Regarding cascade interlocks

I work in the field of process engineering as well. First of all, I think those working in this field really need to understand the concepts involved; it’s not possible to rely on automatic control systems. First, as you can see, this is an Fv type, which indicates that it is a flow control device. If you remove the level signal, it becomes clear that this is a flow control valve. Do you understand? There is also an additional signal input; in production, the DCS determines a flow value based on the difference between the liquid level and the set value, and then sends this signal to the flow controller, which adjusts the flow to that value. You can think of it as follows: when the liquid level changes, it’s necessary to input a different flow control value into the flow control circuit, and the DCS handles this process. Compared to simple level control, cascade control provides more stable performance. None of the above are interlocks; they are all control circuits.

The main circuit is for level control, while the secondary circuit is for flow control. Normal flow fluctuations can be adjusted within the secondary circuit, without affecting the liquid level. The liquid level in the main circuit is used as the setpoint for the secondary circuit, with the primary purpose being to stabilize the liquid level.

The main circuit is designed to maintain a stable liquid level; at the same time, it takes into account the need to prevent large fluctuations in flow rate, which is why flow control is incorporated

It’s fine if the process engineers don’t understand; just have the automation engineers come and adjust it – it takes just a few minutes.

Well, I’ve learned a bit about it. Could you briefly explain the relationship and differences between control loops and interlocks?

Well, I’ve learned a bit about it. Could you briefly explain the relationship and differences between control loops and interlocks?

This is a typical cascade control system for level and flow regulation: the primary loop deals with the level, while the secondary loop handles the flow. The output of the level regulator serves as the setpoint for the flow regulator. The function of the secondary loop is to eliminate first the disturbances caused by changes in flow, which are the main factors affecting level changes; it thus performs an advanced and rough adjustment, preventing large fluctuations in the level. Subsequently, the primary loop makes fine adjustments to any remaining level deviations. In this way, the control quality is significantly improved compared to using a single-loop level control system alone.

To put it simply, in my understanding, a control loop is primarily used for control purposes: by comparing the detected parameters with set values, and using controllable valves to make adjustments according to a predetermined PID algorithm, automatic and stable control is achieved. On the other hand, interlocks function such that when a detected parameter reaches a certain set value, they trigger a series of predefined actions through a SIS or DCS system; these actions can involve valves or anything else. In other words, interlocks are like a row of dominoes – once one is pushed, all the others will fall as well

In fact, whether to use cascade control or not is determined by the process requirements. With the current development of instruments, there is no need for multiple controls over sensitivity and accuracy. In most cases, cascade control is requested by process engineers. For example, in this diagram of yours. By making the valve automatically respond to the liquid level, it can definitely be controlled. And it will definitely be more stable than cascade control. But it is very likely that flow rate is also a very important process parameter. It also needs to be controlled. Current cascade control mainly focuses on the need to control multiple variables, rather than whether the system is stable or not.

What I said makes sense; it’s the answer I want