Combustible gas and its detection
Combustible gas is the most dangerous gas encountered in petrochemical and other industrial occasions. It is mainly organic gases such as alkanes and some inorganic gases.
When the combustible gas (steam, dust) is mixed with air (or oxygen) and reaches a certain volume fraction, it will explode when it encounters a fire source. The range of the volume fraction that explodes in case of fire is called the explosion limit of the gas, and the minimum explosion concentration required for explosion is the lower explosion limit, which is represented by LEL, which is the abbreviation of Lower Explosive Limit. The maximum explosion concentration is the upper explosion limit, which is expressed by UEL, which is the abbreviation of Upper Explosive Limit. The explosion limit concentration is usually expressed by the volume fraction of combustible gas, and the LEL and UEL of different combustible gases are different, such as methane (CH4) LEL and UEL are 5.3%, 15%, hydrogen sulfide gas (H2S) LEL and UEL are respectively 5.3% and 15%. UEL was 4%, 46%, respectively.
The portable gas detector we usually use takes LEL% as the measurement unit (so the combustible gas detector is also referred to as the LEL detector or the explosion detector), that is, the lower explosion limit of a certain combustible gas is the full scale (100%). It is explained that the 100% displayed on the LEL detector does not mean that the volume fraction of combustible gas reaches 100% of the gas volume, but reaches 100% of the LEL. For example, the LEL of methane is 5.3%, 100% LEL = 5.3% volume fraction (VOL), that is, 1% LEL is equivalent to 0.053% methane. For safety reasons, the LEL detector is generally used to issue an alarm when the flammable gas volume fraction is 10% and 20% of the LEL.
Toxic gases and suffocating gases will cause harm to the human body after invading the human body, so they are collectively referred to as harmful gases, but in gas detection, we often call them toxic gases, such as carbon monoxide (CO), hydrogen sulfide (H2S), ammonia (NH3) ), methanol (CH3OH), etc. They are the risk factors that cause the greatest harm to workers, which includes not only immediate short-term harm, such as physical discomfort, morbidity, death, etc., but also long-term harm to the human body, such as disability, cancer, etc.
The degree of harm of harmful gases to the human body is related to the type and quantity of harmful gases. In order to ensure that workers do not suffer from acute and chronic occupational hazards during production, China adopts the highest allowable mass concentration (MAC, mg/m3) as the hygiene standard, and we often see TLV as the standard in the gas detection instrument manual. TLV is Threshold Limit Values (Threshold Limit Values) is developed by the American Conference of Governmental Industrial Hygiene Association (ACGIH), is the American standard, TLV is divided into 3 kinds:
①TWA (8h statistical weight average), mg/m3;
②STEL (15min short-term exposure level), mg/m3;
③IDLH (immediate lethal dose), 10-6.
For toxic gas portable detection and alarm instruments, the peak value of toxic gas concentration, TWA value and STEL value data can usually be displayed to provide specific guidance for worker health and safety. The true value of toxic gas concentration is usually expressed as 10-6, that is, the volume fraction of the gas in the total volume of 1 million parts of the gas. The highest allowable mass concentration in my country's health standards is in mg/m3, and the conversion relationship between these two units is:
Maximum allowable concentration = true value of toxic gas mass concentration × M/24.25 In the formula: M is the relative molecular mass of toxic gas, 24.25 is a constant, which is the molar volume of toxic gas at 25°C and 101.33kPa.
