Flame arrester on the vent pipe at the top of the tank

The standard Q SY 1124.18-2015 \"Code for On-site Safety Inspections in Petroleum Enterprises – Part 18: Atmospheric Storage Tanks for Flammable Liquids in Petrochemical Enterprises\" does not actually require the installation of flame arresters. If a nitrogen seal and single-vent valve system are already in place, along with a configuration for venting to an RTO, it is possible that a flame arrester was added as an additional safety measure. In practical applications, the device configuration can be adjusted according to specific circumstances and safety requirements. .

“Sinopec Refining Letter [2016] No. 127: Guidelines on the Implementation Plan for the Oil and Gas Connection Process and Safety Measures for VOCs Control Projects in Petrochemical Storage and Transportation Areas. V. Oil and Gas Connection System in Storage Areas (2) Single vent valve scheme: A single vent valve is installed upstream of the explosion-proof flame arrester on the oil and gas collection pipeline of the storage tanks.

Urgent warning from the Ministry of Emergency Management: Report on the ‘8.16’ flash explosion incident involving a hazardous chemicals storage tank in xx; excerpts from an expert’s views: Fifthly, the safety facilities of atmospheric-pressure storage tanks are insufficient. For example, flame arresters and breather valves were not installed as required; aluminum floating decks were used in internal floating roof tanks, and a nitrogen seal was not provided. Seventh, the exhaust gas connection and recovery systems for multiple chemical storage tanks have not undergone safety assessment. To cut costs, some companies carry out retrofits to the exhaust gas recovery systems in their hazardous chemicals storage areas without proper design. The combined discharge of different types of hazardous chemicals, in the absence of flame arrestor facilities, poses significant safety risks. I don’t know if these two factors were the causes of that accident.

In accordance with the \"Guidelines on the Implementation Plan for the Oil and Gas Connection Process and Safety Measures for VOCs Control Projects in Petrochemical Storage and Transportation Tank Areas, China Petrochemical Corporation Letter [2016] No. 127\", on the oil and gas collection pipelines of storage tanks, a single vent valve should be installed upstream of the explosion-proof flame arrester. Such a configuration is designed to enhance safety when handling flammable gases, effectively preventing the risk of fires and explosions. The gas flow can be controlled through a one-way valve, while the flame arrester prevents flames from spreading into the tank, thereby enhancing the safety of the system as a whole. .

The author of Q SY 1124.18 believes that once a tank is sealed with nitrogen, the top of the tank is in a condition of oxygen deficiency; therefore, even if flames reach it, it is difficult for the tank to catch fire due to this nitrogen seal. This can basically be avoided in well-managed large refineries. However, in some small and medium-sized enterprises, the selection of nitrogen seal valves is not satisfactory; they may fail, allowing air to enter through the breather valve. At this point, the nitrogen-sealed tank is no longer oxygen-deficient, and a fire may occur. I have discussed this with several engineers from the client side, and they are not confident in the several types of nitrogen-sealing valves available on the market currently. Furthermore, in the \"China Petrochemical Refining Letter [2016] No. 127 – Guidelines on the Implementation Plan for the Oil and Gas Connection Process and Safety Measures for VOCs Control Projects in Petrochemical Storage and Transportation Tank Areas\", pneumatic control valves are used in the accompanying diagrams, rather than self-acting control valves (nitrogen seal valves); it’s unclear whether the authors of Document No. 127 were also concerned about this. Clearly, “HG/T 20570.16—95 Installation of Air Seals” specifies the use of self-acting control valves (nitrogen seal valves), and the selection of nitrogen seal valves in “HGT20559-1993 Piping and Instrumentation Diagrams — Appendix: Basic Unit Patterns for Piping and Instrumentation Diagrams” refers to “HG/T 20570.16—95 Installation of Air Seals”.

In SHT 3007-2014, the Code for Design of Tank Areas in Petrochemical Storage and Transportation Systems, flame arrestor requirements are only specified for vent pipes and breathing valves used to release excess gas; no such requirements exist for pipes leading to RTOs and for oil and gas recovery systems. These practices emerged after 2014.

The selection and application of nitrogen seal valves are indeed a critical factor. A properly designed nitrogen sealing system, along with the use of appropriate nitrogen sealing valves, can effectively maintain an oxygen-deficient environment inside the tank, thereby reducing the likelihood of fires. However, if the nitrogen seal valve is improperly selected or fails, it can indeed allow air to enter the tank, causing the internal environment of the tank to cease being oxygen-deficient. Furthermore, the use of different types of nitrogen seal valves in the two standards you mentioned varies, which may reflect the level of confidence and reliance on nitrogen seal valves in practical engineering applications. Self-acting control valves (nitrogen seal valves) are generally considered more reliable and are the preferred choice in many cases. However, pneumatic control valves may be used in certain specific application conditions due to their rapid response. As for the decision to use pneumatic control valves as stated in \"Sinopec Refining Letter [2016] No. 127\", this may be based on specific engineering requirements or considerations, but the exact reasons need to be clarified by the person who drafted it. You can further discuss in detail the selection of nitrogen seal valves with the engineer from Party A, taking into account various actual operating conditions and safety requirements, in order to choose the most suitable equipment to ensure the safe operation of the storage tank. .

I’ve learned this knowledge; it’s very important to me. Thank you

Originally, the nitrogen sealing requirements for tanks were not as strict as those specified for the process piping and instrumentation systems in workshops; it was sufficient to have a self-acting regulator. If it failed, a bypass could be used temporarily, and another self-acting regulator installed – after all, it was a simple and inexpensive solution. Even when environmental regulations were not stringent, no bypass was needed; one could simply close the valves at both ends and replace them with a self-acting regulator. After all, the vent valve could still provide some protection, ensuring basic safety. Well now, with increasingly strict environmental and hygiene requirements, nitrogen seal valves also need to meet high standards, just like process control valves.

How can a single-acting valve draw in air? If you only exhale and not inhale, where does the air come from?