CM4120
Chemical Plant Operations
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Distributed Control System (DCS)
Architecture
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Used for continuous control of chemical processing
Control functions are distributed throughout redundant, deterministic networked computer architecture
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I/O interface and level 1 control functions advanced control functions human interface
DCS replaced mainframe computer controllers
Added a high level of robustness to control systems
Pre-configured functionality geared toward process industries, but a relatively high cost
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Programmable Logic Controller (PLC)
Architecture
 Used for control where the devices are primarily discrete
(the controlled devices are energized/de-energized and the inputs are open/closed circuit)
 Provide very fast response (< .1 sec) to system changes
 Typical applications are packaging, material conveyance, and waste water treatment
 Very little, if any pre-configured functionality, but PLC’s are relatively low cost
 In chemical processing, often are used along side DCS
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SCADA – Supervisory Control and Data
Acquisition System
 Scalable systems that provide some additional level of control and data acquisition “above” the controller, DCS and/or PLC level
 Typically use non-redundant TCP/IP communications and therefore are not considered appropriate for direct process control
 Built upon non-proprietary, off-the-shelf hardware
 Very useful in large plant environments where data from many sources must be collected
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Rockwell Automation ProcessLogix DCS and OSISoft PI in PSCC
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PI is process data management system
 “External” historized and real-time data requests are served thru PI Server
 Operator Stations and other “internal” data users communicate thru PLX Server
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Control executes at PLX Controller
WE Network
PI Server
Control Room
Network Switch
Plant Area – valves, xmitters
ProcessLogix
Server
PLX Controller and I/O Cards
Operator Stations
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ProcessLogix – How it works
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Control Room network is
Windows 2000 domain – PSCC
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PDC is PI Server
 Control system configuration is done thru PLX Server – creates configuration database
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PLX server stores configuration database and stores/updates an active, real-time database
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Operator stations access process thru PLX server
ProcessLogix
Server
PI Server
Control Room
Network Switch
Operator Stations
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ProcessLogix – How it works
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Communications via ControlNet
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Controller configuration is downloaded to PLX Controller
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PLX Controller continuously updates copy of active database at PLX server
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PLX Controller continuously executes all control logic, alarming,
PID loops, indicators, etc.
 Controller continues to function, even after loss of communication w/ server
ProcessLogix
Server
PLX Controller and I/O Cards
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ProcessLogix – How it works
Plant Area – valves, xmitters
PLX Controller and I/O Cards
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I/O cards are specific to device requirements
•4-20 mA input, 4-20 mA output
•24 VDC input, 24 VDC output, etc.
 A pair of wires runs from each device back to the I/O card
 Typically 8 to 16 devices per I/O card
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PLX Controller Cabinet
PLX controller
I/O cards
Zener barriers
8 pr. cables from field junction cabinet
2 nd I/O chassis
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Field Junction Cabinet
Single pairs from field device
8 pr. Cables to controller cabinet
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Field Devices – Transmitters
Level transmitter
Wiring to field junction cabinet
Thermocouple head
Wiring from transmitter to measuring element
Temperature transmitter
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Field Devices – Transducer
Solenoids for 2-position air-actuated ball valves
Air lines to ball valves
Current to pneumatic transducers
Air lines to control valves
Single pr. cable from field termination cabinet
Wire prs. to solenoids
Wire prs. to transducers
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Field Devices – Regulatory Control Valve
Air line from transducer
Actuator w/ positioner
Control valve
Block valves
Bypass valve
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Field Devices – Ball Valve w/ Actuator
Air line from solenoid
Actuator
Ball valve body
Process line
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PI – Plant Information System
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PI is a process data historization/ data retrieval system
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PI server (Steam) is PDC for control room network and is also on WE network
WE Network
- PIPC
- ProcessBook
 PI data are available anywhere on campus network
ProcessLogix
Server
- OPC Server
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PIPC is Excel add-in
 ProcessBook is graphical tool for viewing process and process data
PI Server
- OPC Interface
Control Room
Network Switch
Operator Stations
- PIPC
- ProcessBook
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PI – Plant Information System
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 Data Historian system is required part of process management system
Long-term data storage/ retrieval system
Optimizes use of hard drive storage space and permits quick recall of archived data
Includes data retrieval software tools
OSISoft, Inc. PI system
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Import process data into supervisory control or process monitoring applications
Link process control systems with business management systems
Provide data for production and compliance reporting
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PI – How it works
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A PI “tag” is configured for every piece of data you want to archive
Collected tags are “scheduled” to retrieve data periodically or upon the occurrence of an event
Data are brought into the archive thru a filtering and compression algorithm
Data are not archived unless they pass the filter and compression tests
When data are retrieved from the archive and presented in a series, the individual points are extrapolated between the stored data
Tags can also be manually entered – lab data
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