THE MANY BENEFITS OF HIGH TECH CONTROL SCHEMES
The color of the world is changing day by day...
Red...A world about to dawn!
Black...The night that ends at last!
("Red and Black," from Les Miserables Soundtrack, music by CM Schonberg music, English lyrics by H. Kretzmer)
HOW ARE THE DIGITIZED AND NON-DIGITIZED
CONTROL ROOMS DIFFERENT?
The DCS Control Architecture
Requires less Space
The non digitized control room is much larger than the DCS control room. A non-digitized control room must include room to put each indicator, controller, recorder and alarm into their respective panels.
In contrast, DCS architecture changed the hardware location and function and added software.
The cabinet standing on the right side of this digitized control room relays data between the field instruments and the computer screen and keyboards.
The picture on the left features a DCS control room design. Standing at the right side of the control room picture is a cabinet.
Just two or three of these cabinets would replace all the panels that a non-digitized control room would require.The cabinets do not have to be located in the control room.
Digitized control systems spread the duty for process control amongst strange-sounding devices like a "Field Input-Output Module,"Analog Input Module," "Process Computer Unit," Unit Control Module," etc. These devices are cabinets that sprang up out in the processing area.
At the time this PTOA segment was written, very modern control rooms use just an ethernet cable to send data from distant operating processes to an ethernet card in a desktop computer located in the Control Room.
The controller would be found in a non-digitized control room. This controller must be housed in a control board panel
Wherever they are located and whatever they are called, the cabinets are full of circuit boards. Circuit boards have replaced the non-digitized controllers like the one shown in the below picture on the right.
Digitized control room technology also incorporates programming software that enhances process control.
The Process Control/Human Being Interface
In the non-digitized control room, the Control Board Operator walks over to the control panel to make an adjustment or observe a process variable.
The DCS Control Board Operator interfaces with process control via computer screens and keyboards from the comfort of his/her desk.
The computer screens and keyboard are sometimes referred to as the "Man-Machine Interface" (MMI) or, more politically correct and gender neutral: "Human-Machine Interface" (HMI).
GUI stands for "Graphical User Interface." That means you don't need to type with a keyboard but can just access icons.
PTOA Readers and Students just need to remember these big-sounding phrases simply refer to how the Control Board Operator receives information from the local/field instruments, makes changes when changes are needed, and then sends output signals to the valves that are located in the processing area.
Older Control Rooms need more Operators
The non-digitized control room requires more Control Board Operators because s/he must physically get up out of his/her chair to view the panel of controllers, assess the status of the process, and make changes.
PTOA Readers and Students can get a fair idea of how a process crises would be handled in a non-digitized control room by watching the move "China Syndrome" through the scene that shows a near-nuclear disaster.
Actors Jack Lemmon and Wilford Brimley in the movie "The China Syndrome" about a nuclear disaster that happened to debut a few days before the USA's worst nuclear disaster at Three Mile Island, PA.
In contrast, the DCS control room has several controller icons on a single screen page.
The Control Board Operator can custom-change a screen to show controllers s/he believes to be the most important for safe and efficient operations. Control Board Operators spend the first few minutes of each shift changing the control parameters to their own selections.
Old Control Rooms have limited Alarm Capability
The non-digitized controllers do not have the same alarm capability to warn a Control Board Operator when a Temperature, Pressure, Flowrate, or Level is out of desired range.
In the DCS control room, the Control Board Operator can set high and low alarms limits on each individual controller to alert him or her when a process variable is in an unexpected range.
Control Board Operators that choose tight limits for alarms will have to tolerate the constant ding-ding-ding of one alarm following another throughout their shift and spend the shift acknowledging and silencing alarms.
New Control Rooms can use Trending
As a Powerful Monitoring/Controlling Tool
The trending capability of non-digitized control rooms is limited. The Control Board Operator has to get up and out of the chair to view the trends of process variables drawn by pens,like the trend shown on the indicating controller to the right.
Digitized trend information has made using history...meaning what happened to the process over a recent time interval...a more valuable tool for monitoring and controlling processes from a desktop.
The impact of a changing one process variable upon another is much easier to observe on a digital trending screen. The Control Board Operator can custom change a screen to include all the process variables s/he is interested in observing and comparing.
Alarms can be set on trend charts to alert the Control Board Operator when one of the process variables has strayed out of desired range.
Control functions can be added to trend screens to allow corrective actions to take place from either the trend screens or the control screens.
Improved Graphics and Graphic Interface
with Digitized Schematics
This pictorial graphic of a Fluid Catalytic Cracker is much easier for the Control Board Operator to understand versus non-attached instrument readings
DCS graphics, graphical interfaces, and user interaction capabilities continue to improve.
The graphic to the right is a pictorial representation of one of the more complicated fuels refinery processes... a Fluid Catalytic Cracker (FCC). FCCs greatly increase the production of motor fuels from a barrel of oil.
In one glance, the Control Board Operator knows the status of critical process variables.The information on the graphic is updated every few seconds. Alarms will sound when the range of a process variable is outside of the desired specification.
Reduced Maintenance with DCS Architecture
DCS have self diagnostic capabilities.
The effort to replace a circuit board is minimal compared to changing out a board mounted controller like the one shown on this PTOA segment.
Take Home Messages: Both digitized (DCS) and non-digitized control rooms perform the same functions.
There are many benefits of Distributed Control Systems compared to non-digitized process control designs. The administrative benefits include economy in footspace and reduced costs for maintenance and operations.
The benefits to the Control Board Operator are the much improved and interactive graphics, expanded utility of using history in the form of Trends for controlling and comparing process impacts, and the ability to custom set alarms and monitoring parameters.
Phrases like Man-Machine Interface, Human-Machine Interface, and Graphical User Interface just refer to how the Control Board Operator interacts with the DCS system.
©2015 PTOA Segment 00013
Process Industry Automation
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