Benefits of foundation in the operational phase

2012 End User SeminarFieldbus Foundation

© 1994 – 2012 Fieldbus Foundation1

Field-based Control : Freedom to Choose the Ultimate DCS

John Rezabek

Process Control Specialist

Ashland, Inc.



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



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



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



The Need For Reliability

ARC Insight

10th June 2010

The global process industry loses $20billion, 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.



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?



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.



Digital All the Way- Digital Closed Loop Control

Fieldbus eliminates the analog signal

Validity:Good/Bad/Uncertain

Initiate Fault-State Command

Limited: High/LowBumpless TransferWindup ProtectionFault-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.



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



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.



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]”



Reliability

Analogue

With CIF

15.9y

48.2y

MTTF



Customer Experience

CIF enabled



Customer Experience

FOUNDATION FieldbusCIF with inherent backup capability

prevented 2 incorrect plant shutdowns, which would have resulted from communication

interruptions.



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



Control In The Field Study

Control in the Field:

Analysis of Performance Benefits



Comparison

CIF enabled

No CIF

125

625

105

ms



Settling Times: Fastest Process

50

54

58

62

Process output (%)

0 4 8 12 16 20Time (seconds)

60%

Setpoint Case 1 - CIFCase 3 - Control in DCS (async)



Settling Times: Very Fast Process

50

54

58

62

Process output (%)

0 4 8 12 16 20Time (seconds)


55%



Settling Times: Fast Process

50

54

58

62

Process output (%)

0 4 8 12 16 20Time (seconds)


66%



Settling Times: Medium Process

50

54

58

62

Process output (%)

0 4 8 12 16 20Time (seconds)


39%



Presence Of Disturbance: Fastest Process

Process output (%)

40 50 60 70 80 90 100Time (seconds)


56

58

60

66

62

64



Presence Of Disturbance: Different Processes

Fastest1.811

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 CIF1.811 control in

DCS (async)



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)



Impact of Tighter Control Loop

Manual

PneumaticAnalogue

Digital

Control limit



Recommended For Fast Loop Response

Control in the field usingFOUNDATION 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.



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”



8 simple loops, CIC




Same 8 loops, CIF (valve positioner)




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

TO

BU

RN

ER

S



Inside “peak pressure” line, flow is unstableInside “peak pressure” line, flow is unstable

Peak Pressure

Desired Flow fell here



Field-based control of blow-off valveField-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



Surge Control SchemeSurge Control Scheme

Blow-off Flow

Flow to Burner

FICInlet

Louvers

Σ FIC(Total Flow)

Blow-off Valve

Discharge Pressure SGCR SP

PT (Discharge)

FT1

FT2

Blow-off Valve Pos.

Inlet Louver Positioner



Blower Anti-surge schemeBlower Anti-surge schemeFlow to Burner

Total Flow Setpoint

Total Flow Controller



10,000 HP Turbine / Blower Anti-surge10,000 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



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)

Technology

Benefits of foundation in the operational phase