【Haichuan Chemical Salary News】Specific techniques for SIL selection—HAZOP analysis research
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It was often said that the ESD system is a category of SIS systems. Safety instrument systems require that instruments in different locations have varying levels of reliability depending on the production needs. Higher reliability comes at a higher cost, while lower reliability results in lower costs; however, in some cases this lower reliability is not sufficient to meet safety requirements. So what is used to measure this reliability? It is SIL. Below is the work carried out to determine the required reliability level (the required grade) for safety instruments in various areas: Specific HAZOP technique studies for SIL selection
I. Introduction: SIF serves as a layer of protection to reduce the risk of process safety incidents caused by deviations in process parameters. It is possible to select SIFs with different SIL levels, depending on the level of risk that needs to be reduced, thereby bringing the process safety risk down to an acceptable level. LOPA is a method used to determine the SIL level, but it cannot be used to identify potential process safety accident scenarios. HAZOP is an excellent tool for identifying such scenarios, yet it cannot calculate the frequency of accidents. By combining HAZOP and LOPA and importing the relevant information from HAZOP into LOPA, it is possible to determine the SIL level of the SIF. However, when these two methods are combined, certain conditions must be met for them to integrate well. The following will explain how to effectively combine HAZOP and LOPA to determine the SIL level of SIFs.
II. Process safety risks and how to reduce them. The core of safety management in hazardous chemicals enterprises is to prevent and reduce process safety accidents, which are accidents caused by the accidental release of hazardous materials or energy. The process of a process safety accident is shown in Figure 1:
A chemical processing plant relies on control models for materials and energy (as shown in Figure 2) to reduce or mitigate process safety accidents: The first level of the materials and energy control models includes devices that directly contain the materials, such as pipes, valves, and fittings; parameter control facilities refer to automatic control circuits, alarms, safety instruments, pressure relief systems, etc.; mitigation facilities include detection probes, fire suppression systems, gas detection systems, etc., in case of leaks. The second level includes structures such as dikes and collection tanks, which are used to collect liquid materials that leak from the first level of containment.
During normal production, to prevent material leakage from the first layer of containment, the following two conditions must be met simultaneously: 1. The parameters of the material are within the designed range； 2. The first layer has adequate capacity; that is, when the parameters are not deviated from, it ensures that the first layer can hold the materials.
Chemical processing units rely on the onion model (as shown in Figure 3) to ensure that the parameters of the materials remain within the design range:
Chemical processing units rely on five layers of control to manage process parameters: process design, basic process control (BPCS), critical alarms with human intervention, safety instrumented systems (SIS), and physical protection. The number of protection layers required for a given unit to control these process parameters must be determined through process safety analysis and evaluation, in order to avoid either insufficient or excessive protection. For a chemical plant, process risks always exist; by balancing safety and economics, we need to keep the process safety risks within acceptable levels.
In accordance with the standard **\"Functional Safety of Safety Instrumented Systems in the Process Industry – Part 3: Guidelines for Determining the Required Safety Integrity Level\" – GB/T21109.3-2007, during the process design it is necessary to reduce process safety risks to levels below those that are considered acceptable, through the use of protective measures; see Figure 4: Process Safety Risk Reduction Model.
In the actual chemical engineering design process, the various protective layers are not typically determined by using the model shown in Figure 4; instead, based on design specifications and engineering experience, these protective layers, including SIS, are established once the preliminary design is completed. This will lead to the following problems: 1. Insufficient protection – the overall average requirement for each protection layer in terms of failure probability (PFD) fails to reduce the risk below the acceptable level ； 2、Excessive protection: too many protective layers are implemented, resulting in residual risks that are far below the acceptable level; this leads to high investment costs as well as elevated maintenance expenses in the future ；
To address the issues of insufficient or excessive protection in chemical engineering design, it is necessary to use Layer of Protection Analysis (LOPA) to calculate process risks. However, during the design of chemical processes, designers do not clearly specify how each layer of protection applies to specific accident scenarios. Therefore, it is necessary to adapt the HAZOP method to identify various process safety hazards from the design drawings, and then use the LOPA method to calculate the residual risks in order to determine whether the protective measures are adequate.
III. The role of safety instrument functions and how to determine the SIL level. A SIF refers to an interlock circuit that possesses a certain PFD and is capable of reducing specific process safety risks. A Safety Instrumented System (SIS) is a combination of all SIFs in a chemical plant. The relationship between SIS and SIF is shown in Figure 5.