Design concept for low liquid level selective control
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Recently, I encountered a problem in a project related to the low-level detection control for a heat exchanger (vaporizer). Typically, in such cases, the process measurement values are first used to determine whether the level is high or low, after which the control valve is adjusted. However, the design approach involves sending out control signals based on these two process values first, and only afterwards performing a low-level selection on the MV or OP values to control the valve. I’m not sure about the intention behind this design; I would appreciate some guidance from everyone.
This post was last edited by zzllyp on 2020-4-16 at 05:10. This is done for process safety; the intention is as follows: the heat exchanger has one flow control mechanism and one temperature control mechanism. When the temperature is above a certain value, the valve can be opened according to the flow PID settings. When the temperature drops below that value, the valve must be closed based on the temperature PID settings. In special cases, it’s possible that the flow rate has not yet reached the set value, but the valve still needs to be closed due to the low temperature. This type of complex control is known as selective control, while in DCS systems it is referred to as override control. In other words, the lower of the two values is assigned to the AO output, and this lower value is also fed back into the PID output circuit. Otherwise, if only the lower value is considered, the other PID will keep producing higher values. For example, if the flow PID outputs 50% because the temperature is high enough, the temperature PID might output 100%. The valve then operates at 50% based on the lower value. But if the temperature suddenly drops, it takes some time for the temperature PID to adjust from 100% back below 50%, and during that time the temperature cannot respond promptly. Some manufacturers of such DCS systems come equipped with functional blocks right out of the box, such as PKS’s OVERRIDE function block, and Yokogawa’s SH block, etc.
The response on the 2nd floor was very comprehensive; there are two ways to implement selective control – by choosing either the PV value or the MV value. Generally, the MV value is chosen more often. It is also necessary to prevent integral saturation (to avoid infinite maximum or minimum outputs in the open-loop system)