Learning Chemical Process PID while working on projects – Part 1
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The company plans to restructure its existing products, increase production capacity and improve control levels, as well as reduce staff while boosting efficiency. It is necessary to create more detailed and accurate PID diagrams for the existing systems (the previous PID diagrams were not detailed enough and even did not reflect reality). Process engineers are not sure where to start when creating such PID diagrams for the first time; they are learning as they go, and hope that everyone can offer some guidance. Current status of the installation: The initial setup was almost entirely manual; after several safety inspections and technical upgrades, a DCS was installed. There are quite a number of instruments and control valves, but automatic control has not been achieved. There seem to be some interlocks related to safety requirements, and on-site operations have become a combination of on-site staff working together with computer operators who manage everything via the DCS, resulting in an awkward situation. First, draw the motor used for stirring in the reactor: currently, there are start and stop buttons for the motor on-site at the reactor, as well as start/stop buttons, current display, and information on the motor’s operation on the DCS system in the control room. There is also a red emergency stop button next to the DCS computer in the control room. Question: 1. How is the emergency stop button symbol represented? 2. How to draw the interlock between the motor current and the stop button? Should the diameters be connected with dashed lines?
To draw this, how many seedlings need to be dug out~~~?
I’m a beginner and not sure if this is the correct way to draw; I hope experts can give me some guidance.
The process mainly involves marking the device identifiers; as for how to achieve control, this usually requires the work of the instrumentation and control team
To be honest, this isn’t quite right… The local start/stop function should have the same designation as the DCS start/stop function; it’s incorrect to use different naming conventions. You can call it HS-1001A for local control, and HS-1001B for DCS control – both functions are used to start and stop this motor. Furthermore, a legend can be designed for these control buttons; whatever name is chosen is fine – HR (HAND RUN), HB (HAND BUTTON), HS, etc. – as long as the legend provides clear explanations. Personally, I suggest that for the process aspect, a simple description of the functions and control strategies that need to be implemented using PID should be provided, leaving the rest to professionals in automatic control instrumentation. I’ll handle all the work related to PID instruments now; for the process-related aspects, I just need to draw a circle and write PG or indicate the local pressure… I’ll take care of the rest. It’s recommended to refer to the national standard HG/T 20505-2014, which specifies the functional symbols and graphical indications for process measurement and control instruments. Let professionals handle professional tasks – you need to clearly specify whether it’s connected to a SIS or DCS system, and whether the valves are of the FC or FO type ； Or how to control it, through interlocks, or whether there is any compensation; after preparing a control description, just hand it over to the instrumentation team to handle.
Thank you for the explanation from the person above; I also think that it’s pointless for the process engineers to do this – if they can’t even manage the standard process flows, then doing this is purely a waste of time.
This is not something that process engineers should do; it requires expertise in automatic control to handle it. You just need the operation instructions, the chain diagram, and the chain values. What you’ve drawn is neither a block diagram nor a PID diagram.
1. The on-site power-on and power-off operations do not need to be illustrated in the PID. 2. The emergency stop button should be placed within the PID logic block, rather than next to the motor
The on-site start/stop switch shown in the diagram above does not need to be displayed in the PID diagram; control signals, current signals, and operation status signals (these signals are represented by solid thin lines), along with interlock emergency stop signals (control and interlock signals are represented by dashed lines). Check the naming rules. Additionally, PID cannot fully display all aspects of control; accompanying documents include automatic control I/O tables, interlock logic diagrams, and electrical control schematics.