Basic knowledge of instruments

Excerpts from the content are as follows: Basics of Chemical Process Instruments (I), Basic Concepts and Unit Conversions 1. What is soft water? That is, softened water. It refers to water from which Ca2+ and Mg2+ ions have been removed, with the required water hardness being ≤0.02 mmol/t. 2. What is desalinated water? Desalinated water refers to water from which anions and cations have been removed using ion exchange. The indicators are: conductivity ≤ 10 s/cm, SiO2 ≤ 100 g/t. 3. What is vaporization? The process by which a substance changes from a liquid state to a gaseous state is called vaporization. 4. What is throttling? The phenomenon in which, as a fluid flows through a pipe, its flow velocity increases suddenly and pressure decreases due to a sudden reduction in the cross-sectional area of the channel is called throttling. 5. What is stress? Pressure is the force acting perpendicularly on a unit area. 6. What is gauge pressure? Absolute pressure? Vacuum level? What is the relationship among the three? The pressure in absolute vacuum is called absolute volume pressure, while the pressure measured with respect to absolute zero pressure is called absolute pressure. The reading on a pressure gauge used to measure fluid pressure is called gauge pressure, which is the difference between the absolute pressure of the fluid and the atmospheric pressure at that location. If the absolute pressure of the fluid being measured is lower than atmospheric pressure, the pressure indicated by the gauge will be negative, and this value is known as the degree of vacuum. Gauge pressure = Absolute pressure – Atmospheric pressure. Vacuum degree = Atmospheric pressure – Absolute pressure. 1 Pa = 1 N/m2; 1 MPa = 106 Pa; 1 mmHg = 133 Pa; 1 mmH2O = 9.81 Pa; 1 kgf/cm2 = 9.8066×104 Pa; 1 atm = 1.013×105 Pa; 1 psi = 6.9 kPa; 1 bar = 105 N/m2. There are three commonly used temperature scales: the Celsius scale (°C), the Fahrenheit scale (°F), and the Kelvin scale (K). The relationship among the three is: ℃=5/9(?F-32) ； ℃=K-273.15 (II) Pressure measuring instruments 1. How to select a pressure gauge? The selection of pressure gauges should be based on the requirements of the production process, with careful consideration given to the type, model, range, and accuracy class of the pressure gauges. The following aspects are mainly considered when making a choice: (1) Different treatments are applied depending on whether the pressure of the medium being tested is stable. When measuring the stable pressure, the maximum pressure value of the medium should not exceed 2/3 of the full scale of the pressure gauge. To ensure the measurement accuracy of the pressure gauge, it is advisable that the pressure value of the medium being measured be no less than 1/3 of the full scale range of the pressure gauge. (2) Determine the accuracy of the pressure gauge based on the maximum measurement error permitted by the production process specifications. Without compromising production requirements, do not choose instruments with excessive precision. (3) When making a selection, it is necessary to consider the properties of the medium being measured, and choose either general-purpose pressure gauges or specialized pressure gauges accordingly. 2. How to check for grease in the oxygen pressure gauge? ------ (VI) Automatic control instruments and valves 1. What is an automatic control system? What is its composition? A system designed to automatically adjust certain key parameters in production, so that when they deviate from their normal values due to external disturbances, they can be automatically brought back within the specified range; such a system is known as an automatic control system. Its components are: the process variable, the measuring element, the transmitter, the automatic regulator, and the actuator. 2. What is the regulated object? Given value and deviation? In an automatic control system, the production equipment whose process parameters need to be adjusted is called the controlled object. The process parameters that need to be maintained during production are referred to as setpoints. In such systems, it is common practice to use the difference between the setpoint and the measured value as the deviation; when the setpoint is greater than the measured value, it is considered a positive deviation, while when the setpoint is less than the measured value, it is known as a negative deviation. 3. What is a control loop? A circuit composed of the measuring elements, controller, and actuator in the measurement process is called a control loop. 4. How is a simple control loop structured? It generally consists of a sensing transmitter, a process variable, a controller, and an actuator. 5. What are the characteristics of a cascade control loop? A cascade control system belongs to a fixed-value control system; the output of the main regulator serves as the setpoint for the secondary regulator, and the output of the secondary regulator is sent to the control valve. The main regulator performs setpoint control, while the secondary regulator carries out follow-up control. Compared to a single-loop control system, a cascade control loop has the following characteristics: (1) It has a relatively rapid and strong ability to overcome disturbances that enter the secondary loop; the introduction of this secondary loop **improves the quality of control. ⑵ It can improve object characteristics. ⑶ It can eliminate the influence of the nonlinear characteristics of the control valve. ⑷ It can take into account both primary and secondary variables. 6. What is automatic control? Automatic control refers to the process of controlling a manufacturing process without any human intervention. On-site measuring instruments such as temperature meters, KTD/TC devices, and transmitters are used to measure process variables like pressure, temperature, and flow rate. These measurements are then converted into standard signals that are sent to actuators, thereby controlling the process variables so that they reach the set values, until any deviations are eliminated. 7. What are some common terms used in automatic control systems? Controlled object: The device, machine, or production process that needs to be controlled is referred to as the controlled object. Controlled variable: A physical quantity within the controlled system that is required to maintain a set value (either close to a constant value or changing according to a predetermined pattern) is called the controlled variable. Operating variable: A physical quantity or energy that is transmitted to the control device to be manipulated in order to maintain the controlled variable at a set value. 8. What are the two types of automatic control systems? There are two types: open-loop control systems and closed-loop control systems. 9. What are the components of a closed-loop control circuit? It consists of several components: detection, transmission, regulation, and actuation. 10. What are the types of closed-loop control systems? ⑴ Fixed-value control system ； ⑵ Servo control system ； ⑶ Program (sequential) control system. 11. What are the types of main control systems? The main types include cascade control systems, proportional control systems, ratio control systems, and split-range control systems. —— 19. What is the OP value? OP stands for output value, the signal that the controller sends to the actuator. 20. What is a PV value? PV refers to the measured value, which is the numerical value of the process variable as detected by the measuring element. 21. How to properly put a cascade control system into operation? First, use manual control to bring the main parameter close to or equal to the set value while keeping the secondary parameter stable. Then switch the secondary regulator from manual to automatic mode. Once the secondary loop is in automatic mode, adjust the main regulator manually to bring the main parameter close to or equal to the set value. After the adjustment of the secondary loop becomes stable, the main regulator can also be switched to automatic mode. During operation, it is necessary to activate the secondary loop first and then the main loop, ensuring a seamless transition with no disturbances. 22. How to standardize the manual-to-automatic switching process? The switch-over should be carried out without any disturbances. Before switching to automatic control, manual operation is required to keep the controlled variable stable at the set value. Only after the operating conditions have been stable for a certain period of time should automatic operation be engaged. During the switch-over process, it is necessary to maintain a constant pressure on the manual control valve, meaning that the opening degree must remain unchanged; otherwise, artificial disturbances will occur. 23. Based on the energy source used by the instrument, into which two main categories can instruments be divided? It can be divided into: (1) Direct-acting regulators (self-acting regulators) ； ⑵ Indirect action regulators are classified into electric regulators, pneumatic regulators, and level regulators, depending on the external power source used. 24. What are the two components of a pneumatic diaphragm control valve? A pneumatic diaphragm control valve consists of two parts: the actuator (diaphragm head) and the control mechanism (valve body). The actuator is used to generate power, while the control mechanism is used to regulate the flow rate of the medium.

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