Regarding whether automatic control is needed
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Recently, I was involved in a project involving the dissolution and emulsification of pesticide active ingredients followed by their decentralized packaging using packaging machines. The design institute recommended the installation of devices for weighing the active ingredients as they are fed in, as well as level sensors in the mixing tank. However, the client prefers to use scale weights to measure the active ingredients on-site, in order to ensure that the liquid level in the mixing tank does not exceed a certain limit. Moreover, all the materials used in the workshop belong to Category C, and the client is reluctant to implement any automated control systems. (The owner does not have any control cabinets such as DCS on site; only some equipment is equipped with on-site interlock controls.) I would like to ask everyone whether this approach is still allowed these days?
Now, risk assessments are required to be conducted in accordance with the guidelines for safety risk assessment of fine chemical reactions, and process safety measures are taken based on the results of those assessments. It makes sense that the design institute has put forward such a requirement. Furthermore, imagine that if there are multiple feedings or feeding errors (such as inadequate handover during shift changes), could it lead to overheating, excessive reactions, the release of toxic gases, and even fires or explosions? For a specific example, please refer to the “7.12” explosion incident at Yibin Hengda Technology. If there is no exothermic reaction occurring during the mixing process, the process poses a low level of risk; then, from the perspectives of occupational health and quality control, could the necessity of installing an automated feeding system be considered? After all, when weighing and feeding materials with the local currency unit, direct human contact with the materials or dust is inevitable; an automated control system enables more precise control of product quality.
Yes! The standard approach to risk assessment is to conduct a safety risk evaluation of the preliminary design during the initial stages of construction. Currently, Hazop analysis and LOPA analysis are commonly used, and the SIL level of the process equipment on site is determined; not only is a DCS required, but also a SIS. And it is re-evaluated every three years upon operation. Regulations were introduced under the Work Safety Law in 2014; China did enact laws, but it was unclear how to implement them. In recent years, HAZOP analyses have also been carried out based on foreign methods, as a model for such approaches.