【Haichuan Illustrated Chemical Equipment】 Series Posts – Understanding Cascade Control Circuits

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Get to know the equipment, understand it, and make good use of it. [Haichuan’s Illustrated Guide to Chemical Equipment] series posts: https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719196. Everyone is welcome to participate in the discussions. ----------------------------------------------------------- This is a simple P&ID diagram that shows the cascade control system for a process heater (Furnace F-101). Cascade control is a powerful framework for handling processes with complex dynamics and external disturbances, and it is very common in chemical manufacturing. Process flow explanation (main pipeline): 1. Process fluid (heating fluid): The process fluid enters the F-101 heater through pipes on the left side; after being heated there, it is discharged through pipes to the downstream sections. 2. Steam (heat source): Steam enters the F-101 through inlet pipes to supply heat to it. Core control system: Cascade Control. Cascade control utilizes two control loops – a primary loop and a secondary loop – that work together in series to maintain the target process variable. Main Loop (Master/Primary Loop): Temperature control. Measurement (sensor): Temperature sensor TT-101 installed on the F-101 heater. Calculation (brain): The temperature indicated in the center is the controller TIC-101. It receives the actual process temperature signal from TT-101 and compares it with the desired setpoint. Output: TIC-101 does not directly control the valve; instead, it calculates the required steam flow rate and sends this value as a setpoint to the flow controller FIC-101. Slave/Secondary Loop: Flow control. Measurement (eye): A differential pressure flow meter installed on the steam pipeline. This includes the flow element (orifice plate) FE-101 and the flow transmitter FT-101. Calculation (brain): The flow indicator controller FIC-101 displayed on-site. It receives the actual flow signal from FT-101 and compares it with the dynamic setpoint sent by TIC-101. Output: Control valve. FIC-101 calculates the control signal required, which passes through an I/P converter (converting current to pressure signal) and the valve positioner FY-101, to ultimately control the opening degree of the steam control valve FCV-101. Why choose cascade control? Compared to single-loop control (for example, TIC-101 directly controlling FCV-101), the advantage of cascade control is its ability to quickly eliminate disturbances in the secondary loop: if there is a sudden fluctuation in the steam supply pressure – a common type of disturbance – the secondary controller FIC-101 will immediately detect the change in flow rate and correct it before it affects the process temperature, without the main controller even needing to know what has happened. This greatly improves the stability of the system. Improving main circuit dynamics: The main controller TIC-101 no longer needs to deal with the non-linearity in steam flow regulation, as it now directly sets the flow rate based on demand. This enables TIC-101 to focus on addressing the non-linearity of the processing process, achieving more precise temperature control.