1Overview of Control System Design Chapter 131. Safety.It is imperative that industrial plants operate safely so as to promote the well-being of people and equipment within the plant and in the nearby communities. Thus, plant safety is always the most important control objective and is the subject of Section 1.!.2. Environmental Regulations.Industrial plants must comply with environmental regulations concerning the discharge of gases, liquids, and solids beyond the plant boundaries.3. Product Specifications and Production Rate.In order to be profitable, a plant must ma"e products that meet specifications concerning product quality and production rate.General Requirements#Chapter 134. Economic Plant Operation.It is an economic reality that the plant operation over long periods of time must be profitable. Thus, the control objectives must be consistent with the economic objectives. 5. Stable Plant Operation. The control system should facilitate smooth, stable plant operation without e$cessive oscillation in "ey process variables. Thus, it is desirable to have smooth, rapid set-point changes and rapid recovery from plant disturbances such as changes in feed composition.%Chapter 13Steps in Control System Design&fter the control objectives have been formulated, the control system can be designed. The design procedure consists of three main steps'1. Select controlled, manipulated, and measured variables.#. (hoose the control strategy )multiloop control vs. multivariable control* and the control structure )e.g., pairing of controlled and manipulated variables*.%. Specify controller settings.+Control Strategies,Multiloop Control:-ach output variable is controlled using a single input variable.,Multivariable Control:-ach output variable is controlled using more than one input variable.Chapter 13!Chapter 13, The degrees of freedom ! is the number or process variables that must be specified in order to be able to determine the remaining process variables. , If a dynamic model of the process is available, ! can be determined from a relation that was introduced in (hapter #,)1%-1*! " E = where " is the total number of process variables, and E is the number of independent equations.13.1Degrees of ree!om for "ro#ess Control,The important concept of degrees of freedom was introduced in Section #.%, in connection with process modeling..Chapter 13/or process control applications, it is very important to determine the ma$imum number of process variables that can be independently controlled, that is, to determine the control degrees of freedom, !#', In order to ma"e a clear distinction between ! and !#, we will refer to ! as the model degrees of freedom and !# as the control degrees of freedom. , 0ote that ! and !# are related by the following equation,Definition. The control degrees of freedom, !#, is the number of process variables )e.g., temperatures, levels, flow rates, compositions* that can be independently controlled.)1%-#*! !# $ = +where $ is the number of disturbance variables )i.e., input variables that cannot be manipulated.*1Chapter 13Example 13.1General Rule. /or many practical control problems, the control degrees of freedom !# is equal to the number of independent material and energy streams that can be manipulated.2etermine ! and !# for the steam-heated, stirred-tan" system modeled by -qs. #-! 3 #-!# in (hapter #. &ssume that only the steam pressure Ps can be manipulated.SolutionIn order to calculate ! from -q. 1%-1, we need to determine " and E. The dynamic model in -qs. 2-50 2-52 contains three equations )E 4 %* and si$ process variables )" 4 .*' %s, Ps, &, %i, %, and %&. Thus, ! 4 . 3 % 4 %. 5Chapter 13/igure 1%.1 Two e$amples where all three process streams cannot be manipulated independently.6Stirre!$%an& 'eating "ro#ess/igure #.% Stirred-tan" heating process with constant holdup, ".Chapter 131Chapter 13, If the feed temperature %i and mass flow rate & are considered to be disturbance variables, $ 4 # and thus !# 4 1 from -q. )1%-#*. ,It would be reasonable to use this single degree of freedom to control temperature % by manipulating steam pressure, Ps.The blending system in /ig. 1%.% has a bypass stream that allows a fraction f of inlet stream to bypass the stirred tan". It is proposed that product composition ' be controlled by adjusting f via the control valve. &naly7e the feasibility of this control scheme by considering its steady-state and dynamic characteristics. In your analysis, assume that '1 is the principal disturbance and that '#, &1, and are constant. 8ariations in the volume of liquid in the tan" can be neglected because 99 &1. Example 13.211Chapter 13/igure 1%.%. :lending system with bypass line.1#Chapter 13Solution,The dynamic characteristics of the proposed control scheme are quite favorable because the product composition ' responds rapidly to a change in the bypass flow rate. ,In order to evaluate the steady-state characteristics, consider a component balance over the entire system'Solving for the controlled variable gives,1 1 # #)1%-%* & ' & ' &' + =1 1 # #)1%-+*& ' & ''&+=,Thus depends on the value of the disturbance variable and four constants )&1, , '#, and &*. ,:ut it does not depend on the bypass function, f. '1'1%Chapter 13,Thus, it is not possible to compensate for sustained disturbances in '1 by adjusting f. ,/or this reason, the proposed control scheme is not feasible.,:ecause f does not appear in )1%-+*, the steady-state gain between ' and f is 7ero. Thus, although the bypass flow rate can be adjusted, it does not provide a control degree of freedom. , ;owever, if could also be adjusted, then manipulating both f and could produce e$cellent control of the product composition.1+Chapter 13(ffe#t of ee!ba#& Control,0e$t we consider the effect of feedbac" control on the control degrees of freedom. ,In general, adding a feedbac" controller )e.g.,