Benefits of foundation in the operational phase

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John Rezabek Presentation from our North American Seminars

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  • Generically, “fieldbus” describes a communication protocol. Foundation fieldbus uniquely adds the user layer. Other fieldbus protocols do not have a user layer.
  • In the late 70’s and 80’s, we started out with digital microprocessor-based DCS’s, which revolutionized our discipline.Not long after, “smart” devices (transmitters and valve positioners) appeared, and it didn’t take long for users to ask them to be integrated. After years of proprietary integration, ISA convened the SP-50 committee to devise a standard. It is the work of this committee that grew in WorldFIP and ISP which later merged to form FF.It was the USERS that wanted a USER LAYER. Vendors were not crazy about it.The “User layer” – device-resident standard function blocks – Are a unique feature of foundation fieldbus.AI, AO, DI, DO (MAI, MDI, etc.) are the minimum required to use data from FF in the host. Even they are configurable to do scaling, alarming, square root extraction, etc. outside the DCS. A lot of power already. In addition to the PV (process variable) they also pass along signal status every time they’re executed.ARTHM, ISEL, SGCR, INT and related FB’s are available in many devices – more chances to off-load computing tasks from the DCS using standard FB’s.PID, CSEL, SPLTR (PID control, control selector & splitter) are among the blocks used for CONTROL IN THE FIELD, a feature end users wanted to achieve truly distributed control . . . ASK – what other control system is vendor-independent and totally distributed? (answer = pneumatics)ASK – Why didn’t we like pneumatics? (answer = sloppy, unreliable, lack of precision)
  • Why do we tolerate “many eggs in one basket” HOST DCS of today?Ask what, if anything, single-loop integrity means to attendees“Functionally” meaning controller, operator interface, network management, historian etc in separate boxes.ASK: Can anyone comment on the number of loops per controller in their system?
  • "The global process industry loses $20 billion, or five percent of annual production, due to unscheduled downtime and poor quality. ARC estimates that almost 80 percent of these losses are preventable, with 40 percent largely due to operator error."ARC Insight June 10 2010Operator error? How much because of acting on invalid information, or plugged lines, failed sensors, failure to detect a “flatlining” level?
  • Discuss mode shedding, bumpless transfer, anti-reset windup, behavior of loops when limited, etc.We are ALREADY relying on the field devices – when your positioner is having a bad day, the “loop” will have troubles NO MATTER WHERE PID is solved.
  • The SAME PEOPLE that forged the robust DCS of the 80’s and 90’s were active on the SP50 committee, and infused their lessons learnt into the design of the user layer. NOT ONLY are there fewer things to fail and no dependence on the DCS or its infrastructure, EVERY TRANSMISSION (nominally, once a second or faster) communicates signal status (good, uncertain, bad, limited, etc.) and the FF FB’s do SMART THINGS by default (shed to manual, bumpless transfer) as well as providing for configurable behaviour (e.g., propagate fault forward and fault state to value).Everyone know what is meant by mode shedding? (go to a safe or stable mode, e.g. manual, on a fault)Everyone know what is meant by bumpless transfer? (smoothly transition from MAN > AUTO > CAS, for example) What is a “bump”? (it’s an upset, a process upset)Any lunatics in the crowd, do not obtain a firearm and shoot your instruments.Compare reliability of components for control in DCS (= Process Control System) vs CIF (Control in the Field)Examine the established MTTF (mean time to failure) figures for each of the components in the systemFor control in the DCS (LHS diagram) there are more partsTransmitterValveCablesTerminationsPower SupplyAI CardAO CardBackplaneControllerController Power SupplyFor CIF (Control in the Field) there are fewer partsTransmitterValveCableTerminations (<half)Fieldbus Power Supply
  • CIF is as reliable as the devices – which you’re relying on already.Plus, FF user layer adds in-built standard thoughtful features to further improve process integrity, availability, and robustness.Value and Status known EVERY (macro)CYCLE – e.g. once, twice, 4 times A SECOND, at the prescribed time within < 1 millisecond. NO OTHER BUS CAN CLAIM THIS.YOU JUST DON’T GET THIS WITH “POLLED” DATA!Status Propagation - GRACEFUL DEGRADATION / FAULT TOLERANCE IS BUILT-IN, STANDARD!ASK: Who would say self-inflicted screw-ups are more common than random hardware or software faults?ASK: What’s a common self-inflicted screw-up in your plant? (Bubba goes out to work on FT-88204 and instead gets on FT-88203) The SECOND he powers it down, any associated loop will shed to manual WITHIN THE SAME MACROCYCLE.Windup protection – “Limited” status propagates to slave PID, up to cascade master if implemented.Bumpless Transfer – hooks are there to prevent process upsets from mode changes.Fault state – configurable; default is “hold last position” = highest availability.
  • Can we apply Safety Integrity Level (SIL) calculations to evaluate basic controls? The same components make up both “loops” – why not?We asked Marszal of Kenexis Consultants to run the numbers, the same way they do for SIL analyses (a sample SIL analysis is shown)Point out that that the FIELD DEVICES (red and yellow pie slices) contribute most to PFD (probability to fail dangerous) and Spurious trip rate.ASK: What should be improved to increase reliability? (answer = devices, diagnostics)
  • Slide 43 shows fault tree for analogue system with control in DCSMTTF = 15.9 yearsSlide 44 shows fault tree for FOUNDATION Fieldbus with Control in FieldMTTF = 48.2 yearsEquipment less prone to fail because of predictive intelligence and proactive intelligenceAbsolute values aren't realistic - as input comes from safety figures; but relative values are robustI’ve heard of projects where “all the critical loops are left 4-20 mA”. In light of this analysis, would you agree to such a strategy?
  • Shin-Etsu plant, the card in DCS (computer) failed, but instead of shutting down the plant, able to use Control in the Field for direct communication between measurement device and valve - continue operating while change card in DCS - avoid shutdownThis is the inherent back-up capability of CIF
  • Following findings based on study by Industrial Systems and Control Ltd - 'Control in the Field: Analysis of Performance Benefits'Industrial Systems and Control Ltd is spin-off from Strathclyde UniversityControl Engineering Consultancy, and TrainingStudy instigated by Fieldbus Foundation EMEA
  • Compare CIF:This example has a Rosemount 8800D FlowmeterFisher Valve Positioner20ms - AI execution in flowmeter30ms - data transfer from flowmeter to PID (proportional–integral–derivative controller) in valve positioner30ms - PID execution in valve positioner25ms - execution of instruction in valve positionerTotal latency - 105msWhich is lower than sample rate - 150ms (how often sample for instructions)With Control in Process Control System:For same set up but with control in process control system, not field20ms - AI execution in flowmeter30ms data transfer from flowmeter to PID in PCS20ms - PID execution in PCS30ms - data transfer from PCS to valve positioner25ms - execution of instruction in valve positionerTotal latency - 125msBut, asynchronous, so latency increases to 625ms because of jitter [ask for clarification on how this works if needed]Sample rate is 500msAssessment was made in simulationA simple continuous process model, coupled to a discrete PI controllerIMPORTANT - controllers tuned to same stability to allow comparisonRepeated for different process dynamicsSpeed of response to set point change and disturbance rejection assessedResults follow on next few slides
  • Only show Setpoint, case 1 - CIF, and case 3 - asyncDifference in performance is difference in time taken for process output to settle at 60% for 1 and 3
  • Only show Setpoint, case 1 - CIF, and case 3 - asyncDifference in performance is difference in time taken for process output to settle at 60% for 1 and 3
  • Only show Setpoint, case 1 - CIF, and case 3 - asyncDifference in performance is difference in time taken for process output to settle at 60% for 1 and 3
  • Only show Setpoint, case 1 - CIF, and case 3 - asyncDifference in performance is difference in time taken for process output to settle at 60% for 1 and 3
  • Only show Setpoint, case 1 - CIF, and case 3 - asyncGraph shows ability of process control to get process output to return to setpoint without deviating to greatly when disturbed[e.g. like ability of refrigerator to get back to 2 degrees without going too hot or cold when the door is opened a lot]CIF deviates far less than case 3, which ends up quite far from the set-pointFor fast process loops (e.g. flow, some temperature) CIF provides 40-60% faster settling time than asynchronous control in the DCS
  • “Control in the field: analysis of performance benefits” study from ISC (industrial systems and control)http://www.isc-ltd.com/
  • If control is not tight, need to leave large margin for error in case disturbance means values go beyond control limitBecause control is tighter with CIF, it's possible to set setpoint much nearer to the control limitCan push control limits harder - aiding e.g. energy efficiency / product quality /raw material utilisation
  • Great for theoretical and simulation, what about real world empirical tests?
  • The test bed was a liquid pressure control in the supplier’s flow lab – some decent sized valves and pipe.Using high-speed monitoring / recording, they measured the “response time” defined as the time from a detectable disturbance or change in the PV, until the air signal to the valve began to change from steady state.RED line is Control-in-fieldYELLOW line is pure 4-20 mA (on their host only)BLUE / GREEN is control in host with various ratios of controller cycle time / FF macrocycle time.DASHED line is response period = configured cycle timeCONTROL RESPONSE PERIOD defined as the time from the introduction of a disturbance (measurement deviation) to detecting a signal to the valve actuator (change in pneumatic signal) using high-speed sensors and recording / test equipment (NOT DCS TRENDS).
  • Like Shell Global Solutions says, when you do control in the controller, you consume a lot of the segment cycle time (macrocycle) UNNECESARILY. FOUNDATION™ Fieldbus is DESIGNED for control in field devices – It’s equally or more reliableIt’s equal or faster than 4-20 mAYou’re relying on the devices ANYHOW . . . When users and their consultants insist on old-school CIC, it’s like choosing an outstanding steak and cooking it WELL DONE . . . It takes too long and when you eat it, you wonder “what’s so great about this? Might as well just get some ole’ HART Hamburger!
  • I don’t know of anyone who actually would install 16 devices including 8 control valves (loops) on a single segment, but this graphic shows that with judiciously chosen instruments, even EIGHT simple loops can execute in less than 0.5 seconds.Q: What do I mean by “judiciously chosen instruments”?(Choose the ones that can go fast) “Cadillac” shown, but Fisker-Karma and Toyota are catching up.
  • Benefits of foundation in the operational phase

    1. 1. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 1 Field-based Control : Freedom to Choose the Ultimate DCS John Rezabek Process Control Specialist Ashland, Inc.
    2. 2. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 2 What is Fieldbus Again? • Fieldbus is a communication protocol, right? • The “OSI Model”: Seven Layers for “Open Systems Interconnection” • Foundation fieldbus uniquely adds the “User Layer” – why? Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer USER LAYER
    3. 3. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 3 Why Have a “User Layer”? • Analog In, Analog Out, Digital In, Digital Out • Do some scaling, linearization, filtering, configure fault states & propagation, etc. outside the host / DCS • Devices are “peers” with the host’s interface • ARTHM, ISEL, SGCR, INT • Relieve the HOST DCS of some compensation, characterization, totalizers, etc. • PID, CSEL, SPLTR • Configure CONTROL IN FIELD DEVICES, using the world’s only vendor-independent totally distributed digital control system
    4. 4. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 4 Why “Distributed” Control? • 1980’s costs for CPU and memory meant many loops-per-controller for reasonable “cost per loop” • Users of the day wanted “single loop integrity” = one fault > loose no more than one loop • Controller, I/O, and Network Redundancy • Commercial model became “functionally” distributed
    5. 5. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 5 The Need For Reliability ARC Insight 10th June 2010 The global process industry loses $20 billion, or five percent of annual production, due to unscheduled downtime and poor quality. ARC estimates that almost 80 percent of these losses are preventable, with 40 percent largely due to operator error.
    6. 6. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 6 Fieldbus = Truly Distributed Control • Foundation Fieldbus allows return to true single-loop integrity • Robust control independent of DCS, needs only power, devices, and schedule (“LAS”) • Intelligence of devices is exploited, employing signal status, diagnostics • But – is it truly more reliable?
    7. 7. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 7 Fewer Components = Longer MTBF PSU PSU  AND Foundation fieldbus also communicates signal status with every scan, which enables smart mode shedding, bumpless transfer and initialization.
    8. 8. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 8 Digital All the Way - Digital Closed Loop Control Fieldbus eliminates the analog signal Validity: Good/Bad/ Uncertain Initiate Fault- State Command Limited: High/Low Bumpless Transfer Windup Protection Fault-state Status Designed for process control Default behavior on Bad or Uncertain PV is shed mode to MAN and hold last output – the same macrocycle the fault is detected.
    9. 9. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 9 Process Variable Validity Hardwired  Device drives current <4 mA or >20 mA on device failure – Looks like process problem  PID counteracts thus tripping the loop  Operator cannot tell the difference between a process alarm and a device alarm Fieldbus  Device health indicated by associated status  Controller holds last position on device failure – Shutdown is optional  Operator can easily distinguish process problem from a device problem
    10. 10. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 10 FF – A Standard for High Availability  Distributed Control, Speed, & Determinism  Real-time value and status  Status Propagation (CIF & CIC)  Validated information Quality and limits  Windup protection  Bumpless transfer for valves  Fault-state for valves  etc.
    11. 11. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 11 SIL - Putting a Number on Reliability • Ed Marszal, ISA 84.001 and IEC 61508 expert, author and president of Kenexis consultants • Entered reliability numbers into reliability models for safety system calculations • “ FF is significantly better – MTTF (mean time to fail) of 48.2 [years] versus MTTF of 15.9 [years]”
    12. 12. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 12 Reliability Analogue With CIF 15.9y 48.2y MTTF
    13. 13. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 13 Customer Experience CIF enabled
    14. 14. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 14 Customer Experience FOUNDATION Fieldbus CIF with inherent backup capability prevented 2 incorrect plant shutdowns, which would have resulted from communication interruptions.
    15. 15. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 15 But . . . Isn’t Fieldbus “Slow”? • Early devices had longer function block execution times • Many early jobs used FF for IO only • Host PID to FF Macrocycle sync is not easily addressed – oversample? • Variable latencies, overly busy segments • Add commissioning, diagnostics, downloads • = Many bogged down segments
    16. 16. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 16 Control In The Field Study
    17. 17. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 17 Comparison CIF enabled No CIF 125 625 105 ms
    18. 18. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 18 Settling Times: Fastest Process 50 54 58 62 Process output (%) 0 4 8 12 16 20 Time (seconds) 60% Setpoint Case 1 - CIF Case 3 - Control in DCS (async)
    19. 19. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 19 Settling Times: Very Fast Process 50 54 58 62 Process output (%) 0 4 8 12 16 20 Time (seconds) Setpoint Case 1 - CIF Case 3 - Control in DCS (async) 55%
    20. 20. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 20 Settling Times: Fast Process 50 54 58 62 Process output (%) 0 4 8 12 16 20 Time (seconds) Setpoint Case 1 - CIF Case 3 - Control in DCS (async) 66%
    21. 21. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 21 Settling Times: Medium Process 50 54 58 62 Process output (%) 0 4 8 12 16 20 Time (seconds) Setpoint Case 1 - CIF Case 3 - Control in DCS (async) 39%
    22. 22. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 22 Presence Of Disturbance: Fastest Process Process output (%) 40 50 60 70 80 90 100 Time (seconds) Setpoint Case 1 - CIF Case 3 - Control in DCS (async) 56 58 60 66 62 64
    23. 23. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 23 Presence Of Disturbance: Different Processes Fastest 1.811 2.132 0.517 0.82 0.642 1.058 0.231 0.53 65% better 50% better 55% better 35% better Very Fast Fast Medium Setpoint CIF 1.811 control in DCS (async)
    24. 24. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 24 ISC Study on Control in the Field “Control in the field: analysis of performance benefits” study from ISC (industrial systems and control) 40-60% faster settling time on setpoint change 30-50% better disturbance rejection Most significant for fast processes (flow and pressure)
    25. 25. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 25 Impact of Tighter Control Loop Manual Pneumatic Analogue Digital Control limit
    26. 26. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 26 Recommended For Fast Loop Response Control in the field using FOUNDATION fieldbus technology is recommended by SGSI for simple and cascading loops, not for complex loops. Major benefits identified by SGSI are reduced process controller loading, reduced network traffic enabling more loops per segment, as well as very fast loop response.
    27. 27. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 27 Empirical Studies: CIF Fast as 4-20 Host Host Host Host Study by Daugherty, Coughran, and Ford, “Effects of Macrocycle Time and Sampling Rates on Control Loop Performance”
    28. 28. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 28 8 simple loops, CIC Study by Daugherty, Coughran, and Ford, “Effects of Macrocycle Time and Sampling Rates on Control Loop Performance”
    29. 29. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 29 Same 8 loops, CIF (valve positioner) Study by Daugherty, Coughran, and Ford, “Effects of Macrocycle Time and Sampling Rates on Control Loop Performance”
    30. 30. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 32 Blower – Combustion Air to Boiler • Keeping the Blower out of the surge region means a more stable flow • More stable flow allows running closer to constraints • Running closer to constraints means less fuel, CO2 AIR TO BOILER AIR BLOWERPREHEATER TOBURNERS
    31. 31. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 33 Inside “peak pressure” line, flow is unstable Peak Pressure Desired Flow fell here
    32. 32. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 34 Field-based control of blow-off valve • Controls total flow • Allows operation close to low-flow BMS trip point • Saves fuel costs and reduces CO2 emissions
    33. 33. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 35 Surge Control Scheme Blow-off Flow Flow to Burner FIC Inlet Louvers Σ FIC (Total Flow) Blow-off Valve Discharge Pressure SGCR SP PT (Discharge) FT1 FT2 Blow-off Valve Pos. Inlet Louver Positioner
    34. 34. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 36 Blower Anti-surge scheme Flow to Burner Total Flow Setpoint Total Flow Controller
    35. 35. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 38 10,000 HP Turbine / Blower Anti-surge • Large critical un-spared asset • Typically runs at 5000 to 7000 RPM • Expensive to repair; lost production even more expensive
    36. 36. 2012 End User Seminar Fieldbus Foundation © 1994 – 2012 Fieldbus Foundation 44 Summary • FF was specified & designed to deliver robust control by default – STANDARDIZED across suppliers • Control in Field Devices is being used in critical process applications with fast cycle time requirements • Modern devices are executing function blocks faster than ever • (note: like cars, there are Yugos and Porches, so choose according to your needs)

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