Instrumentation for Process Measurement and Control, Third Editon

Norman A. Anderson

• Editorial: Routledge
• Año: 2017
• ISBN:
  9781351438100
• Edición: 3
• Categoría: Physical Sciences
• Categoría: Industrial & Technical Chemistry

The perennially bestselling third edition of Norman A. Anderson's Instrumentation for Process Measurement and Control provides an outstanding and practical reference for both students and practitioners. It introduces the fields of process measurement and feedback control and bridges the gap between basic technology and more sophisticated systems. Keeping mathematics to a minimum, the material meets the needs of the instrumentation engineer or technician who must learn how equipment operates. I t covers pneumatic and electronic control systems, actuators and valves, control loop adjustment, combination control systems, and process computers and simulation

SECTION I FEEDBACK PROCESS CONTROL

1

Introduction to Process Control

The technology of process instrumentation continues to grow in both application and sophistication. In 1774, James Watt employed the first control system applying feedback techniques in the form of a flyball governor to control the speed of his steam engine. Ten years later, Oliver Evans used control techniques to automate a Philadelphia flour mill.

Process instrumentation developed slowly at first because there were few process industries to be served. Such industries began to develop at the turn of the twentieth century, and the process instrumentation industry grew with them. However, only direct-connected process instruments were available until the late 1930s. In the 1940s, pneumatic transmission systems made complex networks and central control rooms possible. Electronic instrumentation became available in the 1950s, and its popularity has grown rapidly since. The most recent decade has produced digital computer techniques to improve the performance of more complex processes. However, present trends indicate that future process plants will employ combinations of analog and digital systems.

True control balances the supply of energy or material against the demands made by the process. The most basic (feedback) systems measure the controlled variable, compare the actual measurement with the desired value, and use the difference between them (error) to govern the required corrective action. More sophisticated (feedforward) systems measure energy and/or material inputs to a process to control the output. These will be discussed in Chapter 16.

Fig. 1-1. (A) The process to be controlled occurs in a heat exchanger. All elements of the pneumatic control system are shown—transmitter, controller, valve, input water, output water, and steam, (B) Block diagram of the elements listed in A.

The control loop in Figure 1-1 is shown in both actual and schematic form. The process is a shell and tube heat exchanger, and the temperature of the heated water is the controlled variable. This temperature is measured by a pneumatic temperature transmitter, which sends a pneumatic signal proportional to temperature to the pneumatic analog controller. The desired water temperature is set on the controller’s set-point dial. The controller changes a pneumatic output signal according to the difference between the existing value (temperature) and the desired value (set point). The output signal is applied to the valve operator, which positions the valve according to the control signal. The required quantity of heat (steam) is admitted to the heat exchanger, causing a dynamic balance between supply and demand.

The various control equipment components that may be used to regulate a process and certain aspects of process behavior will be discussed in this text. Examples of some completely instrumented process systems will be given to demonstrate the practical application of the instrument components.

Table 1-1. Analogy Between Characteristics of Basic Physical Systems

Variable	Electrical System	Hydraulic System	Pneumatic System	Thermal System
Quantity	Coulomb	ft^3 or m^3	Std. ft^3 or m^3	Btu or joule
Potential or effort variable	emf E (volt)	Pressure P (psi or kPa) (ft or m of head)	Pressure P (psi or kPa) (m or mm of head)	Temperature T (degrees Fahrenheit or Celsius)
Flow variable	Coulomb/s Current I (amperes)	Flow Q (ft^3/s or L/s) (gal/min)	Flow Q (ft^3/s or m^3/s) (lb/min)	Heat flow dQ/dt (Btu/s or watts)
Resistance	$\text{R}\left(\text{ohm}\right)=\frac{\text{volt}}{\text{amp}}$	psi/(ft^3/s) ft head/ft^3/s) sec/ft^2	psi/(ft^3/s)	deg/(Btu/s) deg/watt
Capacitance	$\frac{\text{q}\left(\text{farad}\right)\text{coulombs}}{\text{volts}}$	ft^3/ft = ft^2	ft^2	Btu/deg
Time	Seconds	Seconds	Seconds	Seconds

Fig. 1-2. Four types of systems: (a) electric, (b) hydraulic, (c) pneumatic, and (d) thermal. Each has a single capacity and a single resistance and all have identical response characteristics.

The physical system to be controlled may be electrical, thermal, hydraulic, pneumatic, gaseous, mechanical, or any other physical type. Figure 1-2 and Table 1-1 compare several common systems. All follow the same basic laws of physics and dynamics.

The behavior of a process with respect to time defines its dynamic characteristics. Behavior not involving time defines its static characteristics. Both static (steady) and dynamic (changing with time) responses must be considered in the operation and understanding of a process control system.

Types of Processes

The simplest process contains a single capacity and a single resistance. Figure 1-2 illustrates a single-capacity, single-resistance process in (a) electrical, (b) hydraulic, (c) pneumatic, and (d) thermal forms. To show how these behave with respect to time, we can impose a step upset (sudden change) in the input to the process and examine the output. The resulting change in process variable with respect to time is plotted in Figure 1-3. The reaction curve of all four types of ...