(a) Classification of Instruments
The most common classification of detection and process control instruments is based on their role in measurement and control systems, generally divided into four categories: detection instruments, display instruments, regulating (control) instruments, and actuators.
Among them, the detection instruments can be divided into temperature detection instruments, flow detection instruments, pressure detection instruments, level detection instruments, and composition analysis instruments according to the five major variables of chemical production, based on the different variables they measure.
(ii) Measurement error
Based on the method of expressing error numerically, errors can be divided into absolute error, relative error, and reference error.
1. Absolute error = Measured value - True value. The true value is generally unavailable and is usually replaced by a conventional true value (the average of a sufficient number of measurements) or a relative true value (the measurement reading of a higher-level standard instrument).
2. Relative error = (absolute error / instrument reading) × 100%.
3. Reference error = (absolute error / instrument range) × 100%, instrument range = upper limit scale value - lower limit scale value.
(III) Instrument function letter codes
The instrument tag number consists of two parts: a combination of letters and a loop number. The first letter indicates the measured variable, and subsequent letters indicate the instrument's function. The loop number can be assigned by unit or section (area) and is generally represented by three to five digits. Examples include EDIT-1230A and FCV-15106.
(iv) The concept of pressure
Atmospheric pressure is the pressure exerted by the weight of the atmosphere itself, often simply called air pressure. Absolute pressure is the differential pressure relative to zero. Gauge pressure is the differential pressure relative to ambient atmospheric pressure, also commonly referred to as pressure. Vacuum is the gauge pressure at which the absolute pressure is lower than atmospheric pressure.
(V) Commonly Used Chemical Instrument Units
1
Flow rate: kg/h, m³/h, Nm³/h, etc., M=Q×ρ
2
Level: m, % etc.
3
Pressure: MPa, bar, PSI (G pressure), kgf/cm2, atm, mmHg, etc.
9
Temperature: K, °C, F
5
Analytical instruments (Analyse): S/m (conductivity, 1/Ω·m), mV (ORP), NTU (turbidity), ppm, % etc.
Remark:
In work, the legally mandated units of measurement (SI base units and their auxiliary and derived units) should be used, 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 or non-legal units of measurement, it is important to understand their conversion relationships with my country's legal units of measurement.
1 bar = 10⁵ Pa, 1 atm (physical atmospheric pressure) = 760 mmHg = 0.101325 MPa.
1 kgf/cm² (engineering atmosphere) = 0.098 MPa, 1 mmH₂O = 9.8 Pa.
1 PSI = 6.895 kPa. PSI refers to pounds per square inch.
(vi) Automatic control system
Common symbols in automatic control systems
SP (Setpoint), PV (Process Value), MV (Manipulate Value), Direct Action, Reverse Action, A/M (Auto/Manual) manual/automatic switching.
P (proportional) output is proportional to the magnitude of the deviation, providing timely and effective regulation, but with residual error. I (integral time) output is proportional to the integral of the deviation over time, eliminating residual error. D (derivative time) output is proportional to the rate of change of the deviation, providing anticipatory regulation. The smaller PI and the larger D, the stronger the regulation, but exceeding critical values can easily cause system oscillations.
(vii) Signal alarm and interlocking system
1
Signal alarm
When process parameters exceed limits or the operating status is abnormal, an alarm will be issued via light or sound to alert operators and prompt them to take timely action. When an alarm occurs, the alarm message will flash in red (critical alarm) or yellow. Once confirmed, the alarm message will stop flashing. After the fault is resolved, the alarm will return to normal, and the alarm message will display green.
2
Interlocking protection
It is an automatic control system that enables relevant equipment to complete operational tasks according to specified conditions or procedures, thereby eliminating abnormalities and preventing accidents.
(viii) Lubrication and corrosion protection of instruments and equipment
When instruments and equipment are installed in open-air locations, they are easily corroded by the atmospheric environment and process media. In addition to carrying out daily inspections and promptly identifying potential safety hazards such as leaks, drips, and spills, further lubrication and anti-corrosion measures should be taken, such as applying grease (commonly known as butter) to connecting bolts, nuts, and connectors, and applying anti-corrosion paint to the outer casing to extend the instrument's lifespan.
