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2.28 Selective, Override, and Limit Controls

F. G. SHINSKEY (1970) J. P. SHUNTA (1985, 1995) J. E. JAMISON, A. ROHR (2005)

INTRODUCTION

In this section some examples are given of control configu-rations where more than one controller can manipulate thesame process and where the selection of the active controlleris dictated by some prearranged logic.

In some control systems, more variables need to be con-trolled than there are variables that can be manipulated. Whenthis is the case, logic must be provided to decide whichcontrollers should have access to the manipulated variablesand which should be temporarily blocked. Switching control-ler outputs can be easily and smoothly accomplished by avariety of hardware and software signal selectors.

In selective control applications, the signal selectorschoose the lowest, highest, or median signal from among twoor more signals. Such selectors are available both as analoghardware or as digital software in DCS control packages.

In most applications, selective control is a form of mul-tivariable control where the selectors facilitate the onlinemodification of control strategies as a function of changingoperating conditions. The selectors allow the control strate-gies to be changed smoothly and without disturbing the pro-cess. Selective control applications include:

Protecting process equipment by keeping operatingvariables within their design limits

Automatic startup and shutdown Protection against instrument failures Selection of one from among several signals

OVERRIDES

In override configurations one controller can take commandof a manipulated variable away from another controller whenotherwise the process would exceed some process or equip-ment limit or constraint.

Selective control is less abrupt than the use of interlocks,which usually shut down equipment in order to avoid exceed-ing a limit or constraint. Overrides usually keep some processvariable from reaching an unsafe condition, and therefore theinterlock trip points are not reached. Thus, selective controlkeeps the equipment running although perhaps at a subopti-mal level.

This concept is illustrated in Figure 2.28a. The hardconstraint denotes the point at which the interlock trips.Overrides come into play at some point before the inter-locks are actuated and therefore are sometimes called softconstraints.

Signal selectors can facilitate the overriding of one control-ler by another. Often, overrides are preferable and therefore areactivated before the safety interlocks would be, but in mostapplications the overrides are backed up by interlocks. Overridescan be defined as:

Controllers that remain inactive until a constraint is aboutto be reached or exceeded, at which point they take overcontrol of the manipulated variable from the normal con-troller through a selector and thereby prevent the exceed-ing of that constraint.

Some common applications of override include theprevention of:

Flooding in distillation columns, by throttling boil-upor feed flow rates

Exceeding level ranges by draining or flooding High pressure or temperature caused by a runaway

reaction when heat input is reduced The development of low oxygen levels in furnace off-gas

streams, by reducing fuel flow

FIG. 2.28aIllustration of the nature of hard and soft constraints. Aninterlock is a hard constraint because it usually shuts down theprotected equipment, while selective overrides are called softbecause they keep the process variable from reaching such hardlimits.

Soft constraint

Normal operatingrange

Interlocktrip point

Overridesetpoint

Varia

ble

Hard constraint

2006 by Bla Liptk

2.28 Selective, Override, and Limit Controls 337

The development of high steam header pressures, bydiverting some of the steam to a low-pressure headeror condenser

Figure 2.28b shows an override control loop in whichthe normal control maintains flow (FIC) while the safety over-ride control is based on pressure (PIC). In this configuration,the outputs of both controllers are fed to a low-signal selector,which selects the lower of the two. The override controller setpoint is set at the maximum steam pressure that the process cantolerate, but below the safety interlock set point.

As long as the pressure controller set point is not exceeded,the output of the override PIC controller (signal A) is blockedby the low selector and cannot reach the steam valve. Whenthe pressure of the steam that is being sent to the users exceedsthe set point of the override PIC, that controllers output signaldecreases, and when it drops below the flow controllers output,it is selected for throttling the steam control valve. This way,the PIC override will prevent the steam pressure from risingabove the controllers set point.

Both the FIC and the PIC control algorithms requireexternal reset feedback to avoid integrating their errorswhen they are idle (reset windup), because their output isnot selected for control. As was discussed in more detail inSection 2.2, the controllers do not receive their own outputsignal as reset feedback, but both of them receive the signalselected by the low signal selector (FY). This makes thetransfer between the controllers bumpless.

Overriding at a Fixed Point

Figure 2.28b illustrates the case when the override variable(steam pressure) had to be limited to a specific value. Anotherexample of this type of application is shown in Figure 2.28c,where the controls involve cooling with river water. In thisapplication the concern is heat exchanger fouling because ifthe water outlet temperature exceeds 50C, exchanger foulingbecomes rapid.

Normally, the river water flow is throttled to control thecondensate temperature by TIC-1. However, if the river

water outlet temperature reaches or exceeds 50C, the high-temperature override controller (TIC-2) takes over the con-trol of the flow of river water and opens the control valve.This limits the river water outlet temperature to a maximumof 50C. As a consequence, the condensate temperature willdrop below the set point of TIC-1, but the fouling of the heatexchanger will be prevented, and that is the higher priority.

Figure 2.28d describes a control system that protectsagainst overpressurizing a reactor. This protection is providedby overriding the temperature controls and reducing the heatinput. In this application a cascade loop throttles the steamflow and controls the reactor temperature if the pressure inthe reactor is safe. On the other hand, when the overheadcondenser becomes overloaded and its capacity to condensethe overhead vapors is insufficient for the rate at which thevapors are generated, the reactor pressure will rise. The pres-sure controller (PC) provides the high pressure override bytaking command of the steam valve (through the low signal

FIG. 2.28bFlow control with pressure override limits the steam pressure to a safe value. Both controllers are provided with external feedback (EF)to prevent reset windup and thereby guarantee bumpless transfer.

Normal controller

SP

EF EF

Override controller

SP

FIC

FT

PIC PT

Steamflow

A/O - air to openA/O

ASteam

pressure atcontrolling

user

Interlock > SPoperating press < SP

FY