Basic knowledge of instrumentation equipment
2024 07/30
First, the classification of instruments
The most common classification of detection and process control instruments is divided according to the role of the instrument in the measurement and control system, which is generally divided into four categories: detection instruments, display instruments, adjustment (control) instruments and actuators.
Among them, the detection instrument can be divided into temperature detection instrument, flow detection instrument, pressure detection instrument, level detection instrument and component analysis instrument according to the five variables of chemical production.
2. Measurement error
Error can be divided into absolute error, relative error and reference error.
1, absolute error = measured value - true value. Where the truth value is generally not available, it is usually replaced by the conventional truth value (the average of enough measurements) or the relative truth value (the measurement value of a higher level standard or instrument).
2, relative error = (absolute error/instrument value) ×100%.
3, reference error = (absolute error/instrument range) ×100%, instrument range = upper limit scale value - lower limit scale value.
3, instrument function letter code (excerpted from HG/T 20505-2000)
The instrument position number consists of two parts: the letter code combination and the loop number. The first letter indicates the variable to be measured, and the following letters indicate the function of the instrument; The loop number can be prepared by device or section (area), and is generally represented by three to five digits. Such as EDIT-1230A, FCV-15106 and so on.
Fourth, the concept of pressure
Atmospheric pressure is the pressure generated by the weight of the atmosphere, referred to as air pressure. Absolute pressure is the differential pressure with zero as the reference pressure. Gauge pressure, referred to as gauge pressure, is the difference pressure of ambient atmospheric pressure as a reference pressure, and is also known as pressure. Vacuum is the gauge pressure where the absolute pressure is lower than the atmospheric pressure.
Five, commonly used chemical instrument measurement units
1, Flow: kg/h, m3/h, Nm3/h, M=Q×ρ
2. Level: m, %, etc
3, Pressure:, MPa, bar, PSI (G gauge Pressure), kgf/cm2, atm, mmHg, etc
4, Temperature: K, C, F
5, analysis instrument (Analyse) : S/m (conductivity, 1/Ω·m), mV (ORP), NTU (turbidity), ppm, %, etc
The legal units of measurement stipulated by the state (the basic unit of the International System of Units SI and its auxiliary units, derived units) should be used in the work, and engineering units and other non-legal units of measurement should not be used. Because many imported instruments and historical reasons may still use engineering units, non-legal units of measurement, it is necessary to understand the conversion relationship between them and China's legal units of measurement.
1bar = 105Pa, 1atm (physical atmospheric pressure) = 760mmHg = 0.101325MPa,
1kgf/cm2 (engineering atmospheric pressure) = 0.098MPa, 1mmH2O = 9.8Pa,
1PSI = 6.895kPa, PSI refers to pounds per square inch.
Six, automatic control system
1. Automatic control model
2. Common symbols in automatic control systems
SP (Setpoint) set Value, PV (Process Value) measured value, MV (Manipulate Value) output control value, Direct action positive action, Reverse action negative action, A/M (Auto/Manual) Manual/Automatic switchover.
P ratio, the output is proportional to the size of the deviation, the adjustment is timely and powerful, but the surplus is bad; I integration time, the output is proportional to the deviation of the integration of time, can eliminate the residual difference; D differential time, the output is proportional to the speed of deviation change, has the role of advance adjustment. The smaller the PI and the larger the D, the stronger the adjustment effect, but it is easy to cause system oscillation when the critical value is exceeded.
Seven, signal alarm and interlock system
1, signal alarm. When the process parameters exceed the limit or the operating status is abnormal, an alarm is issued in the form of light or sound to remind the operator to pay attention and deal with it in time. When the alarm occurs, the alarm information flashes in red (serious alarm) or yellow font. After confirmation, the alarm information stops blinking. When the alarm returns to normal state after the fault is removed, the alarm information is displayed in green.
2, interlock protection. It is an automatic control system, which can make the relevant equipment complete the operation task in accordance with the specified conditions or procedures, and achieve the purpose of eliminating anomalies and preventing accidents.
8. Lubrication and anti-corrosion of instrument equipment
When the instrument equipment is installed in the open place, it is easy to be eroded by the atmospheric environment and