Can a selective control system be used for SIS interlocking?
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The current addition process requires an SIS interlock: if the temperature in the reactor exceeds the set value during the addition process, the interlock will shut off the feed valve for addition. This control keeps the temperature below room temperature. Material needs to be added before the addition process begins, and at that time the temperature of the material is at room temperature, which triggers the SIS shut-off valve for addition. I seek advice on how to control this situation
Is the reaction exothermic? If there isn’t much material, consider introducing coolant in advance to lower the temperature for control, or would it be possible to use an additional material pipe?
The drip addition line is separated from the initial feed line; a shut-off valve A is installed on the drip addition line, and an ‘and’ interlock logic is set for ‘vessel temperature’ and ‘shut-off valve A open’. Before opening the drip cut-off valve A during operation, ensure that the tank temperature has cooled below the interlock temperature.
SIS is a safety interlock and serves as the last line of defense. The SIS interlock is open-loop and illogical. So, it’s simply a matter of high temperature; the valve can be shut off via interlock. As for the feeding process before you add drops of liquid to the system, I believe it is an operation to be carried out before the installation is put into use, and should not be included in the interlock logic of the SIS. If the feeding process is a routine operation, I recommend conducting further analysis to determine whether it is necessary to activate the SIS when the temperature rises and the valve needs to be closed. If it’s not necessary to go into the SIS, it’s much more convenient in the DCS; a button can be added for shutting it down before startup, allowing for manual operation or automatic shutdown/enabling based on other conditions ； If it’s necessary to enter the SIS, then I believe that feeding at a high temperature before the dripping process is problematic and poses safety risks.
Selective control is a \"soft\" protection function; it still falls under the category of basic process control systems, and its protection level is low； The SIS system is a type of \"hard\" protection function that provides emergency intervention in the production process, offering a high level of protection.