to bring the compressed, stored pressure down to a working, usable pressure. Inspect the regulator for signs of damage or foreign materials.
Safe Storage and Transportation
Cylinders must be stored upright and secured using a rack, strap, or chain to minimize the chance of falling over. Cylinders should be transported using a cart or carrier. Never drop or hit cylinders, and never drag, roll or slide cylinders, even for a short distance. Only qualified personnel should refill cylinders. While using cylinders, avoid flammable substances, smoking, open flame, or any other incendiary sources. Cylinders should be stored in a dry, cool, well-ventilated area away from exposure to weather. Cylinders should be stored at temperatures less than 125 degrees Fahrenheit/52 degrees Centigrade.
Oxygen cylinders have more specific storage requirements than other medical gases. While oxygen cylinders can be stored in the same space as other non-flammable medical gases as long as they are segregated properly, full oxygen cylinders and empty oxygen cylinders cannot be stored together. The separation of full and empty cylinders prevents the accidental usage of an empty cylinder during an emergency situation. Partially full oxygen cylinders may be stored in the same location as full cylinders, provided that they are labeled properly.
Use Boyle's law to calculate how much oxygen time is remaining in an E-sized cylinder. Boyle's law states that at a fixed temperature (room temperature) of an ideal gas, the pressure is inversely proportional to volume. Boyle's law can be further rearranged to state that pressure times volume is equal to a constant. The following is the formula:
One could compare a cylinder of gas at filled volume (V1 = 660L) and pressure (P1 = 2200psi) to the current pressure (P2) read on the cylinder. This would provide the information needed to solve for the current volume (V2) remaining in the tank in liters. The following is the formula:
(Measured pressure remaining using the integrated pressure gauge in psi/2200psi) *660L = Liters of oxygen remaining in the tank.
This Volume (V2) can be used to determine the amount of unit time remaining left on the cylinder, given a current flow rate of the gas.
- V2/Flow rate = unit time remaining, or
- Liters of oxygen remaining in the tank/oxygen setting in liters/minute = Minutes of oxygen remaining
The same formulas can be used in cylinders with pure gaseous form. To calculate for nitrous oxide, however, is only applicable once the pressure drops below 745 psi. This is due to having the liquid and gaseous form within the tank. The pressure will remain constant until 75% of the 1590 L gas is consumed, which is approximately equal to 400 L remaining within the cylinder. From this point until empty, the above formulas apply. Prior to this point, the cylinder must be weighed to determine the amount of gas remaining within the cylinder.
The transportation of gas cylinders is highly regulated by both local, state and federal agencies in most countries. In the UD, the Department of Transportation is the governing authority. Further, there are guidelines for the manufacturer to ensure that the cylinders have been tested and are safe. Some of the tests that cylinders undergo include tensile strength, hydrostatic test, impact testing, burst testing and pressure cycling. Once the cylinder is manufactured, it must have all the vital information permanently etched on to the cylinder.