Alarm system

Combustible materials (combustible gases, vapors, and dusts) must be uniformly mixed with air (or oxygen) within a certain concentration range to form a premixed gas; an explosion will occur only when this mixture comes into contact with a source of ignition. This concentration range is known as the explosive limit, or explosion concentration limit. For example, the explosive limit of carbon monoxide mixed with air is 12.5% to 80%. The lowest and highest concentrations at which a flammable mixture can explode are referred to as the lower explosive limit and the upper explosive limit, respectively; these are sometimes also called the lower ignition limit and the upper ignition limit. At concentrations below the lower explosive limit and above the upper explosive limit, it neither explodes nor catches fire. This is because the combustible material concentration in the former is insufficient, and the cooling effect of the excess air prevents the flame from spreading ； The latter is due to a lack of air, which prevents the flame from spreading. The greatest explosive power is achieved when the concentration of the combustible material is roughly equivalent to the stoichiometric concentration for the reaction (i.e., the concentration ratio calculated based on the complete combustion reaction equation). The greater the explosion limit range of a flammable mixture, the lower its lower explosion limit, and the higher its upper explosion limit, the greater its explosion hazard. This is because the wider the explosion limit, the greater the chances of conditions for an explosion occurring ； The lower the explosion threshold, the more that even a slight leakage of flammable material can create conditions for an explosion ； The higher the explosion limit, the more air can penetrate into the container, where it can mix with the flammable substances inside to create conditions for an explosion. It should be noted that when the concentration of a flammable mixture exceeds its upper explosive limit, although ignition and explosion do not occur, it can still burn when it escapes from a container or pipeline and comes into contact with air again, so there remains a risk of fire. The unit for the explosion limit: The unit for the explosion limit of gases or vapors is expressed as a percentage (%) of the volume they occupy in the mixture. For example, the explosion limit of a hydrogen-air mixture is 4% to 75%. The explosion limit of combustible dusts is expressed as a mass ratio of volume in the mixture, in g/m3; for example, the explosion limit of aluminum powder is 40 g/m3. Calculation of the explosion limit: The equivalent concentration for explosive reactions, as well as the lower and upper limits of explosion, and the methods for calculating the explosion limits of mixtures of various flammable gases are as follows: (1) Equivalent concentration for explosive reactions. When the concentration ratio of the combustible material to the oxidizing material in an explosive mixture is such that a complete chemical reaction can occur, the most heat is released during the explosion, resulting in the highest pressure. The actual reaction equivalent concentration is slightly higher than the calculated value, as explosive mixtures usually contain impurities. The molecular formula of combustible gases or vapors is generally expressed as CαHβOγ. Let n be the number of moles of oxygen required to burn 1 mole of such gas; then the combustion reaction can be written as: CαHβOγ + nO2 → products of combustion. With an oxygen concentration of 20.9% in standard air, the chemical equivalent concentration X (%) of a combustible gas in air can be calculated using the following formula. The chemical equivalent concentration of a combustible gas in oxygen, denoted as Xo (%), can also be determined using this formula. Additionally, it is possible to find the chemical equivalent concentration of a combustible gas or vapor in air (or oxygen) directly from Table 1, by using the value of 2n, which represents the number of oxygen atoms required for complete combustion. Among them. Chemical equivalent concentration of combustible gases (vapors) in air and oxygen (2) Lower and upper explosion limits. The explosion limits of various flammable gas and flammable liquid vapors can be determined using specialized instruments, or estimated using empirical formulas. The estimated values of the explosion limit generally differ from the experimental values, as the calculation formulas take into account only the composition of the mixture and cannot account for the influence of a range of other factors; nevertheless, they still hold some reference value. 1) The lower and upper explosion limits of organic compounds are estimated based on the number of oxygen atoms required for complete combustion, using the following empirical formula. Formula for the lower explosion limit: (volume) Formula for the upper explosion limit: (volume) Where L_lower is the lower explosion limit of the flammable mixture ； L upper limit – the upper explosion limit for flammable mixtures ； n —— the number of oxygen atoms required for the complete combustion of 1 mol of combustible gas. The estimated upper and lower explosion limits for certain organic compounds are compared with experimental values in Table 2: Table 2 compares the stoichiometric concentrations of paraffinic hydrocarbons as well as the calculated and experimental values of their explosion limits. As can be seen from the values in the table, there is a certain difference between the experimental and calculated values; however, by using a safety factor, these values can still serve as a reference in actual industrial applications. 2) By calculating the explosion limit and the chemical equivalent concentration at complete combustion of an explosive mixture based on the chemical equivalent concentration, it is possible to estimate the lower and upper explosion limits of organic substances. The calculation formula is as follows: This formula is used for alkanes, and the calculated values show an error of no more than 10% when compared with experimental values. For example, the experimental value of the methane explosion limit is 5%–15%, which is very close to the calculated value. However, when used to estimate H2, C2H2, and flammable gases containing N2, Cl2, etc., the errors are significant, so it cannot be applied. (3) Explosion limits of mixtures composed of various flammable gases. The explosion limit of a gas composed of various flammable gases can be estimated based on the explosion limits of each component, using the following formula: Where Lm represents the explosion limit of the explosive mixture (%) ； L1, L2, L3, Ln —— Explosive limits (%) of each component in the mixture ； V1, V2, V3, … Vn —— the concentration of each component in the mixture (%). V1+V2+V3+…Vn=100. This formula is quite accurate for calculating the explosion limits of mixtures such as gas, water gas, and natural gas. However, it yields significant errors when applied to mixtures like hydrogen with ethylene, hydrogen with hydrogen sulfide, methane with hydrogen sulfide, or mixtures containing carbon disulfide; therefore, it should not be used in such cases. ——Excerpt from the Encyclopedia of Safety Science and Technology (China Labor and Social Security Press, published in June 2003). Explosive limit: The concentration limit at which a homogeneous mixture of flammable gases or vapors and oxidizing gases can cause an explosion under standard test conditions. The oxidizing gas can be air, oxygen, or other oxidizing gases. The explosion limit mentioned in general terms refers to the concentration limits of flammable gases or vapors in air. The lowest concentration of a flammable gas that can cause an explosion is known as the lower explosive limit ； The highest concentration is known as the upper explosion limit. The explosion limit varies depending on the components of the mixture. In the same mixture system, the explosion limit can be altered by factors such as the initial temperature, system pressure, amount of inert medium present, the volume in which the mixture is contained, the material of the container walls, and the intensity of the ignition energy. The general rule is that as the initial temperature of the mixture increases, the explosion limit range widens, that is, the lower limit decreases while the upper limit increases. As the system temperature rises, the internal energy of the molecules increases, turning what was originally an non-flammable mixture into a flammable and explosive one. As system pressure increases, the explosion range also widens. This is because higher pressure brings the molecules closer together, increasing the likelihood of collisions and thus facilitating combustion reactions. As pressure decreases, the explosion limit range narrows ； When the pressure drops to a certain value, its upper and lower limits coincide; the pressure corresponding to this point is known as the critical pressure of the mixture. When the pressure drops below the critical pressure, the system is no longer an explosive system (with some gases exhibiting abnormal behavior). As the amount of inert gas in the mixture increases, the explosive range narrows; when the concentration of inert gas reaches a certain value, the mixture can no longer explode. The smaller the diameter of the container or pipe, the smaller the explosion range. When the diameter of the flame channel becomes small enough, the heat dissipated by the solid cooling surface per unit volume of flame exceeds the heat generated, causing the flame to be interrupted and extinguished. The maximum pipe diameter beyond which the flame cannot propagate is known as the critical diameter of that mixture system. A high ignition energy, a large area of the hot surface, and varying contact time between the ignition source and the mixture can all widen the explosion limit. In addition to the factors mentioned above, the material of the closed enclosure in which the mixed systems are in contact, mechanical impurities, light exposure, surfactants, and other elements can also affect the explosion limit range. The explosive limit of flammable vapors is determined by the vapor concentration generated at the surface of the flammable liquid. For flammable liquids, the flash point temperature corresponding to the lower explosive limit concentration can also be referred to as the lower explosive limit temperature ； The liquid temperature corresponding to the upper explosion limit concentration can also be referred to as the upper explosion limit temperature.

The second floor didn’t answer the original poster’s question; take a closer look

Could you provide me with the full text of the standard “Design Code for Detection and Alarm of Flammable and Toxic Gases in Petrochemical Enterprises” SH3063-1999, which is available upstairs?

The regulated areas include: handshake :handshake

Generally, alarms are designed to detect one or several specific types of gases; therefore, before installing them, it is best to first determine which flammable/hazardous gases are present in the factory area, and then choose the appropriate alarms. I don’t know what constitutes a chain reaction; here we use fixed combustible gas detectors. When the concentration at one location exceeds the limit, the control room is alerted, and the alarm stops once the issue is resolved on site. It’s also possible to silence the alarm after identifying the problem. This post was last edited by limingshuguang on 2008-1-3 20:18.]

For different media, the probe structures vary; does anyone know the principle behind this?

I’m studying hard*, so I won’t give any opinions, hehe

I don’t understand it, but it seems very comprehensive after reading it

When designing the interlocks, two-level alarms and emergency exhaust interlocks must be taken into consideration. As mentioned above, different flammable gases have varying explosion limits. The first alarm level is typically set at 25% of the explosion limit concentration, while the higher alarm level is set at 50%, triggering emergency exhaust ventilation at the same time.