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Appendix G. Control Objectives Analysis

(Presented at the 1977 Computer Conference by Kirby L. Hadley,Chevron Research Company)

SummaryControl engineers are having more and more influence on how refinery processare operated by designing, building, and installing advanced control systems utilizing digital computers. New control systems will be installed which improve plant profitability providing that (1) the process control engineer clearly under-stands the problem to be solved and (2) operating management understands wexpect from the new control system.

In the last two years, Chevron has developed a formal technique for insuring ththis communication occurs without misunderstanding. We call it “Control Objec-tives Analysis.”

The thesis behind “Control Objectives Analysis” is that it is always possible for group representing operations, process engineering, and control engineering todefine and agree on a list of “concise, precise, true-all-of-the-time” statements which define the operating objectives of a process. These objectives are met bymanipulating the valves of the plant. This list of “Control Objectives” serves as tbasis for the control system design, serves as a yardstick for measuring the suof a new control project, and provides a consistent basis for monitoring ongoingperformance.

Contents Page

G1.0 The Game: The Analysis G-3

G2.0 The Goal: The Control Objectives G-3

G3.0 Rules of the Game G-3

G4.0 Role of the Moderator G-4

G5.0 Opening the Game G-4

G5.1 Opening Step #1: The Diagram

G5.2 Opening Step #2: The Overall Objective

G5.3 Opening Step #3: Count the Valves

G6.0 The Game of Defining Objectives G-5

G7.0 Closing the Game G-5

G7.1 Closing Step #1: Categorizing the Objectives

G7.2 Closing Step #2: Back to the Overall Objective

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G8.0 Conclusions G-6

G9.0 Credits G-6

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G1.0 The Game: The AnalysisA “Control Objectives Analysis” session is treated as a game. A minimum of fouplayers is required: a moderator, a control engineer, a process engineer, and anations representative. The game is won when the group reaches the Goal: definlist of carefully stated “Control Objectives” equal in number to the number of control valves in the process area under consideration.

In practice, we’ve made three observations after performing this Game many tim

1. The game is almost always won. Participants come away with a major feeliof accomplishment. There are no individual losers.

2. It takes three or four hours to analyze a portion of a process containing abo15 valves.

3. The analysis is an intense exercise in group dynamics. It’s best to limit sesto half a day.

G2.0 The Goal: The Control ObjectivesA properly defined “Control Objective” is a concise, precise, true-all-of-the-time sentence which defines one of the reasons valves are moved in a process. Thecharacteristics of properly defined objectives are important and need to be undestood by the Players.

Concise means that the objectives statement should be as short as possible yetconvey all of the necessary information. Excessive words may indicate a lack ounderstanding of the true objective.

Precise means that the statement must actually say what the Players are thinkinclearly that everyone has the same understanding of what the objective means

True-all-of-the-time forces the Players to consider all of the conditions which marise in the plant. This is particularly important if an advanced control system opating all-of-the-time is to be designed based upon the “Control Objectives.”

As noted previously, the Game is over, the Goal is reached, when the number oagreed upon “Control Objectives” statements is exactly equal to the number of valves in the process area being considered.

G3.0 Rules of the GameThree rules have evolved which help keep discussions directed toward the Goa

1. The existing control system is not discussed. It is best if a process diagram isavailable which shows only valves, no control loops. If the available diagramshows loops, Players should be instructed to totally ignore them.

2. A new control system is not designed. This happens most frequently when a Player suggests an objective such as “adjust the reflux to hold the tower tem

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ature at some target.” Linking the reflux and the temperature together in a single statement implies a loop configuration.

3. Computers are not mentioned. The Analysis session should not become a forum for the discussion of computer control.

The Moderator of a Game should carefully review these Rules with the Playersbefore the Game begins. We have noted that Players will begin correcting themselves and each other when a Rule is broken.

G4.0 Role of the ModeratorAn important part of this Game concept is that the Moderator is not dictating to Players what the objectives of the process are. In fact, we have found it is not nsary for the Moderator to have any knowledge of the type of process under conation.

There are two requirements for a good Moderator:

1. The Moderator must be able to determine when an objective meets the conprecise, true-all-of-the-time criteria.

2. The Moderator must develop the skill to extract from the Players their knowedge of the process.

We have found that the Moderator works best at a blackboard with chalk and erEach objective proposed by a Player undergoes many revisions before it meetsthree requirements, is consistent with all of the other objectives, and is approveall Players.

G5.0 Opening the GameIf the Players are familiar with the Game, the Moderator can move the group directly into the steps outlined below. If any Players are new, the Moderator shoexplain the Game’s Rules and describe the three criteria of a good objective.

G5.1 Opening Step #1: The DiagramA process diagram should be available, simplified as much as possible, which clearly shows the control valves under consideration. The Moderator should asPlayers to explain to him what the process does.

G5.2 Opening Step #2: The Overall ObjectiveThe Moderator asks the Players to define an overall objective for this portion ofprocess. A long rambling paragraph generally evolves which the Moderator wrion the board uncritically. This opening step is primarily intended to break the iceand allow everyone to talk. When the group agrees on the wording for the “overobjective,” it is copied down and set aside to be used again in the final Closing

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G5.3 Opening Step #3: Count the ValvesThe Moderator returns to the diagram and with the group counts the number ofcontrol valves. The Moderator then announces the goal: the definition of that sanumber of concise, precise, true-all-of-the-time Control Objectives.

G6.0 The Game of Defining ObjectivesMost of the time of an Analysis is spent defining and polishing the individual Control Objectives statements. The persons trained as Moderators at Chevron developed individual styles. We have found, however, a more or less standard squestions Moderators use to stimulate thinking. Here are some of them:

“Is this proposed Objective precise? Is it really true?”

For example, an Objective which says “Minimize fuel consumption” can best bemet by shutting the plant down.

“Is the proposed Objective met in practice in the plant today—or are there impotant constraints or overriding considerations which mean we sometimes relax thObjective?”

A positive answer here has strong implications to the control engineer’s design an advanced control system.

“Do management signals change so that the plant shifts from one set of objectito another?”

“Can the existing valves achieve the objectives?”

“Are there any valves which are normally totally open or totally closed and thus should not be included in the count of valves?”

“Does the list of objectives provide enough information to specify the way in whvalves should be moved and positioned?”

“Is some valve on the diagram not covered by one or more of the objective statements?”

“Can we measure the criteria defined by the objectives with measuring devicescurrently installed?” “New devices?”

The questions above are not asked in any particular order. The skill of the Modator is in directing the group’s thinking toward the completed goal by selecting tappropriate approach at each point through the discussion.

G7.0 Closing the GameWhen the number of agreed upon Control Objectives matches the number of vathe Game is over.

There are two final steps the Moderator may lead the group through to make thresults more meaningful to the participants.

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G7.1 Closing Step #1: Categorizing the ObjectivesIt has been discovered that all single objectives statements fall into one of threegories:

The group should reorganize the list of objectives and force each into one of thecategories.

Here are the types of understanding which can come out of this exercise:

1. Those objectives which we have no control over which are fixed by the requments of the process are identified. These are the Type I objectives.

2. The targets management should be issuing are identified by the Type II staments. This may lead to a re-evaluation of the methods of target setting.

3. The Type III objectives are those which point toward engineering and economic studies.

G7.2 Closing Step #2: Back to the Overall ObjectiveAs a wrapup, we have found it useful to return to that Overall Objective paragragenerated at the beginning of the exercise and compare the detailed Control Otives with that beginning statement. This strongly contrasts preconceived ideasthe finished “Control Objectives List.” Participants frequently choose to rewrite tOverall Objective.

G8.0 ConclusionsThis simple technique has worked successfully for Chevron again and again.

The Company is aggressively moving to implement advanced control projects based on this concept.

Additionally, operating management has recognized a tool to clarify thinking anmotivate operators in plants not scheduled yet for computer control projects.

Control engineering as a discipline has increased its influence on company opetions.

G9.0 CreditsThe “Control Objectives Analysis” technique evolved through the efforts of a number of individuals. Jim Bronfenbrenner and John Westmoreland of ChevronResearch, in particular, developed many of the techniques described in this pap

Type I: Hold material balance, heat balances

Type II: Hold operations at management-set targets

Type III: Balance one objective against another; optimize; minimize or maxmize

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