Technical Sharing | Safety Control Instructions of Chemical Projects
The main ignition sources are listed as follows:
1. Open Flame: Backfire in RTO or in processing equipment.
2. Electric Sparks: The burner inside the combustion chamber employs an electric spark igniter or electrical equipment catches fire.
3. Electrostatic Sparks: The high-speed flow of exhaust gas, friction with the pipe wall, as well as the high-speed rotation of the fan impeller can readily form static electricity, which may cause fires if static electricity cannot be discharged.
4. High Heat Materials: When the temperature of high heat material is higher than the ignition point of explosive components, it might lead to an explosion. The main high heat materials of RTO are the surface of the combustion chamber and ceramics.
Based on the above analysis of reasons, the project safety control measures are as follows:
1. Source Control: Elimination Conditions
1) Concentration:
A. Pipeline: Control the VOCs concentration in the exhaust gas
a. Install an infrared LEL detector on the exhaust gas pipeline with a detection accuracy of ±5% F.S, and configure a pretreatment system to ensure the smooth and accurate operation of the LEL concentration detector. The detection and response time of the infrared concentration detector is approximately 4 seconds, and the flow rate is calculated at 8 meters per second. It is recommended to install the nearest interlocking valve at a distance of more than 32 meters. When the installation distance is insufficient, the distance can be shortened by setting a buffer tank or increasing the pipe diameter. The concentration detector is interlocked with the exhaust gas valve, emergency discharge valve, high-temperature bypass valve and fresh air proportional valve. Regular cleaning and maintenance of the detector is a crucial task in preventing malfunctions. Grounding should be checked regularly. If the grounding fails to meet the standard requirements or there is no grounding at all, it can make the combustible gas detector prone to electromagnetic interference and cause malfunction. If the service life exceeds the requirements of usage regulations, it should be replaced promptly (Choose φ8/φ10 stainless steel/ copper instrument lead wire. If the lead wire is too thin, the medium flow rate cannot meet the requirements; If the lead wire is too thick, the response time is too long).
b. Concentration interlock control: Set up three levels of LEL alarm points (adjustable alarm values). When LEL reaches the first-level alarm point, an alarm is triggered; when it reaches the second alarm point, the interlock control opens the fresh air valve; when it reaches the third-level alarm point, the emergency valve opens, the exhaust gas valve closes, and the fresh air opens.
c. Low Point Drainage: The exhaust gas conveying pipeline should have a slope of 0.15%. The fans and Rto Gas distribution chamber should be equipped with drainage outlets to discharge the low fluidity liquid and condensed liquid during the conveying process from the low point, preventing the liquid from vaporizing and the concentration from suddenly increasing after the temperature rises.
d. Pipeline Viscous Substance: The low fluidity liquid in the exhaust gas should be cleaned before entering RTO to avoid the gasification of viscous substances and a sudden increase in concentration due to temperature rise.
B. Inside the RTO Furnace: Control the VOCs concentration and the flow rate of auxiliary gas (natural gas).
a. RTO Furnace Sweeping: When starting RTO, in accordance with the main program regulations, blow the RTO first and replace at least 10 times of fresh air in the RTO.
b. Blow during operation.
c. Small and large fire ignition of the burner, gas pressure high and low interlock, leak detection, proportional adjustment, UV flame detector, and the control of the natural gas flow rate.
2) Open Flame
A. RTO Backfire
a. There are ceramics inside the RTO regenerative chambers, which are equivalent to a flame arrester.
b. Deflagration flame arresters should be installed horizontally at the inlet pipeline of the RTO main fan. Attention should be paid to preventing particles, low fluidity liquid, or rusts from blocking the arresters.
c. Scrubbers can be installed at the exit of the processing workshop to effectively prevent fires.
B. Static Electricity (The more disordered the gas flow, the more likely it is to generate static electricity)
a. Static Electricity Grounding Elimination: Select materials that are easily conductive to static electricity (such as carbon steel, flame-retardant and anti-static fiberglass, stainless steel). After welding the pipeline, the weld joint should be polished. It is recommended to connect pipelines in different directions at 45° and avoid connecting them at 90°.
b. If the exhaust gas contains a certain amount of saturated water, it can effectively reduce the generation of static electricity.
c. Exhaust Gas Flow Rate Control: Select suitable pipes and fans. It is recommended that the flow rate be less than 10 meters per second.
C. Process Equipment Catches Fire
a. Install flame arresters at the venting ports of process equipment.
b. Install flame arresters or block valves to completely separate the RTO system and the process equipment.
D. Select pneumatic valves as the automatic control valve for the RTO system.
3) Oxygen Content: RTO combustion requires the oxygen content in the exhaust gas to be greater than 3%. If the oxygen meets the combustion requirements, the oxygen content should be minimized as much as possible.
2. Interception: Abnormal Temperature or Pressure.
A. Pipeline
a. It is recommended to install bursting discs every 50 meters on the pipeline to ensure safe pressure relief.
b. RTO Inlet Temperature Interlock Control: When the inlet temperature of RTO is abnormal, the control system will urgently shut down RTO.
c. A pressure transmitter should be installed in front of the main fan to ensure a slightly negative pressure in the pipeline, and the exhaust gas can be drawn into RTO for incineration treatment.
B. Inside RTO Furnace
a. Temperature Interlock Control inside the Combustion Chamber: Install dual thermocouples in the combustion chamber. Set up three levels of alarm points (adjustable alarm values). When the furnace temperature rises and exceeds the first-level alarm point, an alarm will be triggered; when it reaches the second alarm point, the high-temperature bypass valve will open to discharge excess heat; when it exceeds the third-level alarm point, the exhaust gas valve closes, the emergency discharge valve and fresh air valve fully open to fully cool RTO by fresh air.
b. Set an explosion vent for RTO: There is an explosion vent before the exhaust gas enters RTO to release high pressure and reduce losses.
c. The Inlet and Outlet Pressure Difference Interlock Control: Pressure differential transmitters are set at the inlet and outlet of the RTO body to observe the working condition of ceramics and detect if it is blocked or damaged.