Safety design of caustic soda plants

Thread Content

8 Automatic Control and Safety Design 8.1 General Provisions 8.1.1 The control room of the caustic soda plant shall be located outside the explosion-hazardous area, in a zone free from fire hazards. When sharing a control room with other hazardous process production units, the control room shall be designed with an explosion-resistant structure ； When a single caustic soda plant (including auxiliary equipment) is equipped with a control room, the control room building shall adopt corresponding explosion-proof structural design measures, such as using explosion-proof walls on the side facing the process equipment. 8.1.2 The caustic soda plant shall be equipped with an independent process control system (DCS) and a safety instrumented system (SIS) to carry out process operation, control, alarm, interlock, management, and safe shutdown. 8.1.3 The caustic soda plant shall be equipped with detection and alarm systems for toxic gases (chlorine) and flammable gases (hydrogen). 8.1.4 The instrument enclosures in the caustic soda plant shall meet the requirements for resistance to chlorine corrosion. The materials of the instruments that come into contact with the process media shall satisfy the requirements of those media, and shall be no less durable than the materials of the pipes or equipment in which the instruments are installed. In areas at risk of explosion, instruments and equipment must meet the explosion protection requirements for such areas; intrinsically safe instruments should meet the standard of E*aIICT1, while flameproof instruments should meet the standard of ExdIICT1. The protection rating of field instruments is generally IP65. 8.1.5 Measures such as insulation, heat tracing, shading, protection against acid rain, and air conditioning shall be taken based on the site conditions and instrument requirements to ensure the proper operation of the instruments. 8.2 Process Control Systems 8.2.1 Process control systems should be mature systems that have been tested in actual applications. The system should be safe and reliable, easy to expand, and meet the requirements for control, monitoring, and management in the caustic soda production process. 8.2.2 The controllers, power supply units, and communication units of the process control system shall all adopt a redundant architecture. The I/O cards for important control loops and key monitoring points should be configured redundantly. When the control system adopts a client/server architecture, at least one pair of redundant servers should be configured. 8.2.3 The process control system should be able to assign different permissions based on the identity of users or devices, to prevent unauthorized users from accessing network information resources, and it should impose access controls in accordance with the access authorization rules. 8.2.4 Operation access control measures shall be implemented for all data interfaces of the human-machine interface. 8.2.5 The electromagnetic compatibility of process control systems shall meet the requirements of \"China’s Compulsory Product Certification (CCC Certification)\\" or \"European Union Conformity Certification (CE Certification)\“. 8.2.6 The DC power supply units inside the cabinet shall be configured with 1:1 redundancy. 8.3 Safety Instrumented System (SIS) 8.3.1 The caustic soda plant shall be equipped with a Safety Instrumented System (SIS) to implement the plant’s safety interlocks. The design of safety instrument systems shall comply with the provisions of the Code for Design of Safety Instrument Systems in Petrochemical Industries. 8.3.2 The safety integrity of the Safety Instrumented System should be designed according to SIL2, and its logic controllers should be separated from the process control system. The safety instrument system should be designed to be fail-safe. 8.3.3 The safety instrumented system should be equipped with an operator station; in the event of a failure of this operator station, the logical processing functions of the safety instrumented system remain unaffected. The operator station functions shall not modify the programming software of the safety instrumented system. 8.3.4 The system shall be equipped with an engineer station and an event sequence recording station. The engineer station and the event sequence recording station can be shared, with protection via passwords of different levels of access rights. 8.3.5 The maintenance and operation bypass switches installed in process control systems should be connected to the safety instrumented system via communication. In process control systems, interlock outputs should be connected to the safety instrumented system via hard wiring. 8.3.6 Emergency stop buttons and important signal alarms shall be installed on the system’s auxiliary control panel, connected to the Safety Instrumented System (SIS) via hardwired connections; the signal alarms shall have a function to distinguish between different types of alarms. 8.3.7 All signals entering the Safety Instrumented System (SIS) shall be transmitted via communication to the Process Control System (DCS) for display. 8.3.8 Field instruments related to the Safety Instrumented System (SIS) should be certified according to the corresponding SIL level. 8.4 Main Interlock Circuits 8.4.1 The following parameters in the caustic soda plant should be incorporated into the Safety Instrumented System (SIS) for safety interlock protection: electrolyzer rectifier shutdown, brine tank level HH, caustic solution tank level HH, total chlorine pipe pressure HH, total hydrogen pipe pressure HH, pressure difference between the hydrogen and chlorine pipes HH and LL, emergency stop button, instrument air pressure LL, hydrogen compressor shutdown, chlorine compressor shutdown, electrolyzer grounding fault, flow rate of cathode brine into the electrolyzer LL, flow rate of anode caustic solution into the electrolyzer LL, rectifier current HH, pure water flow rate LL. 8.4.2 In the hydrochloric acid synthesis furnace section, depending on the type of furnace, the following parameters should be included in the Safety Instrumented System (SIS) for safety interlock protection: total chlorine pipe pressure entering the synthesis furnace LL, total hydrogen pipe pressure entering the synthesis furnace LL, hydrogen flow rate entering the synthesis furnace HH, hydrogen flow rate entering the synthesis furnace LL, chlorine flow rate entering the synthesis furnace HH, chlorine flow rate entering the synthesis furnace LL, hydrogen/chlorine flow ratio entering the synthesis furnace HH, hydrogen/chlorine flow ratio entering the synthesis furnace LL, drum liquid level LL, drum liquid level HH, circulating water flow rate entering the synthesis furnace LL. 8.4.3 The following parameters should also be included in the Safety Instrumented System (SIS) for safety interlock protection: liquid chlorine tank level HH, liquid chlorine vaporization pressure HH. 8.5 Detection of Flammable and Toxic Gases 8.5.1 Ion-exchange membrane caustic soda plants should be equipped with detection and alarm systems for toxic gases (chlorine) and flammable gases (hydrogen); however, no detection and alarm system is required for hydrogen chloride gas. The design for detecting flammable and toxic gases shall comply with the provisions of the \"Code for Design of Detection and Alarm Systems for Flammable and Toxic Gases in Petrochemical Industries\" GB 50493. 8.5.2 Combustible and toxic gas detection and alarm systems can be installed independently or integrated into the process control system; when integrated, the I/O cards should be installed separately. In addition to the alarms displayed on the process control station, a dedicated control station can be set up for the detection and display of flammable and toxic gases. 8.5.3 The installation of on-site alarms should be determined comprehensively based on the layout of equipment and buildings, the physicochemical properties of the emission sources, the characteristics of air flow at the site, as well as relevant local regulations. 8.5.4 Flammable or toxic gas detectors should generally be installed inside buildings, in areas surrounding equipment prone to leakage such as compressors, pumps, reactors, and storage tanks, as well as in places where gases tend to accumulate. 8.5.5 Electrochemical chlorine detectors are recommended; the range should be 0–10 ppm, with a first-level alarm set at 1 ppm and a second-level alarm set at 3 ppm. 8.5.6 Catalytic combustion type hydrogen detectors are recommended; the range should be 0~100% LEL, with the first-level alarm set at 25% LEL and the second-level alarm set at 50% LEL. 8.6 Instrument Selection 8.6.1 The measuring instruments surrounding the electrolyzer must be suitable for the electromagnetic interference environment present in the electrolysis plant. For flow meters used in the catholyte (alkaline solution) flowing into the electrolyzer, tantalum material should not be used; instead, Hastelloy C is a suitable choice. For flow meters used to measure the flow rate of the anode solution (saltwater), tantalum material is a suitable choice. 8.6.2 For alkali solutions with a temperature above 85°C, stainless steel is not suitable as a material for instruments; instead, Hastelloy C, PTFE-lined steel, or nickel materials are preferred. 8.6.3 The material of instruments for measuring saline should preferably be titanium or steel lined with PTFE. 8.6.4 Titanium is prohibited as the material for instruments used to measure dry chlorine; tantalum or steel lined with PTFE can be used, while titanium can be employed for measuring wet chlorine. 8.6.5 Control valves and on-off valves for chlorine and liquid chlorine shall be equipped with diaphragm sealing or double packing sealing. 8.6.6 When the control valves for the chlorine main pipe and the hydrogen main pipe leaving the electrolyzer are closed, and there is no discharge outlet before these main pipe control valves, they shall not be closed completely; a dead zone of 10%~15% should be left. 8.6.7 Gaskets for instruments used to measure chlorine gas and liquid chlorine media should preferably be metal-wound gaskets or modified polytetrafluoroethylene gaskets; the use of rubber gaskets is strictly prohibited. 8.6.8 For pressure gauges, pressure transmitters, and differential pressure transmitters used to measure chlorine gas and liquid chlorine, inert oil should be used as the filling medium; silicone oil is prohibited. 8.6.9 For liquid chlorine storage tanks, external-type level gauges are recommended for measuring the liquid level, with another level detection method based on a different principle also being installed. 8.6.10 PH or OPR detectors should be products resistant to chloride ion poisoning. 8.6.11 In the event of a failure of the on-site instruments, the output should be brought to a safe state; when there is a failure in the instrument air supply, the control valves and switch valves should be in their safe positions. 8.6.12 Cables in the electrolyzer area shall be high-temperature shielded flame-retardant cables, and conduits shall be made of non-metallic materials. 8.6.13 When hydrogen chloride gas is sent to the downstream unit to be mixed with acetylene gas for VCM production, a free chlorine analyzer for hydrogen chloride gas can be installed on the main pipeline of the hydrogen chloride gas, and the alarm signal from the analyzer can be sent to the VCM production unit. . 8.7 Power and Gas Supply for Instruments 8.7.1 The process control systems (DCS), safety instrument systems (SIS), and field instruments are powered by uninterruptible power supplies (UPS). The UPS capacity should be sufficient to keep the control system and instruments functioning properly for at least 30 minutes. The UPS should have fault alarm and protection functions. 8.7.2 The DCS control station and SIS control station shall have redundant power supply, using at least one UPS and one mains power supply. 8.7.3 The instrument air supply shall be equipped with a backup storage tank with a capacity to maintain pressure from 700 KPaG to 400 KPaG for at least 20 minutes. 8.8 Instrument grounding and lightning protection 8.8.1 Instrument grounding shall adopt the equipotential grounding method. The process control system side is equipped with three types of grounding busbars: instrument signal grounding, intrinsically safe grounding (if available), and protective grounding. These three busbars are each connected to the main instrument grounding plate, which in turn is connected to the electrical grounding network. 8.8.2 The protective grounding of instruments such as field panels, instrument cable trays, instrument equipment, and instrument junction boxes shall be connected to the electrical grounding grid on site ； The signal ground of the instrument should be grounded on the instrument control system side. 8.8.3 For cable trays made of non-metallic materials, conductors should be installed at the bottom layer of the tray, and grounding posts should be provided at both ends of the tray to connect it to the plant’s grounding network, thereby preventing the accumulation of static electricity in hazardous areas that could pose a risk. 8.8.4 Adopt necessary lightning protection design based on the average annual number of thunderstorm days in the area, as well as the economic and safety significance. Explanation of the safety design provisions for automatic control 8.1.1: It is a requirement of current codes and a trend in control room design that control rooms be designed to be blast-resistant. However, in practical engineering applications, many control rooms in small and medium-sized chemical plants are not designed to be blast-resistant. This approach meets both engineering needs and safety requirements, while also taking into account construction costs and facilitating operational management. Therefore, this regulation still retains the requirement that control room buildings should be designed with appropriate blast-resistant structural measures. 8.3.1 The Safety Instrumented System (SIS) is also known as the Emergency Shutdown System (ESD), Safety Shutdown System (SSD), Safety Interlock System (SIS), or Safety Protection System (SPS). Therefore, installing a Safety Instrumented System (SIS) or an Emergency Shutdown System (ESD) in a caustic soda plant is essentially the same thing; it’s just that different terms are used. In any case, the design must comply with the requirements specified in the \"Code for Design of Safety Instrumented Systems in Petrochemical Industries\". 8.3.2 The system safety integrity level of the caustic soda plant is designed as SIL2, as determined through engineering practice; a design at SIL2 can meet the safety requirements of the caustic soda plant. 8.4.1 The shutdown interlock of the hydrogen compressor for the entire plant is designed as if there is no hydrogen gas holder. 8.5.1 The Safety Technical Inspection Regulations for Pressure Pipelines TSGD0001 stipulate that the highly toxic concentration of hydrogen chloride is: Lc50 upon inhalation