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Maximizing the return on your control investment meet the experts sessions part 1

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The design and commissioning of the controls associated with a continuous or batch process directly impact plant operating efficiency and production quality and throughput. In this session we review techniques that may be used to identify control opportunities to reduce production costs, minimize variations in product quality and to maximize production within the limits set by market demand. Several common application examples from the process industry will be used to illustrate how plant production rate and product quality are directly influenced by process control variation and constraints in plant operation. Starting with an assessment of control loop utilization and automatic control performance, a step by step process is outlined that may be used to identifying and addressing areas where it is possible to justified the time and material costs required to improve control performance. In particular, information will be provided on how to quickly tune single loop control of self-regulating or integrating process and to recognize when variations in control loop performance are not associated with loop tuning. An overview will be provided of tools and techniques that may be used to achieve best control performance over a wide variety of operating conditions. Also, guidance will be provided on when it is possible to justify the cost associated with the installation and commissioning of multi-loop techniques such as feedforward control, ratio and override control. The steps required to commission multi-loop control strategies will be address along with common mistakes to avoid. Also, input will be provided on how to recognize when advanced control techniques such as Fuzzy logic or MPC are needed to achieve the desired control performance. At the end of this session a drawing will be held to give away 10 copies of “Control Loop Foundation – Batch and Continuous Processes”. Many of the ideas discussed in this session are addressed in this book.
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Maximizing the Return on Your Control Investment Part 1 of 2 Meet the Experts Sessions
Transcript
Page 1: Maximizing the return on your control investment meet the experts sessions part 1

Maximizing the Return on Your

Control InvestmentPart 1 of 2

Meet the Experts Sessions

Page 2: Maximizing the return on your control investment meet the experts sessions part 1

Presenters

James Beall

Terry Blevins

Page 3: Maximizing the return on your control investment meet the experts sessions part 1

Session Objective

Provide a roadmap that may be used to maximize the return on your control investment

Page 4: Maximizing the return on your control investment meet the experts sessions part 1

Agenda

Assessment of control loop utilization and automatic control performance,

Identifying areas where it is possible to justified the cost of improving control performance.

Tools and techniques to achieve best control performance. Identifying and correcting field problems.

Page 5: Maximizing the return on your control investment meet the experts sessions part 1

Evaluating Control System Utilization

Product quality and manufacturing efficiency may be impacted by variation in key parameters.

When production is process limited, then throughput may be increase by reducing process variation and operating closer to the limit.

The control must operate as designed to achieve these benefits.

Page 6: Maximizing the return on your control investment meet the experts sessions part 1

Control Utilization

Control utilization is an indicator that can be used to quickly determine if control and measurement problems exist within a control system. Surveys indicate that the primary reasons for control not being fully utilized fall into two areas:

Field measurement or control element

Process or control design

An immediate improvement in control utilization will be achieved by addressing these problems. To achieve full utilization, improved communications between maintenance and operations is important.

Page 7: Maximizing the return on your control investment meet the experts sessions part 1

Example – Case StudyAt one pulp and paper plant a snapshot of the control utilization was collected to quantify the state of the process control. This survey showed:

Control Normal Loops Mode Utilization

Bleach Plant 78 60 76%

Power House 185 130 70%

Pulp Mill 174 116 66%

Paper Mill 236 134 56%

An instrumentation team was formed to investigate loops that were not running in their normal design mode. This team was responsible for making sure measurement, control valve, and process problems were addressed in a timely fashion. The reduction in variability led to significant improvements in plant throughput and product quality

Page 8: Maximizing the return on your control investment meet the experts sessions part 1

Example – Another Case Study

At refinery and petrochemical complex a snapshot of the control utilization was collected to quantify the state of the process control. This survey showed:

System Loops Utilization PX 471 67.3% APS&VPS、CLE、Sulfur Recovery 469 59.7% Refinery 478 60.9% IGCC\Auxiliary Boiler 946 52.7% Ethylene 1355 77.5% FCCU 475 48% C4 164 68.9%

Page 9: Maximizing the return on your control investment meet the experts sessions part 1

Examining Control Utilization

Summarizes performance for System or by Area, Cell, or Units

Abnormal Control Conditions indicated for Problem Loops:

– Control Service Status:• Not in Normal mode• Limited control output• Bad/Uncertain input

– Control Performance Status:• Standard Deviation• Variability Index• Oscillation Index• Tuning Index

Page 10: Maximizing the return on your control investment meet the experts sessions part 1

DeltaV InSight Control Performance Reports

Page 11: Maximizing the return on your control investment meet the experts sessions part 1

Roadmap to Improved Control

Control Loop

in Normal

Mode?

Starting

Point

Use DeltaV Insight to

Determine Control

Utilization

No

Yes

Reason?

Transmitter Broken/UnreliableFix or Replace

Transmitter

Poor

Performance

Investigate

Tuning

Valve/Actuator Problem

Changing Process Gain

Process Dynamics

Loop Interaction

Use DeltaV Insight to

examine loop tuning

Page 12: Maximizing the return on your control investment meet the experts sessions part 1

DeltaV Insight – On-demand Tuning

Allows control loop tuning to be quickly established

Page 13: Maximizing the return on your control investment meet the experts sessions part 1

Examine Tuning Impact

Page 14: Maximizing the return on your control investment meet the experts sessions part 1

DeltaV InSight - Adaptive Tuning

InSight automatically calculates dynamic models from operator changes

Model Quality and Learning Status

Tuning criteria and desired speed of response

Tuning Recommendation

No Testing Required

Provides Tuning Based on Normal Operator Changes

Before vs. After

Page 15: Maximizing the return on your control investment meet the experts sessions part 1

Tips for Using On-Demand Tuning

Set step size large enough to insure the response to the PID output changing is larger that the measurement noise and influence of process disturbances.

Select “Integrating” if the process is not self-regulating.

If the process has low or high gain, then selected Default Process and set the process type to match the expected behavior.

Page 16: Maximizing the return on your control investment meet the experts sessions part 1

Manual Tuning (Not a DeltaV System)

When commissioning PID control associated with a self-regulating process, this procedure may be quickly applied to both old and new control systems to determine the tuning for PI control.

The size of the step should be just large enough to easily distinguish the resulting change in the controlled process output.

Page 17: Maximizing the return on your control investment meet the experts sessions part 1

Manual Tuning - (Not a DeltaV System) Alternate

When commissioning PID control associated with a self-regulating process, this procedure may be quickly applied to both old and new control systems to determine the tuning for PI control.

The size of the step should be just large enough to easily distinguish the resulting change in the controlled process output.

Tune as follows:– Place the PV and OUT on a trend.– Place the controller in Manual and allow the process to reach steady sate.– Impose a step change in OUT and observe the response– Set the RESET equal to ¼ of the time it takes to “almost” line out (98% of final

value)– Set the GAIN equal to 1/3 of process gain, Kp where– Kp = ((Δ%PV) / (Δ%OUT) (Be sure to convert to % of span!)– Place the loop in automatic and make small adjustments to the Setpoint and

observe the response. Adjust ONLY the GAIN to achieve the desired response.

Page 18: Maximizing the return on your control investment meet the experts sessions part 1

Impact of Sticky Valve When a control loop is

placed in automatic control, it is easy to detect if a valve or damper is not responding to the control system by observing the response of the controlled parameter to control system changes in the PID output.

Cycling can not be eliminated through tuning.

Setpoint (SP)

Controlled Parameter (PV)

Implied Valve Position (OUT)

Stem Position)

Value

Time

Page 19: Maximizing the return on your control investment meet the experts sessions part 1

Valve Positioner Recommendation

The most common problems in commissioning a control system can often be traced to the fact that a positioner has not been provided with the valve, or the positioner provided with the valve has not been properly installed or has malfunctioned.

– The rule of thumb is that to achieve best control performance, all regulating valves should be equipped with a positioner.

– Without a positioner, the control performance that may be achieved is very limited when a valve is sticking – which is inherent in most valves.

– The cyclic behavior caused by a sticky valve (with no positioner) cannot be eliminated through tuning. Changes in tuning will only impact the period of the cycle that develops. The only way to eliminate this type of behavior is to install a valve positioner.

Page 20: Maximizing the return on your control investment meet the experts sessions part 1

Installed Characteristics.

From a control perspective, it is highly desirable that the process gain be constant. If the process gain is constant, then the same proportional gain may be used over the entire operating range of the control loop.

If the valve characteristic was not been selected based on the process requirements, then the installed characteristic could be non-linear.

As illustrated in this example, the process gain varies from 0.5 to 4; that is, the process gain changes by a factor of eight.

7000

6000

5000

4000

3000

2000

0

Ga

s F

low

(S

CF

H)

0 20 40 60 80 100

Valve Position (%)

Page 21: Maximizing the return on your control investment meet the experts sessions part 1

Impact of Non-linear Installed Characteristics

Non-linear Process Example Process gain and dynamics may change as a function of

operating conditions such as valve position or feed rate.

Page 22: Maximizing the return on your control investment meet the experts sessions part 1

Linearizing Response

To compensate for the changes in process gain, a characterizer block may be installed between the PID and Analog Output blocks. Select the option to inverse the calculation done in the forward path for the PID back calculation to allow bumpless transfer.

Page 23: Maximizing the return on your control investment meet the experts sessions part 1

Figure 12-8

100

90

80

70

60

50

40

30

20

10

0

Pro

cess

Out

put

(%

)

0 10 20 30 40 50 60 70 80 90 100

Valve Position (%)

Characterizer

IN OUT

0 0

5 1.5

10 3

15 5.5

20 8,5

25 10

30 11.5

35 13

40 17

45 20

50 22

55 26

60 31

65 36

70 42

75 48

80 54

85 63

90 74

95 86

100 100

Linear Relationship

Characterizer In-Out

Characterizer Setup The relationship between

the primary inputs and the output of the characterizer block may be defined by 21 x,y pairs over the final control element operating range.

Input values that fall between these points are automatically determined by the characterizer block using linear interpolation.

The curve defined by the characterizer points appears as the inverse of the plot of the final control element installed characteristic.

Page 24: Maximizing the return on your control investment meet the experts sessions part 1

Identifying Model - DeltaV InSight

Last 200 Models automatically stored for each control loop in a model database

Various plot options to analyze impact of operating conditions on process models

Average of selected models may be utilized to establish the recommended tuning

Gain Process Insight with Model Analysis

Page 25: Maximizing the return on your control investment meet the experts sessions part 1

Non-linear Process Models

Page 26: Maximizing the return on your control investment meet the experts sessions part 1

DeltaV InSight – Adaptive Control

Page 27: Maximizing the return on your control investment meet the experts sessions part 1

DeltaV InSight – Adaptive Control

Adaptive Control Enabled

Page 28: Maximizing the return on your control investment meet the experts sessions part 1

Split Range Control

One of the most common ways of addressing multiple process inputs is known as split-range control.

The splitter block may be used to define a fixed relationship between the controller output and each manipulated process input – appearing as one valve to the PID block

The setup must account for the gain associated with each process input to achieve consistent control behavior.

Page 29: Maximizing the return on your control investment meet the experts sessions part 1

Example – Steam Header Pressure

To allow the plant to continue operation if the turbine or generator fails and must be shut down, pressure reducing valves (PRVs) between the high pressure header and the lower pressure header may be adjusted to meet the lower pressure header steam demand and to maintain the header pressure constant.

This may be accomplished by using a splitter block in conjunction with a PID block to adjust the pressure reducing valves.

Page 30: Maximizing the return on your control investment meet the experts sessions part 1

Steam Header – Splitter Characterization If the valve sizes or operating conditions for the valves are different, then it is

necessary to characterize the splitter to compensate for these differences. For example, if the flow ratings in thousand of pounds per hour, KPPH, of the

valves used in split-range control were as shown below:– Valve 1 flow rating = 50 KPPH– Valve 2 flow rating = 150 KPPH

Then the controller output range of adjustment associated with Valve 1 would be:

Page 31: Maximizing the return on your control investment meet the experts sessions part 1

Interactive Loops

The fighting between interactive loops is most often addressed by simply detuning one of the control loops by reducing the proportional gain.

The valve associated with the detuned loop will change very slowly. Thus, the two loops will tend not to interact but at the expense of the detuned loop having slow response.

Mixing

ProcessAdditive

Valve(M2)

Feed

Valve(M1)

Composition

Controller

Flow

Controller

Outlet Flow

(C1)

Composition

(C2)

C2

M1

C1

M1

C2

M2

C1

M2

Page 32: Maximizing the return on your control investment meet the experts sessions part 1

Roadmap to Improved Control

Normal

Mode?

Starting

Point

Determine

Control

Utilization

No

Yes

Reason?

Transmitter Broken/UnreliableFix or Replace

Transmitter

Poor

Performance

Investigate

Tuning

Valve/Actuator Problem

Changing Process Gain

Split range Setup incorrect

Process Dynamics

Loop Interaction

Variation on

Control

Monitor for

Change

Low/acceptable

High

Unacceptable

InvestigateChanging Process Gain

Process Disturbance

Process Dynamics

Loop Interaction

Changing Limit Condition

Page 33: Maximizing the return on your control investment meet the experts sessions part 1

Utilize Process Capacity to Absorb Variability

Lambda

PV Back to SP in 6 x Lambda

Step change in load (inflow)

Controller Output changing outflowsmoothly!

PV

Setpoint

Inflow

Outflow

LIC

Outflow = inflow

Change in PV stopped

Page 34: Maximizing the return on your control investment meet the experts sessions part 1

Utilize Process Capacity to Absorb Variability

Choose the arrest time “slow” enough to provide a variability sink yet maintain level within the allowable variation

Lambda = __2 * ALV___ Kp * MLD

– ALV = Allowable Level Variation

– Kp = Integrating process gain

– MLD = Maximum Load Disturbance (converted to % of level controller output scale)

Page 35: Maximizing the return on your control investment meet the experts sessions part 1

Utilize Process Capacity to Absorb Variability

Level

Manipulated

Variable

Before

Level

Manipulated

Variable

After

Page 36: Maximizing the return on your control investment meet the experts sessions part 1

Reducing Control Variation

When tuning is not sufficient to achieved the desired level of variation in critical control parameter or to maintain it at an operating limit, then multi-loop techniques may sometimes be applied to improve control. Three common multi-loop techniques are:

Feedforward Control Cascade Control Override Control

Page 37: Maximizing the return on your control investment meet the experts sessions part 1

Summary

An on-line measurement of control utilization and variability is provided by DeltaV Insight.

Exploring the causes of poor utilization is the first step in resolving measurement, actuator or control issues.

When single loop control is not sufficient to achieve the desired level of control the multi-loops solutions should be explored.

Page 38: Maximizing the return on your control investment meet the experts sessions part 1

Where To Get More Information

Many of the ideas discussed in this session are addressed “Control Loop Foundation – Batch and Continuous Processes”. Information on this book may be found at the book’s web site:

– http://controlloopfoundation.com/

Also, by going to this web site you can use your web browser to perform the 19 workshops that go with this book.


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