Safety valve trip interlock setting
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The system has an important valve, and its safety interlock is as shown in the figure below. Why is an analog value used? An analog value represents a range of 0% to 100%, while there are also two digital values. Some people say that digital values refer to feedback based on color changes at specific limits – green when everything is fully turned on, and red when everything is fully turned off. But why are two values used? It’s done for greater safety; it’s also possible to use five values. I’m not quite understanding the safety interlock conditions shown in the figure. I hope fellow experts can provide guidance on why the valve needs to be designed in this way, what considerations are taken into account in such a design, and why it’s important to do this
\ud83d\ude1c\ud83d\ude1c\ud83d\ude1c I’ll handle it myself
What is a digital signal? Is an analog signal fake?
The LL90% valve is not fully open. The option of choosing between three options is intended to reduce the risk of shutdowns caused by false alarms from the instruments; using five options would be better, but it increases costs. Moreover, the option of choosing between three options already eliminates the problem of false alarms, so there is no need for more options. The principle behind the slurry circulation valve is the same.
OP, if you think that way, then why is a digital level switch added in addition to the analog level measurement for the storage tank? It’s called a safety interlock – of course it’s for safety reasons, and instruments with different measurement methods need to be used. Moreover, I don’t understand the question posed by the original poster; the digital and analog signals here are likely not related to valve interlocks, but rather to level or flow rate interlocks. The valves should be the points at which interlocks are applied. As for why the same analog signal cannot be used for a 3-out-of-2 system, I think it’s either due to reliability issues or economic considerations. As for a 5-out-of-2 system, it’s possible, but it increases both costs and the level of safety – is that really necessary?
One liquid level is displayed as an analog value, while the two liquid level switches are digital; together they form an interlock logic. When the liquid level is detected at LL90% or HH10%, and one or both of the liquid level switches are activated, with at least one of them being triggered, then at least two out of the three signals are activated, and the interlock is activated. It is recommended to take a look at the interlock logic diagram
To elaborate on it, you might not be able to finish even with thousands of words. I guess you’re not from an instrumentation major either. What is ultimately reflected is reliability and availability; it also encompasses the relationship between cost and risk.