What Are the Best Methods for Hazardous Materials Safety?

Safety evaluation methods are technical terms. Safety evaluation methods are divided into checklist method, expert evaluation method, advance hazard analysis method, failure analysis method, hazard and operability analysis, fault tree analysis method, event tree analysis, safety evaluation method, and danger. Index evaluation method, index evaluation method.

Safety Evaluation Method

Overview of Safety Evaluation Methodology

In 1974, the Mond department of the British Imperial Chemical Industry Corporation (ICI) introduced the concept of toxicity based on the Dow chemical index evaluation method, and developed some new compensation coefficients. Evaluation Method ".

In the study of the hazards of existing plants and planned construction plants, especially when evaluating the potential hazards of new design projects, it is necessary to improve and supplement Dow Chemical's methods. The two most important aspects are:

(1) Introduced the concept of toxicity, and expanded Dow Chemical's "Fire and Explosion Index" to include initial evaluation of "Fire, Explosion, and Toxicity Index" including substance toxicity.

(2) Developed a new compensation coefficient for re-evaluation of the actual danger level of the device.

Safety evaluation method evaluation steps

(1) Determination of evaluation unit

The "unit" is an independent part of the device, not the rest of the device. It can also be used as a unit if it is part of the device separated by a certain distance, fire wall, dyke, etc. When selecting a part of the device as a unit, pay attention to the characteristics of other adjacent units and whether there are areas with different special processes and hazardous substances.

The typical unit types in the device are: raw material storage area, supply area, reaction area, product distillation area, absorption or washing area, semi-finished product storage area, product storage area, transportation loading and unloading area, catalyst processing area, by-product processing area, waste The main piping bridge area of the liquid treatment area and the access device area. In addition, there are filtration, drying, solid treatment, gas compression, etc., and the device can be divided into appropriate units when appropriate.

Dividing the device into units of different types can evaluate the hazardous characteristics of different units of the device. Otherwise, the entire device or a large part of the device will be characterized by the most dangerous unit among them. In addition, through the division of units, the limit of the delay in the occurrence of accidents from the most dangerous units in the installation to other units with a large investment can be considered.

When evaluating storage areas, the unit usually consists of one dam and all storage tanks in a common dam. Other areas separated by dams, such as liquefied gas, highly flammable liquids, flammable liquids, and special hazardous materials such as the danger of self-aggregation, the possibility of generating peroxides, and the danger of condensed phase explosion, can be treated in different units to Can correctly identify its relative danger.

The main piping bridge in the installation area is different from the installation process or storage unit and should be considered as an independent unit. Its danger is mainly the length of the pillar or the bridge bridge erected between the pedestals and the steel pipe supported on it.

(2) Determination of important substances in the unit and their substance coefficients

Select important substances in the unit

There are often many materials such as raw materials, intermediate products, products, by-products, catalysts, and solvents in the unit. The dangerous potential of these materials and the amount of these materials in the unit are different. Different materials are used to evaluate the danger of the unit, and the evaluation results are different. Therefore, in the selection unit, the unit is evaluated by using a relatively large number of substances with a high potential for danger as important substances in the unit.

If more than one kind of important substance exists in the device and unit, different important substances must be evaluated differently, and the most dangerous one is selected as the representative of the danger of the unit as the basis for the final evaluation. If the substance in the device is a mixture and the composition is kept constant, the mixture can also be taken as an important substance when there is a potential danger of major fire, explosion, reaction or toxicity in the device.

Determination of important material coefficients

The material coefficient refers to the scale of the dangerous potential of fire, explosion, or energy release of important substances under standard conditions (25 ° C, 0.1 MPa). The material coefficient (MF) is represented by the symbol B when the total effect calculation is performed.

. The material coefficient of general flammable substances is determined by the combustion heat in the air of important substances under standard conditions.

. The material coefficient of edge flammable material or material that is not flammable under transportation conditions can not be zero because it can be calculated from the combustion heat of the reaction, and its value can be generated by the heat of generation of important materials and the gas phase combustion products The difference between the heat of formation is calculated, and the material coefficient is calculated from the heat of combustion. The marginal flammable substances include trichloroethylene, 1,1,1-trichloroethane, perchloroethane, chloroform, and dichloromethane.

. Combustible substances are substances that do not undergo an exothermic reaction with oxygen, such as water, sand, nitrogen, helium, carbon tetrachloride, carbon dioxide, and hexachloroethane. In order to maintain the validity of the method, for substances with a substance coefficient of zero, MF = 0.1 is given.

. Flammable substance mixture added with diluent If a certain amount of diluent is added to the flammable substance mixture, the material coefficient of the flammable or explosive component can be used; MF = 0.1 of the inactive component can be used And the component ratio of the components to determine the material coefficient of the mixture. For marginal flammable materials, a value higher than the material coefficient of inactive materials is appropriate.

. Combustion heat cannot be obtained for most solids of combustible solids and dust. For example, wooden blocks and large-volume metal solids that are selected as important substances in the unit. Only when such solids are in a particulate, granular or dust state with a much higher danger than large-volume states can MF = 0.1. . For powders and other high hazards, the heat of combustion must be used as the material coefficient.

, the composition of unknown substances such as fuel gas, special-purpose substances, mixed powders such as pharmaceuticals, flour and coal, and other dust-like substances, the combustion value of which must be determined by experiments. In some cases, data on the explosive pressure of the substance in sealed containers are available.

Danger of material mixing When materials are mixed, a large amount of oxidant and reducing agent are mixed in the device and the reaction heat released is greater than the combustion value of combustible materials, such as aluminization reaction, metal powder and halogenated carbon reaction, nitration reaction, sulfur Chemical reaction, etc., the calculated reaction heat must be converted into the material coefficient.

, Potentially hazardous substances with condensed phase explosion or decomposition, such as nitromethane, diylbenzene, acetylene, nitropropane, concentrated hydrogen peroxide, organic peroxides, tetrafluoroethylene, etc. To find out whether the combustion value is greater than the explosion value or the decomposition heat, calculate the material coefficient by the value to be taken.

When an important substance is exposed to the air or under other conditions can become a potentially dangerous mixture or product with condensed phase explosion or decomposition, since the substance changed in the operating unit will not exist at any time, the substance is calculated The coefficient is negligible.

(3) Initial evaluation of unit danger

Danger of special substances

In the case of material hazards, the special properties of important substances and the situation where important substances are mixed with other substances such as catalysts in the unit must be re-evaluated. The criteria for the hazard coefficient of special substances shall be determined according to the quantity of important substances in the unit and the effect on their specific properties under fire or possible fire conditions.

The danger coefficient is a function of the important substances in the specific unit under study in the specific use environment, and cannot be defined by the properties of the isolated important substances. Therefore, the danger coefficient of a certain substance in different units can be strong or weak. If the units are different, even the same important substance needs to change the danger coefficient of special substances.

General process danger

This type of hazard is related to the basic type of process and operation performed in the unit. The operation process includes: purely physical changes, single continuous reactions, single batch reactions, multiple reactions, different process operations in the same device, material transportation, and movable containers.

Dangerous process

Based on the scores of important substances or basic processes and operating properties, some operating processes and processes can increase the overall risk. They include: low pressure, high pressure, low temperature, high temperature, and the danger of corrosion and erosion, the danger of joints and fillers, the danger of vibration and cyclic load fatigue, difficult to control process reactions, operations near the explosion limit, dust or mist explosion Hazard, use of strong gas-phase oxidant process, static hazard, etc.

(4) Danger of quantity

When handling large amounts of flammable and decomposable substances, an additional risk factor is given.

The calculation of the total amount of material in the unit under consideration should take into account the total amount of material in the equipment such as reactors, pipes, feed tanks, towers, etc. It can be calculated directly from the mass of the substance, or from volume and density. Dangerous comparisons can be made based on the mass of the gas, solid, liquid, and mixture.

(5) Danger in layout

The important item examined by the danger factor caused by the unit layout is the height of a large number of flammable substances in the unit. The height of the unit refers to the height from the ground of the top of the device process unit and the conveying material piping. The exhaust pipe and the beam structure of the beam elevator cannot be used to determine the height; however, the main piping positions of the distillation tower and reaction tower must be considered, Product top condenser, upper supply container, etc. In the calculation, the height is represented by H (m).

The normal work area of a process unit refers to the planned area of the structure related to the unit. Pumps, piping, devices, etc., which need to be included outside the above-mentioned work area, can be expanded. The area surrounded by the structures of the surrounding units and related auxiliary facilities with a minimum length of wall can be regarded as the work area, which is represented by N (m2). The normal operating area of the evaluation main bridge unit refers to the area obtained by multiplying the maximum width of the pipe bridge and the distance between the center of the support or the platform. Evaluation of the normal operation area of a tank unit with a dam refers to the actual planned area of the tank itself and the area occupied by the pumps and related piping in the unit. The total area within the dam cannot be regarded as the normal operation area. The normal operation area of an underground storage tank is determined by the location of the underground storage tank. The normal operation area of a deeper storage hole refers to the location of the inlet or piping connection at the surface or underground more than 10m.

(6) The danger of toxicity

It is a relative score on toxicity risk and its impact on comprehensive risk assessment. Health hazards can be considered according to the cause and extent of the cause, and some can be attributed to abnormal process conditions such as maintenance and process control or prone to fire; there are subtle issues that often occur from joints, foundations, process exhaust, etc Leaks; health hazards caused by asphyxiating gases such as nitrogen, methane, and carbon dioxide.

The toxicity of gas, steam, and dust is generally expressed by the time load value (TLV) of 40 hours per week and 7-8 hours of labor per day. For short-term contact, multiply the TLV by a certain factor and use a larger value. Some substances must be controlled in a range lower than the TLV value even if they are contacted within a short period of time.

The danger caused by general leakage and the hazard caused by normal maintenance or process operation are evaluated by TLV value, and abnormally high leakage, device control system failure, and fire conditions are evaluated by high and short time concentration values.

Physical factors such as radiation sources and heat on important installation items must be considered as a compound toxicity risk together with the above direct toxicity. When abnormal chaos occurs, the issues that affect the correct speed and form of action should also be considered.

(7) Calculation of initial evaluation results

Subtotal the various coefficients recorded, and then convert them into all DOW / ICI indicators according to the method originally determined by DOW.