1. Instrument Classification
The most common classification for measurement and process control instruments is based on their roles within measurement and control systems. Instruments are generally grouped into detectors (sensors), display instruments, regulators (controllers), and actuators.
Detection instruments can be further divided according to the measured variable. Based on the five major process variables in chemical production, these include temperature sensors, flow sensors, pressure sensors, level sensors, and composition analyzers.
2. Measurement Errors
Errors can be expressed as absolute error, relative error, or reference error.
1. Absolute error = measured value ? true value. The true value is generally not available and is often replaced by an assigned true value (the average of many measurements) or a relative true value (the reading of a higher-grade standard or instrument).
2. Relative error = (absolute error / instrument reading) × 100%.
3. Reference error = (absolute error / instrument span) × 100%, where instrument span = upper scale limit ? lower scale limit.
3. Instrument Function Letter Codes (excerpt from HG/T 20505-2000)
The instrument tag consists of letter codes indicating function combined with a loop number. The first letter indicates the measured variable; following letters indicate instrument function. Loop numbers can be assigned by unit or area and are usually three to five digits. Examples: EDIT-1230A, FCV-15106.
4. Pressure Concepts
Atmospheric pressure is the pressure produced by the weight of the atmosphere. Absolute pressure is the pressure relative to zero. Gauge pressure is the pressure relative to ambient atmospheric pressure and is commonly referred to simply as pressure. Vacuum refers to gauge pressure below atmospheric pressure.
5. Common Units for Chemical Instrumentation
1. Flow: kg/h, m3/h, Nm3/h, etc. Mass flow M = Q × ρ.
2. Level: m, %.
3. Pressure: MPa, bar, psi (gauge), kgf/cm2, atm, mmHg, etc.
4. Temperature: K, °C, °F.
5. Analytical instruments: S/m (conductivity, 1/Ω·m), mV (ORP), NTU (turbidity), ppm, %.
Work should use the legally mandated measurement units (the International System of Units, SI, and its derived units). Engineering units and other non?standard units should not be used. However, many imported instruments and legacy equipment may still use engineering units or non?standard units; therefore, be familiar with their conversion to China’s legal measurement units.
Common conversions:
1 bar = 10^5 Pa. 1 atm (physical atmosphere) = 760 mmHg = 0.101325 MPa.
1 kgf/cm2 = 0.098 MPa. 1 mmH2O = 9.8 Pa.
1 psi = 6.895 kPa. psi stands for pounds per square inch.
6. Automatic Control Systems
1. Control system model
2. Common symbols in automatic control systems:
SP (Setpoint) set value, PV (Process Value) measured value, MV (Manipulated Value) controller output. Direct action and reverse action. A/M (Auto/Manual) auto/manual switch.
P (proportional): output is proportional to the magnitude of the error; tuning is responsive but may leave a steady-state offset. I (integral time): output is proportional to the time integral of the error and can eliminate steady-state offset. D (derivative time): output is proportional to the rate of change of the error and provides predictive action. Smaller I and larger D increase control aggressiveness, but exceeding critical values can cause system oscillation.
7. Signal Alarms and Interlock Systems
1. Signal alarms. When process parameters exceed limits or operating conditions become abnormal, alarms (visual or audible) notify operators to take corrective action. Alarm messages are typically displayed in red for critical alarms and yellow for warnings; acknowledgment stops flashing. After the fault is cleared, the alarm returns to normal and displays green.
2. Interlock protection. Interlocks are automatic control arrangements that ensure equipment performs tasks according to prescribed conditions or procedures to eliminate abnormalities and prevent accidents.
8. Lubrication and Corrosion Protection for Instrumentation
Instruments installed outdoors are susceptible to atmospheric corrosion and process media attack. In addition to routine inspections to detect leaks and other safety hazards, apply lubrication and corrosion protection measures where appropriate. Examples include applying grease to bolts, nuts, and fittings, and coating enclosures with anti?corrosion paint to extend instrument service life.
