On the explosion of combustibles
Thread Content
Dear seniors, I have a question to ask. Let’s imagine a hypothetical scenario: Natural gas, in large quantities of over 7,000 cubic meters per hour, is sent to container A. This container A is then connected to a discharge flare via an intermediate container B. Initially, the natural gas mixes with the main oxygen, allowing normal combustion; but now there is a problem that causes the system to shut down, and the supply of main oxygen is cut off. The auxiliary oxygen, in smaller amounts of around 200 cubic meters per hour, still mixes with the natural gas. If neither of these gases is burning at this point, could an explosion occur if the supply of main oxygen is restored after too long a delay? Why? Because I think it’s like this: when you get out of the vehicle at first, nitrogen enters and takes away some of the heat, but natural gas still has some heat flowing through it, and this process is continuous. So how can an explosion occur? Isn’t an explosion caused by a sudden increase in pressure? How can there be a pressure buildup when there’s continuous flow? Will putting a large amount of oxygen and natural gas into a torch cause an explosion? I would like to know about this question. Also, if anyone has some understanding of explosions, could they please explain it? Thank you!
The last edit to this post was made by Hansonwen on 2018-7-23 at 15:29. 1) To determine the fire and explosion hazards of the combustible mixture in torches, it is first necessary to study the lower explosive limit (LEL) of the main components of the torch gas under normal temperature and pressure conditions； The combustion and explosion characteristics of the original mixed combustible gas are then analyzed using Le Chatelier’s equation. The combustible components in flare gas are mainly methane, ethylene, ethane, and hydrogen. The explosion limit of a mixture composed of various flammable gases can be calculated based on the explosion limits of each component. Using Le Chatelier’s principle, the formula for calculating the explosion limit of the original mixed combustible gas is as follows: Lm = 100/(φ1/L1 + φ2/L2 + φ3/L3), where Lm represents the explosion limit of the explosive mixture in %, and L1, L2, L3 are respectively the explosion limits of each component in the mixture in % ； φ1, φ2, and φ3 represent the concentrations of each component in the mixture (%v), respectively. Therefore, the calculated lower explosion limit of the torch mixture is A%. 2) If the lower explosion limit of the mixed gas emitted from various parts of the device is very low, it is essentially impossible to eliminate the risk of combustion and explosion by controlling the concentration of combustible components in the process flow. Therefore, controlling the Limiting Oxygen Concentration (LOC) is commonly used to prevent combustion and explosions. To calculate the limiting oxygen concentration for combustible gases, it is necessary to simplify the gas composition by considering only the proportion of combustible gas components and normalizing the concentrations of various combustible gases. The LOC values for each combustible component are determined according to Table 5 in Clause 5.1 of GB/T27862-2011 \"Test methods for classification of chemical hazards – Combustibility and oxidizing capacity of gases and gas mixtures\". The limiting oxygen content of the mixed combustible gas can be determined using the following formula: (LOC)mixt = ∑[XiRi / ∑XiRi/(LOC)i]. Where: (LOC)mixt is the limiting oxygen content of the mixed combustible gas; (LOC)i is the limiting oxygen content of each individual combustible gas; Xi represents the molar or volume fraction of that combustible gas in the mixture; Ri is the number of moles of oxygen molecules consumed during the complete combustion of 1 mole of that combustible gas component; and Li is the lower explosion limit of that combustible gas component. According to this formula, the limiting oxygen concentration of the emitted mixed gas is B%. Based on the above calculations, the lower explosion limit of the emissions from the existing flare is A%, and the Limiting Oxygen Concentration (LOC) is B%. If the oxygen concentration in the mixture after adding main oxygen is C%, and it can be determined that C% is significantly lower than the critical oxygen concentration B%, then it can be concluded that there is no risk of combustion or explosion in the flare gas after adding main oxygen ； Conversely, there is a risk of explosion.
We had a project with similar conditions before; we hired an engineering institute to conduct a safety assessment, using the methods mentioned above