How to design SIS fail-safe interlocks?
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Let’s take an example: for a valve closure via level interlock, the level switch is selected to be normally closed (it closes during interlock); then, based on the SIS logic output (0 or 1), the solenoid valve is activated for closure – but how should the relay be selected, as normally open or normally closed? For pressure interlock valve opening: the level switch should be selected as normally closed (to open during interlock), and then the SIS logic output (0 or 1) is used to interlock the solenoid valve (how should the relay be selected as normally open or normally closed?) When using two-way three-port solenoid valves, does it still matter whether to choose solenoid valves that are normally open or normally closed? It requires normal excitation, right? In general, I just want to know what a fail-safe interlock is On-site liquid level and pressure triggers are required (is it a normally closed switch?) ), must the SIS logic output also be 0 to achieve fail-safe operation? Also, for the relay that outputs the SIS logic, how should one choose between normally open and normally closed operation? Is choosing normally open the option that ensures fault safety? How to choose a solenoid valve? There is also the issue of normally open or normally closed operation. Please ask an expert to answer, thank you!
SIS generally follows input 0 interlock, with the output also being 0 interlock.
Safety interlock: It generally needs to be set to 0, and the output should also be 0. The solenoid valve must be selected according to the pneumatic circuit diagram, but the requirement that \"the solenoid valve must be properly energized while the interlock is powered off\" must be met.
The so-called fail-safe requirement dictates \"zero voltage\" and zero current or low current, as well as disconnection, de-excitation, and spring return, for both the field side (inputs and outputs)； The location of valve failures is determined by the properties of the process medium.