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Most systems can be controlled manually or automatically. The modern trend is to automate the process as much as possible. One reason is that automatic control- lers always respond the same way to changes, whereas men are erratic. Controllers may work for years with only minor maintenance, whereas a man fatigues easily. This means that while controllers may not produce a better product than an alert man, they can, in the long run, produce a more uniform product, with less waste and fewer accidents.

The use of controllers may also reduce over-all expenses. The average operator in a chemical plant, when fringe benefits are included, costs the company over $5.00 per hour. This is equivalent to $43,800 per year for an operating position. By the

methods given in Chapters 10 and 11, this can be shown to be equivalent to a net present value of (assumes money is valued at 8%). The average con-

of controllers can be installed, maintained, and operated for the price of one operating position.

Some plants can be operated essentially without any people. However, for safety purposes there usually are two employees per shift. Then if some mishap should occur to one man, the other can obtain help. This implies that a plant can be over-automated. The operators can become bored if they do not have some tasks to perform. If these are make-work tasks, the operators will rapidly determine this, and either the tasks will be ignored or the reports will be falsified. To keep these men alert, and sufficiently knowledgeable and involved so that they can respond quickly and properly when an emergency arises, it may be best not to automate the plant totally.

CONTROL SYSTEM

A control system consists of four stages. First, the item to be controlled must be measured. This reading must then be compared with some desired value, called the

set point. Depending on the result of this comparison, a decision must be made

whether some variable(s) in the process should be changed. Then if a change is indicated, the amount of change required must be determined and it must be instituted. The comparison, decision making and size change determination are considered part of the controller.

At this stage of design, the details of the whole control system need not be specified. It is only necessary to determine what variables are to be measured, which are to be controlled, and how this is to be accomplished.

The controller and its quantitative interaction with the system will not be covered in this text. Numerous books have been written about this, and most chemical engineering curricula have a course that is devoted solely to the topic.

VARIABLES TO BE MEASURED

The ideal variable to measure is one that can be monitored easily, inexpensively, quickly, and accurately. The variables that usually meet these qualifications are pressure, temperature, level, voltage, speed, and weight. When possible the values of other variables are obtained from measurements of these variables. For example, the flow rate of a stream is often determined by measuring the pressure difference across a constriction in a pipeline. However, the correlation between pressure drop and flow is also affected by changes in fluid density, pressure, and composition. If a more accurate measurement is desired the temperature, pressure, and composition may also be measured and a correction applied to the value obtained solely from the pressure difference. To do this would require the addition of an analog or digital computer to control scheme, as well as additional sensing devices. This would mean a considerable increase in cost and complexity, which is unwarranted unless the increase in accuracy is demanded.

Composition is another variable that is often measured indirectly. A temperature-sensing device is often used at the top of an atmospheric distillation

Control System 163 column to indicate the composition. If the material being measured is a binary mixture, then from the phase rule it can be shown that this is a very accurate procedure, provided equilibrium exists between the gas and liquid phases and the pressure is constant. However, if more than two compounds are present this procedure will be inadequate unless the composition of all but two of the com- ponents are held constant. The process designer must decide whether this is a reasonable assumption.

There are other, more direct means of measuring compositions. One is the use of a semicontinuous device such as a gas These instruments analyze a sample obtained from the process. Until the analysis of that sample is complete, another sample cannot be processed. For some compounds it may take 15 minutes or longer to process the sample. In some processes with short response times such delays may not be permissible. Another disadvantage is that have an installed cost at least eight times greater than that of a temperature-measuring control system. They also require more maintenance and have a greater operating cost.

A list of process analyzers, operating principles, manufacturers, costs, advantages, and disadvantages is given in reference 2. A list of process instrument elements with their accuracy and principles of operation is given in reference 3.

Sometimes it is not possible to use even a semicontinuous analyzer, and a sample must be analyzed in a laboratory. For example, the blow-molding charac- teristics of a plastic must be tested off-line. This often requires that the product be temporarily stored until the laboratory results are obtained.