May I ask, what is the purpose of the [instrumentation circuit diagram] during the implementation of a chemical engineering project?

Instrument loops are primarily used to achieve process control, and all of these functions are carried out by the DCS

Most modern chemical plant installations are turnkey projects; the manufacturer of such plant equipment needs to collaborate with many other related companies in order to bring them to completion. The diagrams for this section are intended to be included in the handover documents, for use in future modifications and optimizations of the procedure. Additionally, some also need to incorporate the conditions proposed by the using units.

Typically, the design firm provides the user with a instrument circuit diagram. In this diagram, there are clear indications of all the instruments, equipment, and components used; details such as the installation location of the instruments and equipment, their identification numbers, model numbers, signal ranges, wiring boxes, terminals, the wires connecting them together, the computer’s I/O connections, the grounding system, power supply, air supply, pneumatic pipelines, as well as the positive and negative actions of regulators and control valves, are all described in detail through written explanations or markings. It is convenient to use instrument circuit diagrams to check for faults in instruments or control systems. The single-line diagrams of instrument circuit diagrams are mostly the schematic diagrams of the instrument circuits, while the double-line diagrams are similar to the wiring diagrams of the instruments. 1. Single-wire instrument circuit diagram: http://yunrun.com.cn/upload/201806/20/201806201750576564.png Figure 1 shows the pressure detection and alarm system. To facilitate fault diagnosis, this system is represented as a schematic diagram of its pressure detection and alarm circuit, as shown in Figure 2. This is a single-line diagram, from which it is possible to clearly see the composition of the pressure detection system, the signal transmission relationships, and the flow of signals. The measured pressure is converted into a 4-20mA current signal by the pressure transmitter PT108, and this signal is sent to the input card of the DCS via a safety barrier. It is then displayed on the DCS. When the pressure falls below the specified value, the IL device activates; the alarm signal, in turn, causes the external relay K to operate, which triggers the red HD indicator light on the control panel to illuminate as a warning. When troubleshooting system failures, by examining or measuring the relevant signal circuits based on the symptoms of the failure, it is possible to identify the fault location. http://yunrun.com.cn/upload/201806/20/201806201752119887.png Figure 2: Schematic diagram of the pressure monitoring and alarm circuit. 2. Schematic diagram of the two-wire instrument circuit. Figure 3 shows the schematic diagram of a compressor anti-surge control system. The working process is as follows: The gas flow entering the compressor is measured through an orifice plate, and the flow rate signal is sent to the FYIC regulator via an FT differential pressure transmitter. After performing a series of calculations on the input signal, the FYIC regulator compares the results with a set value; when the result is greater than the set value, the system is operating normally ； When the value is less than or equal to the given threshold, the FYIC regulator opens the bypass control valve FV, allowing a portion of the gas to return to the compressor inlet and thereby increasing the compressor’s flow rate to prevent surge. http://yunrun.com.cn/upload/201806/20/201806201753427749.png Figure 3: Schematic diagram of the compressor anti-surge control system; Instrument circuit diagram: yunrun.com.cn/tech/1997.html Figure 4: Two-wire instrument circuit diagram of the compressor anti-surge control system. The two-wire instrument circuit diagram of this control system is shown in Figure 4. In the figure, the flow rate signal and pressure difference signal from the field are converted from 4-20mA to 1-5V signals via XP isolation terminals, and these signals are sent to terminals 1, 2, 3, and 4 of the FYIC regulator respectively. The FYIC regulator’s 4-20mA output signal is sent to the FY electrical converter via terminals 5 and 6. When diagnosing faults, it is possible to visually check the signals of each circuit by modifying the diagram. Inspection can be carried out using the terminals behind the dashboard as a dividing point; the signals coming from the field and the output signals from the control room instruments are checked separately, and the current or voltage of each signal circuit is measured to determine whether they are operating normally. This circuit has been simplified; the power supply and grounding circuits are not shown, but they need to be taken into consideration or checked when analyzing and diagnosing faults.

Bro, from my perspective as the person carrying out the construction work, the circuit diagrams serve two main purposes for me: one is to conduct simulation experiments for budgeting, and to determine the numbers involved – specifically, the number of temperature circuits, pressure difference circuits, simple circuits, and complex circuits. Another thing is that the delivery documents must include a table for the instrument circuit test, which requires circuit numbers; if your design office doesn’t provide this diagram, I’ll have no choice but to make things up randomly: lol