Calculation of concentration after leakage of flammable toxic gases
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This post was last edited by timwangwt on 2023-3-3 at 13:53. I have another question for everyone: There is a mixed gas with a composition of 98% vol H2 and 2% vol CO. This gas is compressed by a reciprocating compressor from 40°C and 2.7 MPa to 120°C and 5.5 MPa. The compressor is located in a sealed room, where forced ventilation takes place 16 times per hour, with each ventilation cycle involving a volume of 1500 m3. Now, it is necessary to consider determining the number of detectors for flammable and toxic gases. The question now is: 1. How is the leakage rate of a reciprocating compressor determined? 2. For the process media mentioned above, what determines whether it is necessary to install toxic gas detectors? Assuming the CO content is 0.2%, 0.5%, and 1%, determine whether toxic gas detection is required in each case 3. How is the concentration of the medium leaking from the pipeline into the environment calculated? Which one has, or which standard contains, a more detailed calculation process for reference? I earnestly ask for everyone’s guidance; thank you all.
The concentration of leaking flammable and toxic gases usually needs to be calculated using physical equations, such as PV=nRT (Pascal’s law), taking into account the actual conditions. It is necessary to determine values such as the volume, density, and temperature of these flammable and toxic gases in order to estimate the final concentration. Depending on the circumstances, real-time measurement methods such as those using chemical detectors or gas sensors can also be employed to obtain real-time values of leakage volume and concentration, thereby reflecting the leakage situation of such flammable and toxic gases and allowing appropriate measures to be taken to address the leakage. -
Thank you for answering my questions. For design projects, only the first method you mentioned can be used to calculate the possible concentrations. You mentioned calculating the volume of the gas after depressurization using Pascal’s law, but when gas leaks out, there is also the issue of it being diluted by air. How should this dilution process and the resulting concentration be calculated? How should calculations be carried out separately for workshops without forced ventilation and those with forced ventilation? I would appreciate your further guidance.
1. Check the standards for chemical and petrochemical leaks, as well as the allowable leakage amounts at various sealing points, and then calculate the concentration. 2. Install gas detectors for flammable gases and detectors for toxic gases such as CO. 3. The flow rate required for emergency release is also related to the area of the leakage point
Thank you for your guidance. I would like to ask further: 1. You mentioned that \"at least gas detectors for flammable gases and detectors for toxic gases such as CO should be installed.\" How can we determine whether a CO detector is necessary? For example, is it still essential to install a CO detector when the concentration of CO in the process medium is 0.01%? Is there a quantitative calculation result that can serve as a basis for deciding whether to set it or not? 2. Once the leakage rate at the leak site has been determined (for example, 1 Nm3/h), how can the concentration of flammable and toxic gases at the location where the alarm is installed (2 meters horizontally from the leak site) be calculated quantitatively?
This post was last edited by wycasia on 2023-3-9 at 13:27. 1. Refer to GB/T50493-2019, the Design Standard for Detection and Alarm of Flammable and Toxic Gases in Petrochemical Industries; if the concentration of toxic components exceeds the maximum allowable level, detection systems should be installed. 2. Accurate calculations depend on factors such as the ventilation conditions, wind speed, and the location of the leakage point, and CFD methods can be used for modeling and calculation. This is a similar case I have handled before; the calculation can be simplified by treating it as a jet. Over a distance of 2 meters, it spreads into a circle with a diameter of 1 meter. With an air velocity of 1 m/s, an air exchange rate of 3000 m3/h, and a leakage rate of 1 m3/h, the dilution factor is 3000 times. When the leakage rate reaches 5 m3/h, the CO level exceeds the limit; when it reaches 10 m3/h, the H2 level is found to be above the acceptable threshold
There are still some points I don’t understand, so I would appreciate it if you could provide further guidance: You assumed wind speeds of 1 m/s and a distance of 2 meters, but these parameters seem to not be used in the calculation of the diluted concentrations of CO and H2. What is the impact of these two parameters on the calculation results? For example, will the calculation results be different at a wind speed of 2 m/s? Thank you.
It’s easy to understand: in a closed room where the wind speed is essentially zero, even a small amount of leakage can lead to very high concentrations. When the wind speed is high, diffusion occurs rapidly and there is a large volume of air flow, so the concentration naturally becomes lower. Based on my calculations, a wind speed of 2 m/s is equivalent to an air flow rate of 6000 m3/h
Okay, thank you for your patient guidance.