08_Transmitters_Chap9S10_4

08_Transmitters_Chap9S10_4 - Control System...

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Unformatted text preview: Control System Instrumentation: Week 2 Chapter 9 Chapter 9 • 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 tit th th appropriate for mechanical or electrical processing within the transducer. Figure 9.3 A typical process transducer. 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. Transducers and Transmitters • Figure 9.3 illustrates the general configuration of a measurement transducer; it typically consists of a sensing element combined with a driving element (transmitter). • Since about 1960, electronic instrumentation has come into widespread use. • In the first case, the following correspondence is obtained: Sensors The book briefly discusses commonly used sensors for the most important process variables (See text important process variables. (See text.) Input 50 °C Output 4 mA 20 mA Chapter 9 Chapter 9 150 °C Transmitters • A transmitter usually 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. • In addition, most 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 or 100 to 200 °C. • This instrument (transducer) has a lower limit or zero of 50 °C and a range or span of 100 °C. • For the temperature transmitter discussed above, the relation between transducer output function of input between transducer output = function of ( input ) is ⎛ 20 mA − 4 mA ⎞ Tm ( mA ) = ⎜ ⎟ T − 50 C + 4 mA ⎝ 150 C − 50 C ⎠ mA ⎞ ⎛ = ⎜ 0.16 ⎟ T C + 4 mA C⎠ ⎝ ( ) () 1 The gain of the measurement element Km is 0.16 mA/°C. For any linear instrument: Km = range of instrument output (signal) range of instrument input (physical variable) (9-1) Chapter 9 Chapter 9 Figure 9.4 A linear instrument calibration showing its zero and span. Final Control Elements • Every process control loop contains a final control element (actuator), the device that enables a process variable to be manipulated. most chemical and petroleum processes, the final control • For most chemical and petroleum processes, the final control elements (usually control valves) adjust the flow rates of materials, and indirectly, the rates of energy transfer to and from the process. Control Valves • There are many different ways to manipulate the flows of material and energy into and out of a process; for example, the speed of a pump drive, screw conveyer, or blower can be adjusted. Chapter 9 • However, a simple and widely used method of accomplishing this result with fluids is to use a control valve, also called an automatic control valve. • The control valve components include the valve body, trim, seat, and actuator. Chapter 9 Figure 9.7 A pneumatic control valve (Air-to-Open). If Actuating signal = “Air” pressure , Diaphragm , Valve stem , Valve plug and away from Valve seat = Open Air-to-Open vs. Air-to-Close Control Valves • Normally, the choice of A-O (air-to-open) or A-C (air-toclose) valve is based on safety considerations. 2 • We choose the way the valve should operate (full flow or no flow) in case of a transmitter failure. ATC = “air to close” • Hence, A-C (ATC) valves are referrd to as fail-open and A-O (ATO) valves are referred to as fail-close. • We choose the way the valve should operate (full flow or no flow) in case of a transmitter failure. ATC = “air to close” • Hence, A-C (ATC) valves are referrd to as fail-open and A-O (ATO) valves are referred to as fail-close. Example 9.1 – pg. 216 SEM2 216 SEM2 Example 9.1 – pg. 216 SEM2 216 SEM2 Chapter 9 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) b) Flow rate of reactants into a polymerization reactor. c) Flow of effluent from a wastewater treatment holding tank into a river. a) Flow of cooling water to a distillation condenser. Chapter 9 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. (A-O) b) Flow rate of reactants into a polymerization reactor. (depends, also exothermic must be considered in real reactors !) c) Flow of effluent from a wastewater treatment holding tank into a river. (A-O) d) Flow of cooling water to a distillation condenser. (A-C) 3 ...
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This note was uploaded on 04/17/2010 for the course CHE 461 taught by Professor Staff during the Winter '08 term at Oregon State.

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