Instrument Engineers' Handbook, Volume Two: Process Control and Optimization - Kindle edition by Bela G. Liptak. Download it once and read it on your Kindle device, eBook features: Highlight, take notes, and search in the book; In this. The latest update to Bela Liptak's acclaimed "bible" of instrument right, the fourth edition of Process Control and Optimization continues the. by Béla Lipták INSTRUMENT ENGINEERS' HANDBOOK Fourth Edition Process Control and Optimization VOLUME II Béla G. Lipták EDITOR-IN-CHIEF.
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Part C in Figure 9 illustrates a triple-cascade loop, where a temperature controller is the slave of an analyzer controller, while the refux fow is cascaded to temperature. This confguration is used when maintaining a stable temperature on a particular tray, while the tower operates at a constant and control- lable pressure is desired.
Since temperature is an indicator of composition at constant pressure, the analyzer con- troller serves only to correct for variations in feed composition. Cascade loops will work only if the slave is faster than the master, which adjusts its set point.
Another important consideration in all cascade systems is that the integral mode in the master will cause the output to saturate when that output is blocked from reaching and modulating the set point of the slave when the slave is switched to local set point. This is called reset windup, and it is prevented by providing the integral mode of the master with an input an external reset that is never blocked.
These signals are not shown in Part C selective Control Part D of Figure 9 illustrates a limit control confguration where the analyzer controller is overruled when the temperature reaches its high limit.
The temperature controller is a constraint controller preventing the tempera- ture from exceeding a limit at the bottoms of an absorber stripper. The reason for this limit is energy conserva- tion, because no additional stripping of the light component can be accomplished once the boiling point of the impurity is exceeded.
Therefore, even though an analyzer controller may call for more heat, this heat would only increase the bottoms temperature without removing the impurity, thereby wasting heat. Selective control confgurations also require external feedback to protect them from reset windup. In such a confguration, the external reset ER signal not shown in the fgure is taken from the measurement of the slave controller FRC. First the overhead vapors enter the condenser partial or total , and next the liquid condensate is collected in an accumulator vessel.
Some of the accumulated condensate is returned to the column as refux, while the remainder is withdrawn as overhead product distillate. If the condensation is incomplete, the condenser is called a partial condenser, and the overhead product is withdrawn in both vapor and liquid phases.
Total condensers are usually designed for accumulator pressures up to psia 1. A partial condenser is usually used between psia and psia 1. The condenser pressure drop is usually about 5 psia Most distillation columns are operated under constant pressure, because at constant pressure, temperature measurement is an indirect indication of composition, but foating the operating pressure can have advantages in many applications.
When the column pressure is allowed to foat, the composition must be measured by ana- lyzers or by pressure-compensated thermometers. The primary advantage of foating-pressure control is that one can operate at minimum pressure, and this reduces the required heat input needed at the reboiler.
Other advan- tages of operating at lower temperatures include increased reboiler capacity and reduced reboiler fouling. Cooling Water Control, negligible inerts In distillation processes where the distillate is in the liquid phase and the quantity of inerts is negligible, the column pressure is usually controlled by modulating the rate of condensation in the condenser.
In the control system shown for Column A in Figure 10 p. This control scheme is recommended only when the cooling water is treated, because condenser tube fouling due to high temperature rise across the tubes can occur. In such cases the cost of plugging the software holes can exceed the total hardware cost of the system. Well, in many cases it is not; only its license is. Well, what this language often means is that the cost of integrating these packages into the overall control system is an extra.
So, on the one hand, this age of plantwide digital networks and their associated advanced controls has opened the door for the great opportunities provided by optimization. I will discuss below some of the areas in which the next decade should bring improvements in the quality and intelligence of the components that make up our control systems. I will discuss the need for better testing and performance standards and the improvements needed in the sensors, analyzers, transmitters, and control valves.
Connectivity and Integration Utilizing our digital buses, one can plug in a PC laptop or use a wireless hand tool and instantly access all the data, displays, and intelligence that reside anywhere in a DCS network.
The modern control systems of most newly built plants consist of four levels of automation. We the users should also require that the manufacturers always state not only the inaccuracy of their products but also the rangeability over which that inaccuracy statement is valid. Introduction This is because the performance of the control loop is much affected not only by trim wear in control valves but also by stem sticking caused by packing friction, valve hysteresis, and air leakage in the actuator.
The stability of the control loop also depends on the gain of the valve during the tuning of the loop.
In order for a control loop to be stable, the loop is tuned the gain of the controller is adjusted to make the gain product of the loop components to equal about 0. For this reason, the loop must be compensated for the gain characteristics of the valve; such compensation is possible only if the valve characteristics are accurately known. For the above reasons, it is desirable that the users and the professional societies of ACE engineers put pressure on the manufacturers to accurately determine the characteristics of their valves.
This is particularly true for the rangeability of control valves. Smarter Valves A traditional valve positioner serves only the purpose of maintaining a valve at its intended opening. Digital valve controllers, on the other hand, provide the ability to collect and analyze data about valve position, valve operating characteristics, and valve performance trending.
They also provide two-way digital communication to enable diagnostics of the entire valve assembly and instrument. Section 6. If the sensors of the intelligent control valve are connected to a PID chip mounted on the valve, the smart valve becomes a local control loop.
In many cases there is a need for multiple-range and multiple-reference units. Improved On-Line Analyzers In the area of continuous on-line analysis, further development is needed to extend the capabilities of probe-type analyzers.
More and more analyzers should become self-calibrating, self-diagnosing, and modular in their design. In order to lower maintenance costs, analyzers should also be made more modular for ease of replacement and should be provided with the intelligence to identify their defective modules. The human-machine interfaces HMIs in the control rooms are only as good as the ability of the operators to use them. The hand, the psychological characteristics, the hearing, and color discrimination capability of the human operator must all be part of the design.
Even more importantly, the design should also consider the personality and education of the average operator. Today, when a single operator is expected to oversee the operation of processes having hundreds if not thousands of variables, the operator must be provided with diagnostic, trend, and historical data in an easily understandable and familiar format.
For that purpose, it is advisable to provide in the control room a large display panel, on which as one of the options the operation of the whole plant can be displayed. As the operator focuses in on smaller and smaller sections of the plant, the information content of the display should increase. Another essential feature of modern control systems is their suitability for smooth growth of the plant.
This capability is very important in process control because plants are ever expanding and therefore their control systems must also grow with the expansions. A modular approach to operator stations makes the expansion of the plant easier. Yet in the coming decade much more improvement is expected both in multivariable unit operations control and in model-based optimization. Plant-Wide Modeling and Optimization In addition to its role in providing better control, process modeling and simulation can also improve the training of operators.
If the simulation model is accurate and if it integrates the dynamics of the process with that of its control system, it can be used to train operators for plant startup without risking the consequences of their inexperience.
Needless to say, the building of good models is expensive, but once prepared, they are very valuable.
Business- or enterprise-wide optimization includes the model of not only the manufacturing process, but also the optimization of the raw material supply chain and of the packaging and product distribution chain. Plant-wide optimization also involves more than the optimization of the unit processes because it must also consider documentation, maintenance, production scheduling, and quality management considerations.
Naturally, it should be kept in mind that modeling and optimization can only be achieved when the control loops are correctly tuned, the measurements are sampled fast enough, and interactions between loops have been eliminated. For example, in the section describing the methods of pumping station Introduction optimization, it is pointed out that the lifetime operating cost of a pumping station is about a hundred times higher than its initial cost.
The returns on the optimization of other unit operations are similar, although not that high.
It is for this reason that in the coming decade, optimization is expected to increase. When using multivariable envelopes for unit operation optimization, the individual variables of levels, pressures, and temperatures become only constraints, while the overall goal is to maximize the efficiency or productivity of the controlled process.
Unit Operation Controllers of the Future The next decade could bring a building-block approach to control systems.
Once the particular software package was loaded, the unit controller would be customized by a menu-driven adapter package, organized in a question-and-answer format.
During the customization phase, the user would answer questions on piping configuration, equipment sizes, material or heat balances, and the like. Such customization software packages could not only automatically configure and tune the individual loops but could also make the required relative gain calculations to minimize the interaction among them.
Yet, at the age of 26 I did not dare to hire experienced people because I did not feel secure enough to lead and supervise older engineers.