For combustible gas and toxic gas detection alarms, what is a reasonable setting for the alarm concentration in ppm?
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For combustible gas and toxic gas detection alarms, what is a reasonable setting for the alarm concentration in ppm? Are there any relevant standards? For example, for gases such as ammonia, chlorine, hydrogen, hydrogen sulfide, carbon monoxide, sulfur dioxide, benzene, nitrogen monoxide, nitrogen dioxide, ethylene oxide, etc., what ppm value is appropriate for setting the alarm on gas detection alarms?
This post was last edited by bfdlwolf on 2019-8-6 at 14:49. According to the “Design Code for Detection and Alarm of Flammable and Toxic Gases in Petrochemical Industries” GB50493—2009, section 5.3.3 stipulates that the alarm setpoints shall comply with the following requirements: 1. The first-level alarm setpoint for flammable gases shall be less than or equal to 25% of the lower explosive limit; 2. The second-level alarm setpoint for flammable gases shall be less than or equal to 50% of the lower explosive limit; 3. The alarm setpoint for toxic gases shall preferably be less than or equal to 100% of the maximum allowable concentration/short-term exposure limit. When it is difficult to prepare standard gases for testing, the alarm setpoint may be set at less than 200% of the maximum allowable concentration/short-term exposure limit. When the measurement range of existing detectors does not meet the requirements, the measurement range for toxic gases can be set from 0 to 30% of the immediate harmful concentration, and the second-level alarm setpoint for toxic gases shall not exceed 10% of the immediate harmful concentration value.
There was a previous version of the approval draft for 50493, which included recommended values that can be used as a reference
Those with calculation formulas are the best; it’s troublesome during inspections when it comes to safety design documents
Sinopec standard: hydrogen sulfide at 7, 15 ppm
What is flammable is a mixture of 25% H2 and 50% HH and H2S; previously it was 7 and 15, but now it has been changed to 0.85 and 1.7. The factory made these changes after conducting an inspection. I forgot the rest.
1. The combustible limits for the two stages are 25% LEL and 50% LEL respectively； 2. Toxic 50% MAC, or STEL, or the excess factor ； 3. The same gas can be both flammable and toxic; it is only necessary to indicate that it is toxic. Check the specifications yourself; I’m not sure if there are any mistakes.
There are 2 draft submissions for approval; you can take a look at them – there are some minor differences!