Safety design of caustic soda plants

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6 General Layout and Safety Design 6.1 Site Selection 6.1.1 The site should be located on the upwind side of the wind direction with the lowest annual wind frequency in towns or residential areas. 6.1.2 The plant site should be avoided in areas prone to the formation of inversion layers and those affected by a high frequency of calm winds throughout the year. 6.1.3 Plant sites should not be selected in the following locations or areas: (1) Seismic fault zones and earthquake areas with a basic seismic intensity of 9 degrees or higher ； (2) Areas with severely poor engineering geology ； (3) Areas of distribution of important mineral deposits and mining subsidence (displacement) zones ； (4) Areas that have an impact on aircraft takeoff and landing, radio communication, television broadcasting, radar navigation, as well as astronomical, meteorological, and seismic observations, and **related facilities ； (5) Areas prone to flooding or requiring substantial flood control measures ； (6) Within the blasting hazard area ； (7) Below the dam of large tailing ponds and waste dumps (ponds) ； (8) Areas affected by severe radioactive contamination ； (9) Areas that may be flooded in the event of a breach in the dam of reservoirs that cannot ensure safety ； (10) Areas where the frequency of calm winds throughout the year exceeds 60% ； (11) Areas affected by tsunamis or lake surges. 6.2 General Layout 6.2.1 The general layout of the factory should be planned in a way that groups facilities by function, taking into account the process flow, the production characteristics of various units, their toxicity levels and fire hazards, as well as factors such as the terrain and wind direction. 6.2.2 The area in front of the plant, as well as areas with a high concentration of people such as laboratories and control rooms, should be located on the downwind side of the location where the lowest wind frequency occurs throughout the year for the process units. 6.2.3 Internal equipment of the process plant and building areas shall be separated by roads, such that the equipment and building areas each cover an area not exceeding 10,000 m2 ； 6.2.4 Equipment, buildings, and structures should preferably be arranged on the same horizontal plane ； When constrained by the terrain, the control room, cabinet rooms, substation, laboratory, offices, etc. should be located on a higher elevation ； Process equipment, units, storage tanks, etc., should be located at a lower elevation. 6.2.5 Salt piles, liquid chlorine areas, tank areas, solid alkali warehouses, etc., should be located relatively close together and near the relevant equipment and transportation routes. They must be situated on the upwind side of the area with the lowest wind frequency throughout the year, and must comply with fire prevention, explosion protection, safety, and hygiene regulations. 6.2.6 The layout principles for the main substation shall meet the following requirements: (1) When overhead power lines are used to supply power to or remove power from the plant area, they should be located at the edge of the plant area. (3) It should be located in an area with a higher elevation; (4) It should be placed on the downwind side of the wind direction with the lowest frequency of occurrence throughout the year in the area where process equipment and storage facilities are located, as well as on the upwind side of the wind direction that prevails in winter in areas with mist ； (5) The distance between the outermost edge of the outdoor main substation’s framework and the units for electrolysis, chlorine treatment, liquid chlorine, and hydrogen chloride synthesis should be greater than 25 m ； (6) Avoid placing it near strong vibration sources. 6.2.7 The plant substation should be located downwind of the liquid chlorine, chlorine gas treatment, and hydrogen chloride synthesis units, at the location with the lowest annual wind frequency, and should not be situated in close proximity to them. 6.2.8 The layout of liquid chlorine storage tanks, bottle storage areas, and filling stations shall meet the following requirements: (1) They shall be located on the upwind side of the wind direction with the lowest frequency of occurrence throughout the year in the plant area, as well as in low-lying, open areas ； (2) It should be kept away from the main traffic routes within the factory area, as well as from areas where flammable and explosive materials are produced, stored, and where large numbers of people gather ； (3) The floor of the above-ground liquid chlorine storage tank should be 0.3~0.5 m lower than the surrounding ground, or a dike 0.3~0.5 m higher than the floor should be constructed around the tank ； 6.2.9 The greening of the factory area shall comply with the following requirements: (1) Trees with high oil content should not be planted in the production area; it is advisable to choose tree species with higher water content ； (2) It is not advisable to plant hedges or dense shrubs between process units or tanks for combustible gases and liquids and the surrounding fire lanes ； (3) The greening of the factory area should not hinder fire-fighting operations. 6.2.10 The fire separation distances for the overall layout of the caustic soda plant shall be in accordance with the Code for Fire Protection Design of Buildings GB50016. 6.3 Factory roads 6.3.1 Circular roads should be provided within the factory area; where there are ends to such roads, a turning area should be installed at those ends. The size of this turning area shall be determined based on the turning requirements of the vehicles that use it. 6.3.2 The entrances and exits for personnel and goods in the factory should be separated ； There should be no fewer than two main entrances and exits, which are preferably located in different directions. 6.3.3 The roads for transporting goods should be located at the edge of the factory area, in safe areas that facilitate transportation, to prevent delivery vehicles from entering the interior of the facilities. 6.3.4 The maximum vertical slope of the roads within the plant used for transporting liquid chlorine on a regular basis shall not exceed 6%. 6.3.5 An annular fire road shall be provided outside the process plant area, with a width of not less than 6 meters and a turning radius of not less than 12 meters. 6.3.6 The layout of fire roads within the process plant area shall comply with the following requirements: (1) There shall be through roads within the plant, and these roads shall have no fewer than two exits, with the two exits preferably located in different directions. When the distance between the fire roads on either side of the facility is not more than 120 m, it is not necessary to provide a through road within the facility ； (2) The width of the road surface should not be less than 4m, and the clear height above the surface should not be less than 4.5m ； The turning radius at the inner edge of the road surface should not be less than 6m. 7 Safety Design of Buildings and Structures 7.1 Classification of Fire Hazards, Fire Resistance Ratings, and Fire Compartmentation of Buildings and Structures 7.1.1 The production processes related to the electrolysis of caustic soda, hydrogen treatment and compression, as well as the synthesis of hydrochloric acid and hydrogen chloride are classified as Category A ； The production processes related to chlorine treatment, chlorine compression, waste chlorine treatment, brackish water dechlorination, chlorine liquefaction, chlorine packaging, and chlorine vaporization are classified as Category B. A detailed list of the characteristics of typical facilities is provided in Table 7.1.1, “List of Hazardous Buildings and Structures in Caustic Soda Plants”. Table 7.1.1: Sequence Number, Building/Structure, Common Structural Types, Production Category, Fire Resistance Rating, Remarks. 1. Electrolysis (including rectification): Truss structure, Frame structure. Category A (electrolysis), Category C (rectification). Fire resistance rating: II, II. The hazardous substance in the electrolysis section is hydrogen; therefore, the electrolysis plant needs to be equipped with ventilation facilities such as skylights or ventilated rooftops. It should be separated from the rectification section by fire-resistant and protective walls, with two fire compartments. 2. Brackish water dechlorination: Frame structure. Category B. Fire resistance rating: II. The hazardous substance is chlorine; it is not explosive but is toxic and corrosive, so an open-type structure is appropriate. 3. Chlorine treatment: Frame structure. Category B. Fire resistance rating: II. The hazardous substance is chlorine; it is not explosive but is toxic and corrosive, so an open-type structure is appropriate. 4. Waste chlorine treatment: Frame structure. Category B. Fire resistance rating: II. The hazardous substance is chlorine; it is not explosive but is toxic and corrosive, so an open-type structure is appropriate. 5. Hydrogen treatment: Frame structure. Category A. Fire resistance rating: II. The hazardous substance is hydrogen, so an open-type structure is suitable. 6. Hydrogen chloride synthesis and hydrochloric acid production: Frame structure. Category A. Fire resistance rating: II. The hazardous substance is hydrogen, so an open-type structure is suitable. A fire and explosion-proof wall with a fire resistance rating of not less than 3.00 hours should be used to separate the operation control room for hydrogen chloride synthesis and hydrochloric acid production, which is at risk of explosion, from the site of the synthesis furnace. The control room for separate operations should not be located directly below floors where corrosive liquids are present, nor should it be in direct communication with rooms where corrosive substances are produced. For liquid chlorine, a frame structure of Category B, Class 2 is appropriate; the substance in question is chlorine, which poses no explosion risk but is toxic and corrosive. An open-type structure is suitable for such facilities (while a closed-type structure should be used for liquid chlorine plants). Note: The minimum required fire resistance rating for the plant is Class 2, although a Class 1 fire resistance rating can be adopted depending on the specific conditions of the project. 7.1.2 The combustion performance and fire resistance rating of the components in factory buildings shall meet the requirements specified in Tables 3.2.1 and 3.2.2 of the Code for Fire Protection Design of Buildings GB50016. 7.1.4 The production category, fire resistance rating, number of floors, area of fire compartments, etc., shall meet the requirements specified in Table 3.3.1 of GB50016 \"Code for Fire Protection Design of Buildings\" ； The fire resistance rating of the factory building should not be lower than grade II. 7.2 Safe evacuation from buildings and structural aspects 7.2.1 The number and spacing of safe evacuation exits in buildings shall meet the following requirements: (1) The safety exits in factories should be located in various places. In each fire compartment, and on each floor of a fire compartment, the horizontal distance between the nearest edges of two adjacent safety exits shall not be less than 5.0 m. (2) The number of safety exits in each fire compartment of the factory building, and on each floor within a fire compartment, shall be determined through calculation, and shall not be less than 2 ； A single safety exit can be provided when the following conditions are met: 1) For Class A factories, the floor area is less than or equal to 100 m2, and the number of workers present at any given time does not exceed 5 ； 2) For Class B factories, the floor area per floor is less than or equal to 150 m2, and the number of workers present at any given time does not exceed 10 ； 3) Class C factories, with a floor area of 250 m2 or less, and no more than 20 workers present at any given time ； (3) The safety exit doors of buildings should open outward. Class A, B, and C rooms should have no fewer than two safety exit doors ； Rooms with an area of 100 m2 or less can have only one. Interpretation of the provision: This provision is derived from Articles 3.7.1 and 3.7.2 of GB50016 “Code for fire protection design of buildings” ； Article 5.2.25 of GB50160 “Code for Fire Protection Design of Petrochemical Enterprises” stipulates this. 7.2.2 The evacuation distances shall meet the following requirements: the distance from any point within the factory building to the nearest safe exit shall be in accordance with the table in Table 7.2.2, Evacuation Distances for Caustic Soda Plants. Production category, fire resistance rating, single-story factory building, multi-story factory building, high-rise factory building, underground or semi-underground factory building or the basement/semi-basement of a factory building: Class A – Grades 1 and 2: 30.0; 25.0. Not allowed. Not allowed. Class B – Grades 1 and 2: 75.0; 50.0; 30.0. Not allowed. Class C – Grades 1 and 2: 80.0; 60.0; 40.0; 30.0. Explanation of the provisions: These requirements are derived from Table 3.7.4 of GB50016, Code for Fire Protection Design of Buildings. 7.2.3 Fire compartments of factory buildings are separated by firewalls. Explanation of the provisions: In caustic soda production facilities, only the electrolysis buildings are separated into fire compartments by fire walls. Note 1 to Article 3.3.1 of the Code for Fire Protection Design stipulates that fire compartments in Class A buildings must be separated by fire walls. Article 7.2.4 requires that pressure relief areas in Class A buildings with explosion hazards meet certain requirements; according to Table 3.6.3 in the Code for Fire Protection Design GB50016, the C value for hydrogen should be no less than 0.25 ； Pressure relief facilities can include lightweight roofs, lightweight walls, as well as doors and windows that facilitate pressure release. 7.3 Corrosion protection design of buildings and structures 7.3.1 The corrosion protection design of buildings and structures shall meet the requirements specified in the Code for Corrosion Protection Design of Industrial Buildings GB50046. 7.3.2 Corrosive media in the caustic soda plant: Gaseous corrosion is mainly caused by chlorine (Q2, with a chlorine concentration of ≤1 mg/m3) and hydrogen chloride (Q4) ； Liquid corrosion includes acid corrosion such as Y1 sulfuric acid and hydrochloric acid, alkali corrosion such as Y7 20% NaOH and dilute alkaline solutions, Y16 NaCl at pH=8, and Y12 NaCLO 10wt% sodium hypochlorite ； Solid-state corrosion is G2 chlorides of potassium and sodium. 7.3.3 Detailed Table of Corrosion Characteristics of Structures and Buildings in the Caustic Soda Plant Table 7.3.3: Corrosion Characteristics of Structures and Buildings Table 7.3.3 Sequence Number Structure/Building Corrosion Characteristics 1 Raw salt storage area Corrosion due to solid salts (corrosion occurs when they become soluble upon moisture exposure) 2 Primary brine Brine corrosion, NaCl-induced corrosion, corrosion caused by the strong oxidizing properties of sodium hypochlorite, and corrosion due to the strong alkalinity of caustic soda 3 Secondary brine Liquid-phase corrosion: 29–32% caustic soda, 18% hydrochloric acid, brine 4 Electrolysis Gas-phase corrosion: Chlorine gas, strong acidic corrosion from high-purity hydrochloric acid, strong alkaline corrosion from ion-exchange membrane caustic soda, and strong oxidizing corrosion from chlorine-containing fresh brine 5 Chlorine gas treatment Gas-phase corrosion: Chlorine gas, 78% dilute sulfuric acid, 98% concentrated sulfuric acid, chlorine water (a saturated solution of chlorine in water, acidic in nature, and causes severe corrosion on materials such as carbon steel) 6 Waste chlorine gas treatment 2–10 wt% sodium hypochlorite (strongly oxidizing, liquid is alkaline), dilute alkaline solutions, 15% NaOH 7 Hydrogen chloride synthesis and hydrochloric acid production Gas-phase corrosion from chlorine gas and hydrogen chloride, as well as 31% hydrochloric acid 8 Liquid chlorine Gas-phase corrosion and liquid-phase corrosion from chlorine gas 9 Evaporation 32% liquid caustic soda, 50% liquid caustic soda 10 Tank areas Sulfuric acid tank area: 75–98% concentrated sulfuric acid ； Sulfuric acid unloading: 98% concentrated sulfuric acid ； Hydrochloric acid tank area: 31% hydrochloric acid ； Caustic soda tank area: 32% caustic soda, 50% caustic soda ； Sodium hypochlorite tank area: 10% sodium hypochlorite. Explanation: Due to differences in the process flow, the layout and names of various units may vary; therefore, the corrosion characteristics should be determined based on the specific project. 7.3.4 Other measures for corrosion protection of the caustic soda plant building in addition to the specified requirements: (1) The downspouts of the factory building should be made of fiberglass-reinforced plastic or UPVC. (2) The doors and windows of the factory building should be made of corrosion-resistant plastic windows and wooden doors. (3) In areas where corrosive liquids may drip, corrosion-resistant floor drains should be installed—such as PVC floor drains, fiberglass floor drains, or ceramic floor drains. Its drainage pipes should preferably be made of acid-resistant ceramic pipes, rather than cast iron pipes. (4) Principles for selecting anti-corrosion materials: Acid-resistant bricks should be used for solutions with a caustic soda concentration greater than 30% ； For the combined action of acids and bases, epoxy and vinyl ester materials are the preferred choices ； For sodium hypochlorite solutions with a concentration greater than 5%, vinyl ester materials are recommended ； For high-concentration acids and those used at higher temperatures, potassium silicate-based materials are recommended; whereas for acids with a hydrochloric acid concentration greater than 30%, phenolic and vinyl ester-based materials are suitable. (5) For the buildings and structures in caustic soda plants that are in contact with acids and alkalis, anti-corrosion treatment must be applied to their floors, platforms, walls, columns, beams, and roofs in accordance with relevant regulations ； For anti-corrosion coatings or materials, it is necessary to use materials with excellent resistance to acids and alkalis and reliable quality, with a service life of 5 to 10 years. Provision explanation: Selection should be made with reference to the material properties on pages 158–159 of 08J333. (5) Applying anti-corrosion coating to the walls, columns, beams, and ceiling of the facility: Coatings with excellent acid resistance should be used, with a service life of 5 to 10 years. 7.3.5 Forms of reinforced concrete structures: The strength of structural concrete must meet the requirements specified in Table 6.3.5 and Item 7.3.5 below.
Item | Corrosiveness level | High | Medium | Low
Minimum concrete strength grade | C40 | C35 | C30
Minimum cement content (kg/m³) | 340 | 320 | 300
Maximum water-cement ratio | 0.40 | 0.45 | 0.50
Maximum chloride content (% of cement content) | 0.08 | 0.10 | 0.10
Explanation: This requirement is derived from Table 4.2.3 of GB50046 “Code for Design of Corrosion Protection in Industrial Buildings” ； 7.3.6 The thickness of the reinforcement cover shall meet the requirements specified in Table 6.3.6, Thickness of Reinforcement Cover. Table 7.3.6 Component Types: Severe Corrosion, Moderate/Weak Corrosion. For surface components such as slabs and walls: 35; 30. For linear components such as beams and columns: 40; 35. For foundations: 50; 50. For underground exterior walls and floor slabs: 50; 50. Explanation of these provisions: These requirements are derived from Table 4.2.5 of GB50046, Code for Corrosion Protection Design of Industrial Buildings ； 7.3.7 When a steel structure is used, the rust removal grade of the steel substrate shall meet the requirements of Article 5.2.4 of the Code for Corrosion Prevention Design of Industrial Buildings GB50046-2008; the rust removal standard shall be no lower than Sa2 level. For important components that are not suitable for repair, the rust removal grade shall not be lower than Sa2½ level, and anti-corrosive coatings shall be applied. 7.4 Selection of building structure types 7.4.1 Due to the strong gaseous corrosivity in caustic soda plants, steel structures and load-bearing brick masonry structures are not suitable in areas or locations where the relative humidity is above 75%; instead, cast-in-place reinforced concrete structures should be used. 7.4.2 In corrosive environments, roof trusses and crane beams made of a combination of steel and concrete should not be used ； Concrete composite structures using profiled steel sheets as formwork along with reinforcement should not be adopted. Explanation of the provision: This provision is taken from Section 4.4 of the \"Code for Corrosion Prevention Design of Industrial Buildings\" GB50046; the reason for this requirement is that when corrosive substances enter the gap between steel and concrete, it becomes very difficult to address the corrosion of the steel structure. 7.4.3 The electrolysis plant should be equipped with doors, windows, lightweight roofs, or lightweight walls to allow for explosion venting; in addition, structural measures should be put in place to prevent the accumulation of hydrogen gas. For pitched roofs, ridge vents should be installed, while for flat roofs, ventilation vents should be placed at the center of each beam section. 7.4.4 In areas with cold winters where lightweight roof designs are used, it is necessary to implement proper insulation measures for the roofs, or to add snow and ice melting facilities, in order to prevent the formation of large amounts of ice formations and icicles, which could pose safety hazards. 7.5 Fire protection design for building decoration: The combustion performance grade of the decorative materials used in various parts of caustic soda production plants shall not be lower than the requirements specified below: 7.5.1 For plants of Class A and Class B, the decorative materials used in all areas must be of Grade A ； 7.5.2 The ceilings, walls, and floors of Class C underground plant rooms shall be of Grade A, while partitions shall be no lower than Grade B1 ； The floors of Class C high-rise factories should be of Grade A; the walls and floors should be at least Grade B1, while partitions should be at least Grade B2 ； The ceilings and walls of Class C multi-story and single-story factories should be at least B1 grade, while the floors and partitions should be at least B2 grade. Explanation of the provision: This provision requires compliance with the requirements specified in Table 4.0.1 of GB50222 (revised in 2001), the Code for Fire Protection Design of Interior Decoration in Buildings.