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Prepared by A. H. Burns, July 2019

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Introduction

SCADA is a system that is implemented plant wide to supervise controls and collect data

Data can be used in a number of ways:

➢ To connect various business activities

➢ To monitor plant performance

➢ To provide information for a real time, client supplier interface

➢ To maximize plant availability though diagnostic and maintenance assistance

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SCADA

A typical screen shot from a Siemens system

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Design philosophy

➢ The system design must enable operation off-line (local mode)

➢ When the control is off-line, local manual operation, set-up and troubleshooting operations must be possible, without affecting operations in the rest of the plant

➢ Control room feedback must make it clear that components are off-line

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Distributed control systems

➢ The key to success of an integrated system is the way the data is collected, moved around, presented, and used.

➢ The most common interface is the analog 4/20mA signal. This signal is scaled to control or to transmit data too and from a field device

➢ Digital communications provide more information and greater operational flexibility

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Typical communications

A typical Profibus wireless

Gateway connecting HART

/IO-Link devices

Typical safety rated

Profibus Gateway

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Digital comms via Profibus

➢ Modular system with a single consistent protocol

➢ Runs control and safety systems on the same bus

➢ Device behavior is consistent for all devices connected to the bus

➢ Diagnostic data is sorted in accordance with the NAMUR NE107 standard

➢ Digital communications provide more status and set-up data to the system operator

➢ Provides a direct link to SCADA systems

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Digital comms via Modbus

➢ Runs with a standard RS 484 cable network

➢ May run a little slower than Profibus

➢ Can operate on a wireless network

➢ Easier to use than Profibus (One cable, Profibus requires in & out cables)

➢ Modbus provides a direct link to SCADA systems

➢ Modbus can not be used in hazardous or multi vendor applications

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IO-Link

➢ Is a protocol that defines the way instruments, actuators and sensors interact with a master controller connected via any fieldbus

➢ Remote configuration and monitoring are simplified.

➢ A richer dataset enables more detailed information to be shared with the DCS network

➢ Advanced diagnostics probably means that this protocol will eventually dominate the market

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IO-Link an example

➢ Devices have features that provide

➢ Characterization, response control, alarms, configuration and diagnostic information.

➢ These options are usually set up on commissioning using a local interface

➢ An IO-Link device uploads its internal set-up file to the server

➢ The stored file is downloaded when the device is replaced

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DCS connectivity

➢ Connection of a digital device to a DCS usually requires only a single communication cable and a local power source.

➢ A DSC server operates as a master supervisor, polling each device for information. Typically:

➢ Diagnostic & current condition feedback

➢ Set-up parameters

➢ Operating data

➢ Etc.

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Advantages of a digital DCS

➢ Control is via a digital signal

➢ Device feedback is also via a digital signal

➢ Installation wiring is simplified

➢ Signals are less prone to errors resulting from electrical interference in the plant

➢ Extensive operating and diagnostic information exchange and test signal quality which enable a timely status-based intervention

➢ Engineering, installation and commissioning are simplified

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DCS Basics

➢ A DCS is implemented from a plant computer server running customised software

➢ The software interface is developed for each operation in the plant

➢ Implementation of a DCS is usually an evolutionary process

➢ In a zinc oxide plant controls are considered to be Level 2

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Software

➢ Software should be open architecture

➢ There are some important exceptions

➢ Controls that interface with safety devices

➢ Systems that must be certified by an inspection agency, such as a burner management system

➢ Systems that are proprietary to the supplier and are sold on a single user license basis only

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DCS interface

➢ If closed architecture modules are operating in a network, it is important to properly define the points of interface

➢ For a burner control unit (BCU), the logic that kills the gas flow to a burner is closed architecture, however, feedback or performance data may still be available without compromising the intent of the BCU and the fuel safety code

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Software development

➢ Clear, early identification of data required by the SCADA system will save development costs

➢ Supervisory requirements must be well defined and limited to important functions

➢ The software for the DCS should be open architecture and should be modular

➢ Local programmers will be required to keep the software current and up to date

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Modular software development

➢ Each module is a standalone piece of software.

➢ Each module will have properties and methods intended to process and store information

➢ The main program, running on the plant server, calls each module as required, processing data and transmitting instructions

➢ Once implemented and deployed, a module may be called by any application running on the network

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Advantages of modular design

➢ A module is written and debugged separately

➢ A module may be added to the system without affecting modules previously written

➢ A module may use an already written and deployed module

➢ A module usually contains error handling to check the quality of the data and the instructions

➢ A module only gets added to the main program after it has been locally debugged

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Software evolution

➢ Using the modular approach, it is possible to start with a basic system during commissioning, to minimize initial costs

➢ This approach provides tools for local programmers to add functionality after the plant has been handed over to production

➢ Modular software design allows local programmers to respond to changing requirements, minimizing system crash dangers

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Open architecture

➢ To make open architecture software design work, it is necessary to provide good documentation for the plant

➢ A programming team will usually consist of a program designer, a project manager, and specialist coders

➢ Each member of this team has to have access to the plant documentation to be effective

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Definition of control functions

➢ An important task during the FEED process is the development of the specification for the DCS

programming team comprising:

➢ Material flow control

➢ Equipment control tasks

➢ Supervisory tasks

➢ Archival tasks

➢ Documentation tasks

Development of a detailed P&ID is part of this process

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P&ID Examples

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P&ID - Process and Instrument Diagram

➢ P&ID illustrates the interconnection of process equipment using a standard set of symbols and a methodology defined by the IEEE standard

➢ P&IDs are developed during the design stage of the project and are used as the basis of the control system design

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P&ID

➢ The diagram provides the reference ID for each device across all engineering deliverables

➢ Schematics; Electrical, Piping, etc.

➢ General arrangement and detail drawings

➢ Installation drawings, scopes

➢ Hazop studies

➢ Datasheets, instructions, set-up data

➢ Software development

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P&ID Scope

➢ These are important documents that are used to define and describe

➢ The overall process layout in a graphical form

➢ Startup and shutdown sequences

➢ Safety and regulatory requirements

➢ Operational procedures

➢ It is very important to keep P&IDs up to date when making modifications or plant upgrades

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Conclusion

➢ Control options are rapidly evolving. Some of the options discussed may not be attractive for all plant operations.

➢ For a new plant, the level of functionality described can be built into the early phase of the plant design for minimal cost

➢ This presentation is intended to start dialog so that a DCS can be properly discussed and evaluated during the FEED process.