Key Exam Points for Safety Engineering – “Chemical Process Safety”: Safety Design Techniques for Chemical Processing Projects
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I. Summary of key examination points 🌟 Examination point 1: Inherently safe design of chemical processes Definition and levels of inherent safety: Inherent safety refers to making production equipment or systems inherently safe through design measures, so that accidents cannot occur even in cases of human error or equipment failure. Its core is to eliminate or reduce hazard sources at their source, rather than relying on additional safety safeguards and management controls. Intrinsic safety can be divided into 3 levels, forming an onion structure: Core layer: Process intrinsic safety. Prevent accidents based on the basic physical and chemical characteristics of materials (such as the quantity, properties, and process routes of chemicals), rather than relying solely on control systems and interlocks. For example: replacing toxic and hazardous materials with safe and non-toxic ones, and developing new catalysts to make reaction conditions milder and more controllable. Intermediate layer: Intrinsic safety of equipment instruments. Safety provided by the equipment’s instruments due to their design characteristics. They are mainly divided into two categories: fail-safe and fault-safe. Outermost layer: Safety protection measures and management measures. It mainly includes site selection, overall plant layout, fire and explosion prevention facilities, emergency rescue measures, and the establishment of safety management systems.
Test Point 2: Methods for determining the external safety protection distance (a frequently tested key topic). Definition: The external safety protection distance is the distance or risk control line established between hazardous chemical production units and storage facilities and the objects that need to be protected outside the facility, in order to prevent and mitigate the impact of potential accidents such as fires, explosions, and poisonings on those protected objects. Procedure for determination: The method should be determined based on the specific type of hazardous chemical production facilities and storage installations. In the case of explosives, the accident consequence method should be used to determine the external safety protection distance. Involves toxic gases or flammable gases, and constitutes a major hazard source. A quantitative risk assessment method should be used to determine the external safety protection distance. Involves toxic gases or flammable gases, but does not constitute a major hazard source. Comply with the distance requirements specified in relevant standards and specifications. Process of quantitative risk assessment method (commonly tested in short-answer questions) ① Collect data and information → ② Determine the assessment unit → ③ Identify hazards and leakage scenarios → ④ Analyze accident probabilities and consequences → ⑤ Calculate quantitative risks → ⑥ Determine external safety protection distances using risk benchmarks. Classification of protection targets: Based on the main nature of their actual use, they are classified as follows: Highly sensitive protection targets: such as kindergartens, schools, hospitals, nursing homes, etc. Important protective targets: such as detention centers, military facilities, diplomatic sites, major transportation hubs, etc. General protection targets: They are classified into Category 1, Category 2, and Category 3 protection targets based on their size (for example, a community with 35 households belongs to Category 1, while a restaurant with an area of 3500 m² falls under Category 2).
🌟Test Point 3: Safety in the layout of chemical plants – Functional zoning of the plant area and layout principles: Process unit area (core hazardous area): Generally located in the central part of the plant area. It should be located on the upwind side of the wind direction with the lowest annual frequency at places where people gather and at locations where open flames or sparks are generated. In mountainous or hilly areas, it should be avoided to be located in wind-trapped zones. Process equipment that requires a clean environment should be located downwind of the pollution source that emits harmful gases, smoke, mist, and dust, in the direction of the wind with the lowest frequency of occurrence throughout the year. Storage tank area: Warehouses and tank areas used for storing Class A and Class B materials, as well as liquefied hydrocarbon tanks, should be located in designated zones at the periphery of the factory site. Finished products and filling stations should not be located in areas that pass through hazardous zones such as production areas and tank areas. Liquefied hydrocarbon or flammable liquid tank farms should not be located adjacent to areas that are at a higher elevation than the plant, other key facilities throughout the plant, or areas where people gather. Layout of factory roads: The factory’s traffic routes should be arranged in a circular pattern as much as possible, and the width of the roads should generally be sufficient to allow two cars to pass each other. The clear height above the surface of fire lanes shall not be less than 5 m. The width of the road surfaces within installations and facilities should not be less than 4.0 m, the radius of curvature at the inner edge of the road surfaces should not be less than 7.0 m, and the clear height above the road surface should not be less than 4.5 m. When the road within the factory area reaches an end, a turnaround area should be provided, or a turning space with a floor area of at least 12m×12m.
🌟Exam Point 4: Safety design of process procedures (HAZOP/LOPA/SIL) (a core and frequently tested topic). Hazard and Operability Study (HAZOP): Definition: A structured analytical method used to identify design flaws, process hazards, and operational issues. Analysis is conducted by examining deviations resulting from the combination of guiding words (blank, excess, reduction, accompanying, partial, opposite, abnormal) and process parameters (temperature, pressure, liquid level, flow rate, etc.). Analysis process: ① Define objectives and scope → ② Preparation for analysis → ③ Conduct analysis → ④ Record results → ⑤ Track actions taken. Layer of Protection Analysis (LOPA): Definition: It is a continuation of the HAZOP analysis, used to determine whether the existing protections are sufficient to address risks with extremely severe accident consequences, and to decide whether additional SIS system protections are necessary. The conditions that an Independent Protection Layer (IPL) must meet are: effectiveness, independence, and auditability. Typical measures that can be used for IPL include: intrinsically safe design, BPCS, critical alarms and personnel response, Safety Instrumented Systems (SIS), physical protection (safety valves, blast discs, etc.), and physical protection after release (fire dikes, barriers, etc.). Measures that are not suitable as independent protective layers (common exam pitfalls): training and certification, operating procedures, regular testing and inspection, maintenance, and warning signs. Safety Integrity Level Analysis (SIL): Definition: Verification of the results of the LOPA analysis to determine whether the SIS system meets the required safety integrity level. SIL is divided into 4 levels (SIL1 is the lowest, SIL4 is the highest). Correspondence between SIL levels and PFDavg (commonly tested in calculations/multiple-choice questions). SIL1: PFDavg ranges from 10⁻² to 10 to 100. SIL2: PFDavg is 10⁻³ ~ 100 ~ 1000. SIL3: PFDavg is 10⁻⁴ ~ 1000 ~ 10000.
II. Practice Questions 1. 【Single-choice】The core of intrinsic safety is to eliminate or reduce hazard sources at their source. Among the following measures, which one contributes to achieving intrinsic process safety? A. Adopt a fail-safe instrument system. B. Install fire dikes in the plant area. C. Develop new catalysts to make the reaction conditions milder and more controllable. D. Equip operators with anti-static work clothes
1. 【Answer】C Explanation: Intrinsic safety is divided into 3 levels. Option A (fail-safe instruments) falls under the intermediate layer of “intrinsic safety of equipment instruments”” ； Option B (fire dike) and Option D (anti-static work clothes) fall under the outermost category of “safety protection measures and management measures”. Option C (developing new catalysts to make the reaction mild and controllable) addresses the inherent hazards of the process at its core, and constitutes intrinsic process safety at the fundamental level. Therefore, option C is correct.
2. 【Single-choice question】According to the \"Method for Determining the External Safety Protection Distance for Hazardous Chemicals Production Facilities and Storage Units\" (GB/T 37243), for a company’s production facilities that deal with flammable gases and constitute a level-3 major hazard source, the external safety protection distance should be determined using ( ). A. Accident consequence method B. Quantitative risk assessment method C. Safety checklist method D. Preliminary hazard analysis method
2. 【Answer】B Explanation: The process for determining the external safety protection distance: ① For cases involving explosives, the accident consequence method is used； ②For situations involving toxic or flammable gases that constitute major hazard sources, a quantitative risk assessment method should be employed ； ③In the case of toxic or flammable gases that do not constitute a major hazard source, the relevant standards shall be applied. Since this enterprise deals with flammable gases and constitutes a major hazard source, a quantitative risk assessment method should be employed. Option B is correct.
3. [Single-choice] Regarding the layout of various functional areas in a chemical plant, which statement is correct? ( ) A. The process unit area is a hazardous area and should be located at the periphery of the plant site.
B. LPG tank farms should be situated at a higher elevation than the process units to facilitate gravity flow.
C. The process unit area should be placed on the upwind side relative to the direction of the wind with the lowest frequency of occurrence throughout the year, in areas where people gather.
D. The finished product filling station should be located close to the production area to minimize transportation distances
B. LPG tank farms should be situated at a higher elevation than the process units to facilitate gravity flow.
C. The process unit area should be placed on the upwind side relative to the direction of the wind with the lowest frequency of occurrence throughout the year, in areas where people gather.
D. The finished product filling station should be located close to the production area to minimize transportation distances
3. 【Answer】C Explanation: Option A is incorrect. The process unit area is the core hazardous area and is generally located in the central part of the plant site. Option B is incorrect; groups of tanks for liquefied hydrocarbons or flammable liquids should not be located adjacent to areas where equipment or personnel are concentrated, at a higher elevation than those areas. Option C is correct; the process unit area should be located on the upwind side of the direction corresponding to the least frequent wind direction throughout the year, relative to areas where people gather and places with open flames. Option D is incorrect; finished products and filling stations must not be planned in hazardous areas such as production zones and tank farms.
4. [Single-choice] HAZOP analysis is an important method for the process safety design of chemical plants. Which of the following are HAZOP analysis trigger words? A. Pressure B. Temperature C. Accompanying D. Flow rate