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Process Control and Data Acquisition Systems

CM4120

Chemical Plant Operations

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Distributed Control System (DCS) Architecture

Used for continuous control of chemical processing Control functions are distributed throughout redundant,

deterministic networked computer architecture– 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

WE Network

PI Server

Control RoomControl RoomNetwork SwitchNetwork Switch

Operator Stations

ProcessLogix Server

PLX Controllerand I/O Cards

Plant Area – valves, xmitters

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

Control executes at PLX Controller

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Control Room network is Windows 2000 domain – PSCC

PDC is PI Server

Control system configuration is done thru PLX Server – creates configuration database

PLX server stores configuration database and stores/updates an active, real-time database

Operator stations access process thru PLX server

ProcessLogix – How it works

Control RoomControl RoomNetwork SwitchNetwork Switch

Operator Stations

ProcessLogix Server

PI Server

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ProcessLogix – How it works

ProcessLogix Server

PLX Controllerand I/O Cards

Communications via ControlNet

Controller configuration is downloaded to PLX Controller

PLX Controller continuously updates copy of active database at PLX server

PLX Controller continuously executes all control logic, alarming, PID loops, indicators, etc.

Controller continues to function, even after loss of communication w/ server

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ProcessLogix – How it works

PLX Controllerand I/O Cards

Plant Area – valves, xmitters

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

8 pr. cables from field junction cabinet

Zener barriers

I/O cards

2nd 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

Wiring to field junction cabinet

Thermocouple head

Temperature transmitter

Wiring from transmitter to measuring element

Level transmitter

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Field Devices – Transducer

Wire prs. to transducers

Current to pneumatic transducers

Air lines to control valves

Single pr. cable from field termination cabinet

Solenoids for 2-position air-actuated ball valves

Air lines to ball valves

Wire prs. to solenoids

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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 is a process data historization/ data retrieval system

PI server (Steam) is PDC for control room network and is also on WE network

PI data are available anywhere on campus network

PIPC is Excel add-in

ProcessBook is graphical tool for viewing process and process data

PI – Plant Information System

PI Server- OPC Interface

Control RoomControl RoomNetwork SwitchNetwork Switch

Operator Stations- PIPC- ProcessBook

ProcessLogix Server- OPC Server

WE Network- PIPC- ProcessBook

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PI – Plant Information System

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

– 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

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