The fundamental difference between C and S in instrument function symbols

The control loops are denoted by C; they are typically implemented in DCS systems and are used for ordinary PID control and other similar control functions. Interlocks are denoted by S; they are usually implemented in SIS systems and are used for emergency shutdowns, as well as for starting or stopping units or closing isolation valves. In short, that’s what it means

....It’s actually quite detailed; different functions are represented by different symbols. In other words, ‘C’ denotes normal production, while ‘S’ indicates safety measures. There’s no need to worry about this, nor is it necessary to learn it – you’ll understand it over time; it’s quite easy to get started with. . .

Generally, C stands for control, which is applicable to control valves; S stands for interlock, which is used for shut-off valves, pumps, and the solenoid valves built into control valves. For example, LICA(H) refers to the high-level alarm for level display control (control valves)； LIAS (H HH) level indication high limit alarm – high-high limit interlock to stop the pump or valve ； TICAS (H HH) temperature display control – high limit alarm; high-high limit interlock: under normal control, the control valve is operated, while the high-high limit interlock causes the solenoid valve associated with the control valve to send a current signal in order to close the valve ； S is also used in DCS; SIS interlocks are only required when significant safety is at stake.

All those that can be controlled at valve positions from 0 to 100% use C, and two-position on/off valves also use C. For control valves with interlock, add S after *IC; for two-position on/off valves with interlock, only S is used, no C.

Based on what you said, may I understand that C is used when there is regulatory control, and S is used when there is on/off or start/stop control? Can it be understood this way?

You mean that when control is within the 0–100% range, use C; whereas for starting or stopping, or turning on or off – when there is no value at all – use S instead of C. Is that the right way to understand it?

First, you need to distinguish between interlock concepts and control concepts, as well as between interlocks in DCS systems, DCS controls, and SIS interlocks. Then you need to figure out when to use C: Control and S: Chain

Seeking science popularization. Please explain in detail. :handshake

LZ can be understood in this way: C generally represents a regulating function, that is, what you refer to as control. You can often see symbols such as LICS or FICS in PID controllers; these indicate that the level gauge or flow meter is used to regulate a certain valve or other device. By regulation, is meant variation within a certain range – it’s a concept of range. S usually stands for DCS interlock, so one might wonder, “Why can’t interlock be considered a form of regulation?” ”In fact, regulation involves not only a range but also two specific points, namely 0% and 100%. It’s as if the level gauge gradually adjusts the valve from 100% to 0%. As for S, it operates in a binary manner, and its speed is faster than that of C; therefore, S is used for interlocks in emergency situations. Additionally, some organizations* are accustomed to using Z instead of S to represent interlock symbols within the SIS system.