ERT 313 BIOSEPARATION ENGINEERING LEACHING Prepared by: Miss Hairul Nazirah Abdul Halim.
ERT 210/4 Process Control CHAPTER 9 Control System Instrumentation Hairul Nazirah bt Abdul Halim...
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Transcript of ERT 210/4 Process Control CHAPTER 9 Control System Instrumentation Hairul Nazirah bt Abdul Halim...
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ERT 210/4Process Control
CHAPTER 9Control System Instrumentation
Hairul Nazirah bt Abdul HalimEmail: [email protected]
Office: 04-9798840
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Control System Instrumentation
Figure 9.3 A typical process transducer.
Transducers and Transmitters
• transducer - typically consists of a sensing element combined with a driving element (transmitter).
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• Transducers for process measurements convert the magnitude of a process variable (e.g., flow rate, pressure, temperature, level, or concentration) into a signal that can be sent directly to the controller.
• The sensing element is required to convert the measured quantity, that is, the process variable, into some quantity more appropriate for mechanical or electrical processing within the transducer.
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Standard Instrumentation Signal Levels
• Before 1960, instrumentation in the process industries utilized pneumatic (air pressure) signals to transmit measurement and control information almost exclusively.
• These devices make use of mechanical force-balance elements to generate signals in the range of 3 to 15 psig, an industry standard.
•Since about 1960, electronic instrumentation has come into widespread use.
•Signal range: 1-5 mA, 4-20 mA, 10-50mA, 0-50V.•Most industrial analog instrumentation use 4-20
mA
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Sensors
• Main categories of measurements used in process control:
1)Temperature
2)Pressure
3)Flow rate
4)Liquid level
5)Composition
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Temperature Sensor• On-line measurement by:
– Thermocouples & resistance temperature detector (RTD) – up to 10000C
– Pyrometer and filled – above 9000C– System thermometer.
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• Differential Pressure
On-line measurement by:
- Liquid column- Elastic element: Bourdon tube- Optical fiber sensor: measure pressure in high- temperature environments (>11000C)
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Liquid or gas flow rate
• Flow rate can be measured indirectly using pressure drop across orifice or venturi
• Other measurements: – Rotameter– Turbine flowmeter– Ultrasonic– Magnetic
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Liquid level• Level can be measured using:
– Float-activated– Head devices– Electrical (Conductivity)– Radiation– Radar
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Chemical Composition• On-line measurement by:
– Gas-liquid chromatography (GLC)– Gas chromatography (GC)– Infrared (IR) spectroscopy– UV spectroscopy (UV spectrophotometer)– Magnetic resonance analysis (MRA)– Refractive index (RI)– Electrophoresis
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• Converts the sensor output to a signal level appropriate for input to a controller, such as 4 to 20 mA.
• Transmitters are generally designed to be direct acting.
• Commercial transmitters have an adjustable input range (or span).
• For example, a temperature transmitter might be adjusted so that the input range of a platinum resistance element (the sensor) is 50 to 150 °C.
Transmitters
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• In this case, the following correspondence is obtained:
Input Output
50 °C 4 mA
150 °C 20 mA
• Lower limit or zero of 50 °C
• Range or span of 100 °C
• The relation between transducer output and input is
20 mA 4 mAmA 50 C 4 mA
150 C 50 C
mA0.16 C 4 mA
C
mT T
T
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For any linear instrument, the gain (Km):
The gain of the measurement element Km is 0.16 mA/°C.
range of instrument output(9-1)
range of instrument inputmK
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Figure 9.4 A linear instrument calibration showing its zero and span.
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Final Control Elements• Final control element (actuator) is the device that
enables a process variable to be manipulated.
• There are many different ways to manipulate the flows of material and energy into and out of a process.
• Final control elements (usually control valves) adjust the flow rates of materials, and indirectly, the rates of energy transfer to and from the process.
• Variable speed air compressor – adjust air flow rates
• Variable speed pump - adjust the flow rates of material
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Control Valves
• Automatic control valve - Simple and widely used method to adjust flow rate of fluid.
• The control valve components include the valve body, trim, seat, and actuator.
• Control valves are either linear or rotary in design.
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Globe valve• Linear valve• Open and close by moving a plug vertically away
from the orifice and seat
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• Ball valve :
• Plug valve:
Rotary valve- Closed by a 900 turn of the closing element- Used for both on-off and flow modulating control
valve
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Figure 9.7 A pneumatic control valve (air-to-open).
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• Air-to-open: output signal , pressure on the diaphragm compresses the spring
Pulling the stem out open the valve further
• Air-to-close: output signal , pressure on the diaphragm compresses the spring
Valve stem downward close the valve further
Air-to-Open vs. Air-to-Close Control Valves
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• Normally, the choice of A-O or A-C valve is based on safety considerations.
• We choose the way the valve should operate (full flow or no flow) in case of a transmitter failure.
• Hence, A-C and A-O valves often are referred to as fail-open and fail-closed, respectively.
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Example 9.1
Pneumatic control valves are to be specified for the applications listed below. State whether an A-O or A-C valve should be used for the following manipulated variables and give reason(s).
a)Steam pressure in a reactor heating coil.
b)Flow of effluent from a wastewater treatment holding tank into a river.
c)Flow of cooling water to a distillation condenser.
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Valve Positioners
Pneumatic control valves can be equipped with a valve positioner, a type of mechanical or digital feedback controller that senses the actual stem position, compares it to the desired position, and adjusts the air pressure to the valve accordingly.
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Specifying and Sizing Control Valves
A design equation used for sizing control valves :
(9-2)vv
s
Pq C f
g
q = flow rate,
= flow characteristic,
= pressure drop across the valve,
gs = specific gravity of the fluid.
f
vP
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• Specification of the valve size is dependent on the so-called valve characteristic f.
• Three control valve characteristics are mainly used:- linear- quick opening- equal percentage
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Figure 9.8 Control valve characteristics.
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where R is a valve design parameter that is usually in the range of 20 to 50.
1
Linear:
Quick opening: (9-3)
Equal percentage:
f
f
f R
0 1f f •For a fixed pressure drop across the valve, the flow characteristic is related to the lift , that is, the extent of valve opening, by one of the following relations:
0 1
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• The rangeability of a control valve is defined as the ratio of maximum to minimum input signal level.
• For control valves, rangeability translates to the need to operate the valve within the range 0.05 ≤ f ≤ 0.95 or a rangeability of 0.95/0.05 = 19.
Rangeability