instrumentation basic
TRANSCRIPT
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• Type and application of instrumentation devices , selection and finalization of equipment depends largely on the process , system and plant.
• Process refers to manufacturing / refining / distribution / storage etc
• Plant size, location and availability of logistics form a major part in decision making regarding finalisation and selection of field and control instruments .
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• Philosophy of operation and Size of plant contribute directly to development of design and engineering.
• Owner’s range of selection are :
• Pneumatic controls
• Electric controls
• Electronic systems
• Electro-pneumatic controls
• Electro-hydraulic controls
• Automation• Warning and trip devices
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SELECTION OF INSTRUMENTATION SYSTEM
Owner’s range of selection are :• Fail-safe control devices
• Prime movers
• Valves
• Smart devices
• Emergency shut down systems
• Supervisory functions
• Data acquisition and remote control functions
• Local ( station ) and Master control centres
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SELECTION OF INSTRUMENTATION SYSTEM
• Location of various processes in a plant allows the Owner to have a range of selection as below:
• Centralised
• Distributed
• Hot and cold stand by
• Local
• Upon finalisation of above, communication links are developed such as processor to processor, peer communication and various interface protocols
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Utility of various processes in a plant allows the Owner to have a range of facilities as below:
• Diagnostic messages
• Remote calibration and maintenance checks
• MIS reports
• Data storage and retrieval facilities
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• With these guidelines and requirements we will be in a position to step into the exact instrumentation device and control equipment.
• For a typical pipeline and pump station application , we need the following:
• Fail-safe , reliable instrument devices
• Integrated auto control system
• SCADA systems
• ESD system
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• Before we go to the selection of instruments ( both field and control ) we need to understand the prevailing industry safety guidelines and standards for solid state control .
• All field equipment are to necessarily comply with certification for use in hazardous areas. These compliances are discussed in latter part of the session.
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FIELD INSTRUMENTATION :-
- Types
- Characteristics
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FIELD INSTRUMENTATION :-
There are three types .
1) Monitoring : These are used to monitor the health of equipment and operation as well. Basically they are alarm devices such as level / pressure / flow / temperature transmitters or switches.
2) Accounting : ( Measuring / Indicating ) : Basically to record values eg. Pressure / temperature indicator / gauges, level indicator, flow meter
3) Protective : are used to achieve fail safe operation and are used as shut-down or trip devices such as level / flow / vibration switches. These can be called field control type also.
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FIELD INSTRUMENTS :-
- Turbine meter
- Pressure transmitters
- Temperature transmitters
- Density meters
- Relief valves
- Control valves
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CONTROL FIELD INSTRUMENTATION :-
- These are generally discrete devices which generate contact for
signal interlocks or alarm or tripping of equipment.
- Analog control of equipment through control systems is also
possible from various field transmitters such as pressure and
temperature transmitters.
- Control equipment include pressure or flow control valves.
These are configured in auto control loop of a process.
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Apart from measuring / protective field instrument, we have calibrating type field devices / equipment such as Meter prover. These are used to calibrate turbine flow meters . Construction and design of these provers are as per API codes and standards. This is one critical equipment used in pipeline industry
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Besides analog / digital field instruments, there are certain instruments which are located in control room. These are generally as follows:
- Data loggers
- Flow computer
- Vibration monitor
- Recorders
- ESD devices
- Alarm annunciators
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Objective of a Pipeline Control system:
The system is designed to facilitate an effective , safe and smooth operation of pumping as well as delivery / storage system. The system takes care of opening and closing of motor operated valves ( MOVs ), starting and stopping of mainline pump, booster pumps, monitoring sump tank levels, provide audio and visual alarms for various parameters and status . It interfaces with analog control system for throttling of control valves.
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Various other functions of SCC and logic control system in a pipeline application:
• Starting and stopping of tank stirrers
• Starting and stopping of auxiliary equipment such as air compressors, centrifuges, control valves, pig launch / receive system
• Auto control of mainline pumping units
• Continuous monitoring of process parameters, alarming and tripping . This includes batching, monitoring of tank levels.
• Maintaining line balance and line integrity through application such as pressure profile, flow monitoring etc.
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Station control centre
Pipeline system ( consisting of tank farm, pump station and terminal facilities ) is operated by established procedures so as to have a smooth and fail-safe operation. To ensure these tasks at each site ( originating tank farm cum pump station, intermediate station or terminal ) a station control centre is installed.
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Objective of a Pipeline station control centre :
The logic system also ensures that the station is operated safely or the station is left in safe state in the event of failure .
It is the nerve of the station control centre which in turn communicates with the Master control centre.
Application software, hardware to be used depends on the application, I/O capacity and system time responses desired. Scan cycle and other updation times will depend upon the owner’s design with a vision for future developments / replacements / addition / augmentation etc.
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Main features of Station /Master control centre worth consideration during design and selection:
• Dual redundancy at logic controller processor as well as work station / operator station processor , memory, power supply and communication level so as to have minimum downtime
• High speed communication bus, WAN and interfaces with logic control system
• Real time tagging of data
• System design for open connectivity
• Serial interfacing on MODBUS protocol
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Main features of Station /Master control centre worth consideration during design and selection:
• Operating system – Windows NT
• Computer architecture – 64 bit for server and 32 bit for operator station
• Programming terminal to handle station control in case of exigencies
• Portability and remote downloading of software
• SCADA and control system commands shall have SEL-CHK-EXE features. This is select-checkback-execute instruction on all digital commands and analog set point controls .
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Main features of Station /Master control centre worth consideration during design and selection :
• Dual redundant PLC or intelligent RTU in hot stand by mode for logic control
• Open architecture and distributed intelligence type
• Bumpless switch over between processors
• High speed data transfer rate ( state of the art available )
• Reports shall be generated on MS Excel
• MMI updation 2 seconds or better
• Highest ingres protection rating
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Main features of Station /Master control centre worth consideration during design and selection :
• On-line servers shall hold alarms, events and trend, history for minimum 30 days.
• Networking as per IEEE standards
• Continuous on-line diagnostics
• Easy archival and retrieval systems
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Main features of Station /Master control centre worth consideration during design and selection :
• Colour RAL 7032
• Temperature withstand ( specified by owner )
• Electro-magnetic interference suppression
• Surge protection, transient protection, RFI fiters
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Design considerations of PLC / SLC based control system : • Design for the dual redundant field interrogation power supply shall be
such that one power supply can take full load at worst loading condition. Regulation shall be .05% or better for both line and load.
• All outputs shall be individually fused
• Standard 19 inch modules
• Spare 20% capacity in I/Os and other installed items like relays etc
• Switch-on protection
• IEC 65 design procedures
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Design considerations of PLC / SLC based control system : • Flow algorithm compliant to API
• Minimum I/O handling capacity 4096 any mix
• Floating point processing
• On-line editing without effecting outputs
• On-line self-diagnostics
• Processor fail, redundancy failure, rack failure , I/O module failure and communication failure signaling to operator MMI
• Individual power supply for each I/O rack
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Design considerations of PLC / SLC based control system : • Time of scan/K word length of application shall be better than 0.5 ms/K
• Very high noise immunity
• On-line replacement of module possibility
• Isolation level between logic and I/O min. 1 kv dc
• Reverse polarity protection in each I/O
• Contact de-bouncing
• Lightning protection
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Design considerations of PLC / SLC based control system : • Digital filtering
• Oscillation suppression
• Conversions from BCD to Binary and vice-versa
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Design considerations of field instruments • Accuracy of pressure gauges 0.5 % or better
• Accuracy of pressure switches 0.5 % or better
• Accuracy of Pressure transmitters 0.1 % or better
• Linearity and accuracy of turbine flow meters 0.15 % with repeatability also at 0.15 %
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Design considerations of field transmitters • Intrinsic safety interface must be met.
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Design considerations of field transmitters • A certified IS ( intrinsically safe ) interface passes signals to and
from the process, but limits the voltage and current that can reach the hazardous area under fault conditions.
Usually the interface is made up of a number of discrete devices such as shunt-diode safety barriers.
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Design considerations of field transmitters • Shut diode barriers must be suitably earthed to the
equipotential bonding system for the plant via an insulated conductor whose resistance measures more than one ohm.
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Design considerations of pressure switches
• Hermetically sealed micro-switch for clean environment
• Corrosion resistant material design
• Integral sensor design : Diaphragm sensors with welded construction eliminates O-rings
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Other Design considerations
• Life cycle of operations, switching time cycles, surges, pulsations, over-pressures, process temperatures can reduce sensor life.
• Dead band analysis
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Design consideration for selection of flow meter:
- Type of device ( capacity and accuracy related )
- Length and diameter of upstream and downstream piping
- 3D and 2 D concept
- No . Of pulses / cubic meter
- Floating rotor principle
- Output signal type and level . Distance criteria
- Flow counting method using flow computers
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Design consideration for selection of flow computer - Repeatable analog outputs
- Input handling ( pulse atleast two streams and other process parameters)
- Accuracy
- Isolation
- Displays
- Diagnostics
- Meter proving function
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Design consideration for selection of Meter Prover:
Definitions
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Design consideration for selection of Compact Meter Prover:
{ as per API }
• Pulse generation of minimum 10000 pulses from meter under test
• Usage of double chronometry
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Double chronometry
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D D
Count - A
Count - B
No. of Integral Flow meter pulses Count - C
Total Flowmeter Pulses = A * C / B
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Low wear and tear of piston and seals
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