When the cryogenic method is used for air separation, the whole process is carried out in closed containers and pipelines from the air compressor to the final gas and liquid products such as oxygen, nitrogen, argon and so on. During the start-up, shutdown and normal operation of the equipment, the values of relevant parameters must be strictly controlled in each process, including pressure, temperature, flow, resistance, liquid level, rotate speed, voltage, current, component analysis, etc. The following introduces some instrument control components for measuring temperature, pressure and flow for reference.
1.Temperature measurement:
In the temperature measurement of air separation plant, the main measuring elements are platinum thermistor thermometer, temperature transmitter and bimetal thermometer. Platinum thermistor thermometer is widely used in air separation plant due to its high measurement accuracy at low temperature (13.81K) and no temperature compensation at cold end. Temperature transmitter converts platinum resistance and thermocouple signal into standard 4 ~ 20mA signal and outputs it, which is divided into integral type and split type. Bimetal thermometer has simple structure, good precision and small lag It has the advantages of low cost and not affected by ambient temperature and atmospheric pressure. It is mainly used for local temperature indication in air separation.
2.Pressure measurement:
In the pressure measurement and control of air separation plant, the main measuring elements are pressure transmitter, spring tube pressure gauge and pressure controller. The process pressure of the measured point is detected by the pressure transmitter through a certain detection method, and the signal is transmitted to 4 ~ 20mA. Generally, it can be indicated by the on-site meter, which is convenient for maintenance and debugging. The pressure measuring points that need to be displayed and controlled by DCS need to be measured by pressure transmitter; the spring tube pressure gauge is mainly used for local pressure indication in the air separation plant because of its simple structure, good accuracy, low cost and not affected by the ambient temperature and atmospheric pressure; the pressure controller, also known as the pressure switch, is mainly used for the alarm and interlock of process pressure.
3.Flow measurement:
In the flow measurement of air separation plant, orifice plate and vortex street are mainly used. Orifice plate throttling device is a flow measurement device without scale, which is used to measure the flow rate of liquid, gas and steam with medium temperature ≤ 400 ℃. The pressure difference generated by the orifice plate is converted into a proportional output signal by the transmitter, and then the measured flow is recorded, indicated or regulated by the secondary instrument or regulator; the thermosensitive vortex flowmeter is used to measure the clean, non corrosive liquid and gas medium with the temperature of - 196 to + 400 ℃. It is suitable for pipes with nominal diameter of DN25-DN200. Air separation is often used to measure the flow of temperature liquid, its output 4 ~ 20mA DC signal into PLC display; electromagnetic flowmeter, air separation for water measurement at room temperature, select electromagnetic flowmeter, its output 4 ~ 20mA DC signal into PLC display.
