Why can’t the remaining gas in an experimental gas cylinder be used up completely? Why is it necessary to maintain a certain pressure?
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1. Remaining gas in the cylinder after use: There are many different types of gases stored in cylinders, but regardless of the type of cylinder or gas, it is necessary to leave a certain amount of pressure as residual gas during use, so that other gases cannot get in. For example: There should be a gauge pressure of 1-2 kg/cm² remaining in the oxygen cylinder ； The acetylene cylinder should have a gauge pressure of 0.5–1 kg/cm². Generally speaking, the remaining pressure in the gas cylinder should not be lower than 0.5 kg/cm of gauge pressure. If such pressure has already been applied, the bottle valve should be closed immediately to prevent any residual gas from escaping. 2. Why is it necessary to leave some gas remaining in the cylinder? The purity of the gas contained within the cylinder must meet certain standards in order to ensure the quality of the gas and safety during use. If there is no residual gas left in the cylinder, air or other gases will enter it. As a result, the gas inside the cylinder becomes impure, leading to accidents during use. For example: Nitrogen itself is non-flammable and non-explosive, which is why it is widely used to displace flammable and explosive gases such as hydrogen for chemical reactions or for carrying out maintenance work that involves open flames. If air gets into the nitrogen cylinder, and since air contains oxygen, there will also be oxygen in the nitrogen. When using shock gas for displacement, oxygen is introduced; if oxygen mixes with flammable gases, it will inevitably lead to danger. In some situations, such as chemical analysis, very high purity is required for the gases used; even a slight impurity can affect the accuracy of the data and lead to the failure of the experiment. More importantly: if there is no residual gas in the cylinder, gases with conflicting properties may enter. For example, when cutting steel plates with a nitrogen-oxygen flame, if there is no residual pressure in the hydrogen tank or oxygen tank, hydrogen may flow into the oxygen tank or oxygen may flow into the hydrogen tank, resulting in an explosion on the spot. Even if backflow occurs after the hydrogen-oxygen flame goes out and no explosion takes place on the spot, an oxygen cylinder containing hydrogen or a hydrogen cylinder containing oxygen still poses a risk of explosion upon refilling and use next time. When using oxy-acetylene welding and cutting, if the oxygen cylinder is completely emptied with no residual pressure, acetylene gas will flow back into the oxygen cylinder, which can lead to an explosion of the oxygen cylinder when it is used again. Before filling the gas cylinders, the filling facility must check each cylinder for any remaining gas; cylinders with no residual gas do not meet the requirements for inspection. Gas filling units will not fill cylinders that lack killing gas blindly; strict cleaning is required first. This increases the amount of work, results in a waste of manpower and resources, and affects the production order of the gas filling units ； If care is not taken and an empty bottle slips through, and a gas with conflicting properties is filled into it, it can lead to serious problems. Therefore, this remaining air volume should not be underestimated, as it is of great significance for fire safety and improving production efficiency. When using gas cylinders, we must ensure that there is some air left in each cylinder to prevent unnecessary accidents and losses.
In short, leaving some gas in the cylinder is necessary to ensure the purity of the gas and its safe use. If there is no residual gas, other gases may enter, resulting in impure gas, accidents, or safety hazards. Therefore, when using gas cylinders, it is essential to maintain a certain amount of residual pressure to ensure safe operation and proper use. .
This post was last edited by qugd on 2023-4-23 at 16:14. In fact, the recommendation to leave some residual pressure in gas cylinders stems from the fact that gas supplies were not as abundant in the past, and it was either required by the gas suppliers or even intentional on their part to encourage users to do so. Let me share my opinion: First of all, the gas in a gas cylinder cannot be used up entirely in one go; even after using it several times, it still cannot be exhausted, because it’s not possible to connect a vacuum pump between the cylinder and the device that uses the gas. When the pressure inside the cylinder approaches the pressure in the device that uses the gas, it becomes impossible to use up the remaining gas in the cylinder without some other method ； Secondly, it is the gas suppliers who deliberately ask users to do this in order to save on the costs and time associated with cleaning the cylinders. Since the gas cylinder retains a certain amount of residual pressure, from the supplier’s point of view it is possible to fill the cylinder directly with the product gas without cleaning it; however, the user actually loses a considerable amount of gas from the cylinder. Third, as for any safety concerns, they are completely nonsense. Aside from certain toxic gases that require strict control, the lower the residual pressure in an used gas cylinder, the safer it is, as leaks are less likely to occur. And even if a leak does happen, the lower the residual pressure, the less gas will leak out; thus, it is safer to use up all the gas in the cylinder. Are cylinders with higher pressure safer than those with lower pressure? Another reason is that a small amount of residual pressure remains in the gas cylinder. In the past, when gas supply was inconvenient, if a certain type of gas was not available, a cylinder with residual pressure of that same gas could still be used for some time. Furthermore, the current management of gas cylinders covers all aspects such as supply, use, cylinder labeling, documents or certificates accompanying the cylinders, cylinder caps, and straps – all of which are regulated. If there are any safety issues, they stem from non-compliance on the part of the suppliers and users of the gas cylinders; these fall under the categories of management and personal competence, rather than being problems with the cylinders themselves. Between individuals, it is because of keeping up with the times that there arise rather unique perspectives.
This post was last edited by qugd on 2023-4-24 at 11:33. The filling methods and technologies used by current gas suppliers ensure that, even when the residual pressure in the cylinders is at atmospheric level, the methods for evacuating and purifying the filling ports are far more advanced than those used by ordinary users to prevent contamination; this was simply a requirement due to the imperfect technology available in previous years. As for safety, a higher residual pressure results in greater safety.
In laboratories (such as those using flame atomic absorption), there is often a shortage of acetylene, with some residual pressure left in the cylinders; although ignition is still possible, such practices do pose risks. Attention should be paid to gas preparation, and it should be replaced in a timely manner.