TECHNOLOGY

Process Control Equipment Continues Fast-Pace Development

Biennial Chem Show attracted many visitors

Instruments highlighted at Chem Show have wider range of uses, reflect influence of microprocessors and concepts of distributed control

James H. Krieger, C&EN Washington

The rapid evolution of process con­trol instrumentation that began sev­eral years ago shows no signs of abating. A number of current offer­ings provide new, higher levels of flexibility and sophistication in a range of applications.

Introduction of the new develop­ments came at the 40th Exposition of Chemical Industries—the Chem Show—held early this month at New York City's Coliseum. They were among the broad array of pro­cess equipment and instrumentation showcased by the biennial event.

Differing widely in application, the new developments bear no com­mon thread. But as a group, they do reflect recent influences permeat­ing process control. On the one hand, there is the direct influence of computers and microprocessors. On the other, there are the con­cepts of distributed control, the idea of operating a control system from the local control elements through a hierarchy of computer control up to a supervisory level.

Among the new introductions are a microcomputer-based gas moni­toring system from Mine Safety Ap­pliances Co., Pi t tsburgh, and a microprocessor-based interface sta­tion for use in level monitoring and control from Drexelbrook Engineer­ing Co., Huntington Valley, Pa. Leeds & Northrup, North Wales, Pa., has developed an operating system for its general-purpose, single-loop

controllers based on an IBM person­al computer. And Westinghouse Electric Co., Pittsburgh, has devised a graphic process control language for configuring control strategy for distributed control systems.

The gas monitoring system from MSA is designed to interface with sensors for toxic and combustible gases, as well as oxygen monitors, temperature and pressure indicators, and other such input devices. The entire system—called the DAN 6000 data acquisition network—consists of a central microcomputer station linked to as many as 14 micropro­cessor-controlled remote stations, each of which is capable of moni­toring up to 16 sensing points.

Making up the central station are two full-color, 19-inch-diagonal vid­eo display terminals, a keyboard for data entry and control functions, and a floppy disk drive. A high­speed printer is optional.

Remote stations handle the input from sensors such as MSA's diffu­

sion heads for combustible gases, carbon monoxide, oxygen, hydro­gen sulfide, chlorine, and hydro­gen cyanide. Any sensor with an analog output of 0 to 1 volt dc can be used. Sensors can be located up to a mile from each remote station, and remote stations can be located a mile away from the central sta­tion (or up to 20 miles through use of repeaters). Since each remote sta­tion is under control of its own microprocessor, it will sound alarms automatically and function indepen­dently of the central monitoring station.

Data from remote stations are transmitted digitally to the central monitoring station. At the central station, one video display screen pre­sents at-a-glance information from as many as the full complement of 224 sensing points. The data are orga­nized on the screen by rows, with each row representing inputs from all sensors from a single remote station. Chemical symbols on the

December 19, 1983 C&EN 19

Technology

screen indicate what gas is being sampled from each point. Colors in­dicate degree of gas concentrations— green for normal, yellow as a warn­ing level, and flashing red for an alarm.

The second terminal presents all readings from a given remote sta­tion in bar-graph form using the same color scheme. Exact values from the sensors are displayed with the graphs. If there is a warning or alarm condition, written operator instruct ions are flashed on the screen. Such instructions are among the information that the user can program into the computer.

A microprocessor also plays a role in Drexelbrook's new DE8000 inter­face station. As part of a level monitoring and control system— along with level measurement sen­sors and single-adjustment electron­ic units—the interface station is de­signed to simplify calibration and to reduce equipment and labor costs.

Cost reduction comes about by means of elimination of separate re­ceivers for level sensors. The DE8000 provides a l ight -emi t t ing-diode (LED) display of up to eight differ­ent level measurement channels at a cost for the basic unit of $2250 to $3350. The display provides both level status and channel number, with entry prompting and echoing. The system can be programed to produce readouts in usable engineer­ing units.

For calibration, Drexelbrook has devised a simplified method it calls Magi-Cal. The company explains that a keypad eliminates the need for calibration in the field, making it possible for a user to calibrate each channel from the central loca­tion of the interface station. The system is programed so that only two known levels need be entered into the keypad for each process vessel. The vessels need not be emp­tied or filled. As a temporary mea­sure, the unit will accept a single level for calibration until such time as a second level is known and can be entered to provide the necessary precision.

For security, a battery-operated backup power supply protects the memory if there is a power failure. And the 4-inch X ,4-inch keypad can be detached to help prevent unauthorized changes.

In addition to its eight channels of analog input and output, the sta­tion has 16 assignable set points for relay contacts, providing and, or, and not logic. There is also an RS-232 digital data port.

L&N has provided a number of improvements in standard features and a broader selection of applica­tion-oriented options for its Electro-max VPLUS general-purpose, single-loop controller. Among improve­ments, for example, are scaling of a remote set-point span to the process-variable span and tracking of the

Westinghouse exhibitor demonstrates graphic process control language for configuring control strategy for distributed control system

process variable by the local set point when operating with remote set point, for supervisory control. Among new application options are dual-input capability to make it easier to use the controller for ratio, set-point bias, summer, mass-flow, or feedforward control appli­cations.

Going a step further, the compa­ny has developed an operating sys­tem for the controllers based on an IBM personal computer. Included are driver and typical applications software. The system has a provi­sion for four ports, each of which can accept 30 controllers. L&N sees the system as being useful and eco­nomical in such applications as pi­lot plants, where there are a limited number of controllers.

Westinghouse's new graphic pro­cess control language was designed for use with the Westinghouse dis­tributed processing family, a micro­processor-based distributed control system called WDPF introduced by Westinghouse early in 1982. The system incorporates components for operator interface, informat ion management, and continuous, se­quential, and batch control along a

Visitors view Leeds & Northrup's new operating system for its general-purpose single-loop controllers based on an IBM personal computer

20 December 19, 1983 C&EN

In its plants, and in its research centers, Elf-Aquitaine develops today your chemistry of tomorrow.

Indispensable to life, sulfur is the wellspring of a rich and diversified chemistry of vital interest to many industrial branches.

Situated on one of the world's largest natural gas fields, Elf-Aquitaine produces a broad range of thioorganic chemicals :

methylmercaptan • ethylmercaptan • higher linear mercaptans • tertiododecylmercaptan • Aquitaine TPS • dimethylsulfide • dimethyldisulfide • diethylsulfide • tetrahydrothiophene (Alerton 88) • dimethylsulfoxide (DMSO) •

thioglycolic acid and esters • thioacetic acid • liquefied hydrogen sulfide. With its worldwide network of affiliates and sales representatives,

Elf-Aquitaine provides its customers with complete service : transport, packaging, new applications and safe use of its products.

Over 20 years' experience makes Elf-Aquitaine a world leader in thioorganic chemicals.

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single 2-megabit data highway that can be either a coaxial or fiber optic cable. Operators deal with the sys­tem through standard and custom graphic displays that are maintained in bubble memory for quick access.

The system is designed with high speed in mind. It can report process information to plant operators at a rate of 2 million bits per second— much faster, Westinghouse says, than a typical distributed system. Under critical conditions, the com­pany says, the WDPF gets essential information to an operator in one second or less, making faster reac­tion possible.

A WDPF system consists of vari­ous processing units distributed throughout a plant. These drops are connected to control devices. A typi­cal system, for example, would in­clude a distributed processing unit to convert raw data into engineer­ing units, an operator's console, a data logger, a calculator, and a programable controller. These drops communicate with each other over a data highway that can extend to nearly 4 miles and can accommo­date up to 254 drops. Every 100 milliseconds, each drop has access to the data highway, allowing it to report process values. In turn, each drop listens to reports made by oth­er drops about process points of in­terest to it, and pulls the reports off the highway for storage in its shared memory.

Microprocessor-based control­lers handle all the data reporting so the microcomputer in each unit is free to do its specified job uninter­rupted. The microcomputer controls, scans, monitors, and computes data before reporting the data to print­ers and via color graphics on video screens.

It is this system for which the new graphic process control lan­guage is designed. Westinghouse claims the language reduces appre­ciably the amount of time required to create and modify process con­trol schemes. With the language, a process engineer can configure con­trol schemes by simply drawing functional control loops and ladder diagrams on cathode-ray-tube con­soles.

For creating and making on-line changes to these control schemes,

Westinghouse has designed a digi­tizing tablet it calls Graphics Plus. In using it, an engineer places a stylus on the pad, choosing one of the many high-level algorithm sym­bols and repeating the operation un­til the control loop is completed and all desired algorithm symbols are connected to the proper inputs and outputs. This completes the con­trol strategy generation, and the WDPF system begins execution.

The process can be monitored, controlled manually, or tuned us­ing the graphic block diagrams on any system console. The graphic al­ways reflects the latest strategy or tuning constant changes.

Chemicals may improve Scientists from the Geophysics Lab­oratory at Hanscom Air Force Base, Bedford, Mass., have injected chemi­cals into the ionosphere to create disturbances they hope will improve communication with satellites. This fall the National Aeronautics & Space Administration fired three rockets containing these chemicals over the Atlantic Ocean from its Wallops Island, Va., site in separate experiments. Though two rockets malfunctioned, success with a third encouraged scientists to continue chemical release experiments.

The successful rocket was a Nike Tomahawk, which injected 40 lb of sulfur hexafluoride into the F-layer of the ionosphere, about 220 miles up. As chief project scientist Rocco Narcisi of the Geophysics Laborato­ry explains, that layer contains a plasma of oxygen ions ( 0 + ) and electrons. Earth's magnetic field sometimes causes striated irregulari­ties of electron density to occur in the layer, and these irregularities absorb ultrahigh-frequency radio signals used to communicate with satellites.

Sulfur hexafluoride reacts with 0 +

to yield SF5+ ions and with elec­trons to give SF5~ ions. Further, SFs"1" ions recombine rapidly with more electrons. The net effect is to pro­duce a "hole" in the ionosphere, depleted of electrons, that is intend­ed to abolish the irregularities of e lectron densi ty . In the recent experiments, scientists observed for-

A similar procedure is used to create logic control diagrams, with the engineer using the stylus on the pad and choosing contacts, coils, and special funtion blocks to build the control ladder. And for those loops not following standard prac­tices, the language has a lower-level text mode for constructing very spe­cial loops as well as user-defined special-purpose algorithms.

Because the language operates on line, all CRT consoles on the data highway show up-to-date control configuration displays. Control engi­neers can be sure of monitoring the currently executing process control strategy, Westinghouse says. •

satellite communication mation and motion of such a hole by fluoresced light and radar.

Two other chemical payloads were carried on board rockets whose sec­ond stages failed to ignite. These rockets were to have released bromo-trifluoromethane at an altitude of 200 miles, followed by an injection of atomized samarium on descent to 115 miles. It may be possible to repeat these experiments in early 1985 at Syindre Str^mfjord, Green­land, during a program to study creation and destruction of iono­spheric irregularities in polar re­gions.

The bromotrifluoromethane was intended to abolish electron irregu­larities by reaction with electrons to form mainly bromide ions, in reactions similar to those of sulfur hexafluoride. The samarium was to have been injected to create irregu­larities for study. Reaction of atom­ized samarium with oxygen atoms was to have produced SmO+ ions and electrons. Earth's magnetic field then would orient the resulting re­gion of high electron density into s tr iat ions like those of natura l irregularities. Scientists had planned to follow progress of the samarium injection by the blue fluorescence of atoms and ions. Because this fluo­rescence has never been observed from the ionosphere, one payload included samarium doped with 5% strontium. Strontium is known to fluoresce with a characteristic red light. •

December 19, 1983 C&EN 21