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Effective Use of Measurements, Valves, and PID ControllersISA Edmonton Conference 4-18-2012

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Presenter

– Greg is a retired Senior Fellow from Solutia/Monsanto and an ISA Fellow. Greg was an adjunct professor in the Washington University Saint Louis Chemical Engineering Department 2001-2004. Presently, Greg contracts as a consultant in DeltaV R&D via CDI Process & Industrial and is a part time employee of Experitec and MYNAH. Greg received the ISA “Kermit Fischer Environmental” Award for pH control in 1991, the Control Magazine “Engineer of the Year” Award for the Process Industry in 1994, was inducted into the Control “Process Automation Hall of Fame” in 2001, was honored by InTech Magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of 20 books on process control, his most recent being Advanced Temperature Measurement and Control. Greg has been the monthly “Control Talk” columnist for Control magazine since 2002 and has started a Control Talk Blog. Greg’s expertise is available on the Control Global and Emerson modeling and control web sites: http://community.controlglobal.com/controltalkbloghttp://modelingandcontrol.com/author/Greg-McMillan/

3

Resources

2005 2010 2011

• “Without deadtime I would be out of a job”• Fundamentals

– A more descriptive name would be total loop deadtime. The loop deadtime is the amount of time for the start of a change to completely circle the control loop and end up at the point of origin. For example, an unmeasured disturbance cannot be corrected until the change is seen and the correction arrives in the process at the same point as the disturbance.

– Process deadtime offers a continuous train of values whereas digital devices and analyzers offer non continuous data values at discrete intervals, these delays add a phase shift and increase the ultimate period (decrease natural frequency) like process deadtime.

• Goals– Minimize delay (the loop cannot do anything until it sees and enacts change)

• Sources – Pure delay from process deadtimes and discontinuous updates

– Piping, duct, plug flow reactor, conveyor, extruder, spin-line, and sheet transportation delays (process deadtimes set by mechanical design - remaining delays set by automation system design)

– Digital device scan, update, reporting, and execution times (0.5∗ΔT)– Analyzer sample processing and analysis cycle time (1.5∗ΔT)– Sensitivity-resolution limits– Backlash-deadband

– Equivalent delay from lags– Mixing, column trays, dip tube size and location, heat transfer surfaces, and volumes in series (process

lags set by mechanical design - remaining lags set by automation system design)– Thermowells– Electrodes – Transmitter damping – Signal filters

(1) - Delay

Top Ten Concepts

• “Speed kills - (high speed processes and disturbances and low speed control systems can kill performance)”

• Fundamentals– The rate of change in 4 deadtime intervals is most important. By the end of 4 deadtimes,

the control loop should have completed most of its correction. Thus, the short cut tuning method (near-integrator) is consistent with performance objectives.

• Goals– Make control systems faster and make processes and disturbances slower

• Sources– Control system

– PID tuning settings (gain, reset, and rate)– Slewing rate of control valves and velocity limits of variable speed drives

– Disturbances– Steps - Batch operations, on-off control, manual actions, SIS, startups, and shutdowns– Oscillations - limit cycles, interactions, and excessively fast PID tuning– Ramps - reset action in PID

– Process– Degree of mixing in volumes due to agitation, boiling, mass transfer, diffusion, and migration

(2)- Speed

Top Ten Concepts

• “All is lost if nothing is gained”• Fundamentals

– Gain is the change in output for a change in input to any part of the control system. Thus there is a gain for the PID, valve, disturbance, process, and measurement. Knowing the disturbance gain (e.g. change in manipulated flow per change in disturbance) is important for sizing valves and feedforward control.

• Goals– Maximize control system gains (maximize control system reaction to change) and

minimize process and disturbance gains (minimize process reaction to change).• Sources

– PID controller gain – Inferential measurements (e.g. temperature change for composition change in distillation

column) – Slope of control valve or variable speed drive installed characteristic (inherent

characteristic & system loss curve)– Measurement calibration (100% / span). Important where accuracy is % of span– Process design– Attenuation by upstream volumes (can be estimated)– Attenuation by upstream PID loops (transfer of PV variability to controller output)

• For a discussion of unifying concepts check out Deminar #9 “Process Control Improvement Primer” Sept 8, 2010 Recording

– Go to “Deminars” under “Archives” heading on top of web page– http://modelingandcontrol.com/author/Greg-McMillan/

(3) - Gain

Top Ten Concepts

(4) - Resonance

• “Don’t make things worse than they already are”• Fundamentals

– Oscillation period close to ultimate period can be amplified by feedback control• Goals

– Make oscillation period slower or control loop faster • Sources

– Control loops in series with similar loop deadtimes (e.g. multiple stage pH control)

– Control loops in series with similar tuning and valve stiction and backlash– Day to night ambient changes to slow loops (e.g. column temperature control)

Top Ten Concepts

(4) - Resonance

Top Ten Concepts

1

UltimatePeriod

1

1FasterTuning

Log of Ratio ofclosed loop amplitudeto open loop amplitude

Log of ratio ofdisturbance periodto ultimate period

no attenuationof disturbances

resonance (amplification) of disturbances

amplitude ratio isproportional to ratio ofbreak frequency lag to

disturbance period

1

no better than manual worse than manual improving control

For all of you frequency response and Bode Plot Fans

(5) Attenuation

• “If you had a blend tank big enough you would not need control”• Fundamentals

– Attenuation increases as the volume of the blend tank increases and the ultimate period of the control loop decreases.

• Goals– Maximize attenuation by increasing volume and mixing and making loops faster

• Sources– Mixed volume size and degree of mixing– Control loop speed

Top Ten Concepts

f

oof

tAAτπ ∗

=2

*

The attenuation of oscillations can be estimated from the expression of the Bode plot equation for the attenuation of oscillations slower than the break frequency where (τf) is

the filter time constant, electrode or thermowell lag, or a mixed volume residence time

Equation is also useful for estimating original process oscillation amplitude from filtered oscillation amplitude to better know actual process variability

(measurement lags and filters provide a attenuated view of real world)

(5) Attenuation

Top Ten Concepts

(6) Sensitivity- Resolution

• “You cannot control what you cannot see”• Fundamentals

– Minimum change measured or manipulated - once past sensitivity limit full change is seen or used but resolution limit will quantize the change (stair step where the step size is the resolution limit). Both will cause a limit cycle if there is an integrator in the process or control system.

• Goals– Improve sensitivity and resolution

• Sources– In measurements, minimum change detected and communicated (e.g. sensor

threshold and wireless update trigger level) and quantized change (A/D & D/A)– Minimum change that can be manipulated (e.g. valve stick-slip sensitivity and

speed resolution)

Top Ten Concepts


Top Ten Concepts

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Top Ten Concepts

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(7) Hysteresis-Backlash

• “No problem if you don’t ever change direction”• Fundamentals

– Hysteresis is the bow in a response curve between full scale traverses in both directions. Normally much smaller and less disruptive than backlash

– Backlash (deadband) is minimum change measured or manipulated once the direction is changed - once past backlash-deadband limit you get full change

– Both Hysteresis and backlash will cause a limit cycle if there are 2 or more integrators in the process or control system.

• Goals– Minimize backlash and deadband

• Sources– Pneumatic instrument flappers, links, and levers (hopefully these are long gone)– Rotary valve and damper links, connections, and shaft windup – Variable speed drive setup parameter to eliminate hunting and chasing noise

Top Ten Concepts


Top Ten Concepts

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Top Ten Concepts

Backlash (Deadband)

Deadband is 5% - 50%without a positioner !

Deadband

Signal(%)

0

Stroke(%)

Digital positionerwill force valve

shut at 0% signal

Pneumatic positionerrequires a negative % signal to close valve

(8) Repeatability-Noise

• “The best thing you can do is not react to noise”• Fundamentals

– Noise is extraneous fluctuations in measured or manipulated variables– Repeatability is difference in readings for same true value in same direction – Often repeatability is confused with noise

• Goals– Minimize size and frequency of noise and do not transfer noise to process

• Sources– Noise

– Bubbles– Concentration and temperature non-uniformity from imperfect mixing– Electromagnetic interference (EMI)– Ground loops– Interferences (e.g. sodium ion on pH electrode)– Velocity profile non-uniformity – Velocity impact on pressure sensors

– Repeatability– Sensitivity and resolution

Top Ten Concepts


Top Ten Concepts

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Official definition of repeatabilityobtained from calibration tests


Top Ten Concepts

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Practical definition of repeatabilityas seen on trend charts


Top Ten Concepts

Noise as seen on trend charts

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• There is always an offset and drift, it is matter of size and consequence• Fundamentals

– The deviation of the peak in the distribution of actual values from true value– Drift shows up as a slowly changing offset

• Goals– Minimize offset and nonlinearity by smart transmitters and sensor matching and

smart tuned digital positioners with accurate internal closure member feedback• Sources

– Manufacturing tolerance, degradation, de-calibration, and installation effects (process and ambient conditions and installation methods and location)

(9) Offset-Drift

Top Ten Concepts

(9) Offset-Drift

Top Ten Concepts

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(9) Offset-Drift

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xDrift = 1% per month

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Drift (Shifting Bias)

(10) Nonlinearity• “Not a problem if the process is constant, but then again if the process is

constant, you do not need a control system”• Fundamentals

– While normally associated with a process gain that is not constant, in a broader concept, a nonlinear system occurs if a gain, time constant, or delay changes anywhere in the loop. All process control systems are nonlinear to some degree.

• Goals– Minimize nonlinearity by process and equipment design (e.g. reagents and heat

transfer coefficients), smart transmitters and sensor matching, valve selection, signal characterization, and adaptive control

• Sources– Control valve and variable speed drive installed characteristics (flat at high flows)– Process transportation delays (inversely proportional to flow)– Digital and analyzer delays (loop delay depends upon when change arrives in

discontinuous data value update interval)– Inferred measurement (conductivity or temperature vs. composition plot is a curve)– Logarithmic relationship (glass pH electrode and zirconium oxide oxygen probe)– Process time constants (proportional to volume and density)

Top Ten Concepts

(10) Nonlinearity

Top Ten Concepts

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Good Accuracy and Good Precision2-Sigma

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Accuracy and PrecisionAccuracy and Precision

Top Ten Concepts

Sensor SelectionSelection Objectives

• Maximize threshold sensitivity, resolution, and repeatability reducing undetected and extraneous changes over the whole operating range. The sensitivity of RTDs is more than an order of magnitude better than TCs. The sensitivity of Coriolis meters are more than an order of magnitude greater than vortex meters. Differential head meters may have good repeatability but suffer from noise plus uncertainty from pipe inside diameter and roughness and orifice edge wear.

• Minimize nonlinearity that cannot be corrected by a smart transmitter. RTDs can be consistently linearized by the use of Callendar-Van Dusen equation eliminating the error when sensors are changed. The interchangeability error for TCs is much greater than RTDs due to greater nonlinearity and unpredictability.

• Minimize maintenance by eliminating drift by the use of the aforementioned advances in smart transmitters and sensors and by eliminating impulse (sensing) lines, sample lines, wires, and terminations. In-line flow meters, close coupled differential pressure and pressure transmitters, in-situ probes, retractable insertion pH electrodes, and wireless transmitters greatly reduce the time spent analyzing real or perceived problems. Analyzer shelters should be used for sophisticated at-line analyzers. For maximum on-stream time and reliability use middle signal selection of 3 measurements that is capable of inherently riding out a single failure of any type and eliminating unnecessary maintenance by recognition of relative performance. The use of middle signal selection is particularly important for pH.

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Measurements

Sensor SelectionSelection Objectives

• Minimize drift eliminating loss of process knowledge, running at the wrong operating point, and the need for recalibration. Drift results in an offset of the measured value from the true value. An offset can be automatically corrected by upper level loop in cascade or model predictive control. Thus loops with a cascade or remote cascade setpoint are less affected by drift. However, knowledge of the process is degraded. For example, while the offset in a flow measurement is corrected by a setpoint change in a cascade loop, the error messes up material balances (process flows), energy balances (utility flows), and online process metrics for process analysis. Flow ratio control must be corrected by a composition loop for flow measurement drift. For custody flow meters, an offset is unacceptable. Smart transmitters and advances in sensor design have in many cases reduced drift and the effect of extraneous process and ambient conditions on installed accuracy by an order of magnitude. Drift in analytical, temperature, or pH is particularly troublesome because these are upper level loops often closely related to product quality. Operations may have adjusted setpoints to compensate for offsets in upper level loops but such adjustments are ad hoc and undone by the replacement of a sensor or transmitter. The drift of TCs is unpredictable and can be one to two orders of magnitude larger than the drift of RTDs. The drift of new pH electrode designs from sterilization and high temperature exposure has been greatly reduced. Solid state pH reference electrodes tend to drift for hours to days after installation due slow equilibration of the reference and high reference junction potential.

28

Measurements

Sensor LocationLocation Objectives

• Maximize the detection of changes in the process from disturbances and setpoint changes. For composition, pH, and temperature choose the location that shows the largest change in both directions for a positive and negative change in the ratio of the manipulated flow to the feed flow realizing there are cross sectional and longitudinal temperature and concentration profiles in pipes and equipment. Areas behind baffles or near the surface or bottom of an agitated vessel or at the outlet of inline equipment may not be as well mixed. Temperature and pH sensor and analyzer sample tip should be near the center of pipe and extend well past equipment walls. Packed and fluidized bed equipment may have uneven composition and temperature distribution from channeling of flow. A series of temperature sensors across a fluidized bed at several longitudinal distances is often necessary with averaging and signal selection to get a representative measurement and prevent hot spots. The insertion length of the thermowell should be more than 5 times the diameter of the thermowell to minimize thermal conduction errors from heat conduction along the thermowell wall between the tip and process connection. Calculations should be run with program supplied by manufacturer on the allowable maximum length in terms of preventing vibration failure from wake frequencies. Resistance temperature detectors (RTDs) are more prone to vibration failure than thermocouples (TCs). Programs today may only be looking at thermowell failure. The tip of a pH electrode must be pointed down at a 30 to 60 degree angle to prevent the internal bubble in the glass electrode from lodging in the tip.

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Measurements

Sensor LocationLocation Objectives

• Minimize noise over the whole operating range reducing extraneous changes. The real definition of measurement rangeability must take into consideration the increase of noise at extremes of the range. Noise at low flow is the principle limitation to the rangeability of a differential head meter. Sufficient straight runs upstream and downstream have a critical effect. Purging can cause transients from changes in the process pressure and purge flow. A location with good mixing and a single phase will minimize fluctuations in temperature and concentration and the disruption of bubbles or solids in liquids and liquid droplets in gasses hitting temperature or pH sensors or getting into sample lines for analyzers or into impulse lines for pressure and level measurements. Pressure probes in high velocity gas streams and furnaces must be designed to minimize momentum and vacuum effects. Sensors and sample probes tips should not be on pump suctions. The spikes from ground potentials and electromagnetic interference (EMI) can be eliminated by wireless transmitters. RTDs are less susceptible to EMI than TCs.

• Minimize sensor deadtime and lag by reducing transportation delays and increasing velocities. The transportation delay in a pipe or sample line is the volume divided by the flow rate or the distance divided by the velocity. The lag time of temperature and pH sensors decreases with velocity by an increase in the heat transfer and mass transfer coefficient. Fouling also decreases with velocity. A liquid velocity of 5 to 7 fps has been shown to greatly reduce fouling of probes. Velocities less than 1 fps significantly increase the lag time of sensors.

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Measurements

The following checklist is not intended to cover all the specification requirements but some of the major application details to be addressed for inline meters (Coriolis, magnetic, turbine, and vortex meters). The following list assumes the materials of construction have been properly specified and that the meter will work safely and reliably with acceptable accuracy for the maximum possible temperature. For more information on flow measurement see the March 2012 Control Talk column “Going with the Flow” http://www.controlglobal.com/articles/2012/mcmillan-wiener-going-with-flow.html For a detailed understanding see Chapters 4 in the ISA book Essentials of Modern Measurements and Final Elements in the Process Industrieshttp://www.isa.org/Template.cfm?Section=Books3&template=/Ecommerce/ProductDisplay.cfm&ProductID=10764Do the meters’ threshold sensitivity, repeatability, and drift meet application requirements?Does the meters’ rangeability and permanent pressure loss meet application requirements?

Maximum rangeabiblity:15:1 vortex, 50:1 turbine, 100:1 magmeter, 200:1 Coriolis (actual rangeability depends sizing)

Is the permanent pressure loss for the meter size acceptable?Do O-rings and gaskets meet worst case corrosive and temperature operating conditions?Are gaskets not projecting into flow stream?Is meter centerline concentric with piping centerline? Do the upstream and downstream straight run lengths for vortex meters meet the ASME guideline for 0.8 beta ratio flow tubes (e.g. 20 pipe diameters upstream for long bends)?Do the upstream and downstream straight run lengths for turbine meters meet the ASME guideline for 0.6 beta ratio flow tubes (e.g. 10 pipe diameters upstream for long bends)?Do the upstream and downstream straight run lengths for magnetic flow meters meet the ASME guideline for 0.4 beta ratio flow tubes (e.g. 5 pipe diameters upstream for long bends)?

31

Inline Flow Meter Checklist

Measurements

Have asymmetric profiles and swirling been minimized by piping design and straightening vanes by special conditioners for profile distortion for turbine and vortex meters?Is the maximum kinematic viscosity less than the maximum for vortex meters?Is the maximum and minimum velocity within limits for magnetic, turbine, and vortex meters?Is the minimum Reynolds number greater than the minimum for vortex meters?Are flow meters in vertical lines installed with flow up?Is maximum vacuum (e.g. after steam cleaning) less than maximum for lined magnetic meters? Is the minimum fluid conductivity (e.g. low water) greater than minimum for magnetic meters?Are there no bubbles in magnetic, turbine, and vortex meters?Is maximum % bubbles and solids less than maximum permitted by Coriolis meter software?Is particle abrasion negligible for U-tube Coriolis, magnetic, turbine, and vortex meters?Is particle concentration high enough to require a straight tube Coriolis meter?Is the minimum fluid lubricating effect greater than minimum for turbine meter bearings?Is the fluid always a liquid (e.g. no flashing) for magnetic meter? Are Coriolis and magnetic flow meters completely full at zero flow?Is signal grounded to zero when no flow to prevent sloshing errors?Is maximum piping vibration less than the maximum permitted by Coriolis and vortex meters (e.g. is there a vibration damper for isolation)?Are bubbles or solids not trapped in U-tube Coriolis, magnetic, turbine, and vortex meters?Are magnetic meters properly grounded to earth and for lined pipe are there ground straps?Do electrical connections and enclosure meet electrical area classification and codes in plant?

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Inline Flow Meter Checklist

Measurements

τm τm

τm

τm

Effect of Transmitter Damping or Filter for SurgeEffect of Transmitter Damping or Filter for Surge

Measurements

The following checklist is not intended to cover all the specification requirements but some of the major application details to be addressed for automation component. The following list assumes the materials of construction have been properly specified and that the sensor will work safely and reliably with acceptable accuracy for the maximum possible temperature. For a detailed understanding see Chapters 3-5 in the ISA book Essentials of Modern Measurements and Final Elements in the Process Industrieshttp://www.isa.org/Template.cfm?Section=Books3&template=/Ecommerce/ProductDisplay.cfm&ProductID=10764 and the Control Global Knowledge Centers Flow Forum http://www.controlglobal.com/knowledge_centers/flow_forum/index.html?DCMP=ILC-mainnav_to_ffkcFor gas, is transmitter mounted above process connection to prevent accumulation of liquids?For liquid, is the transmitter below process connection to prevent trapping gases?Do impulse lines have vent and drain valves?Do impulse lines have a continuous slope with no bends or smooth long radius bends?Does a DP have an equalization valve?Does process pressure connection and probe design prevent appreciable velocity head?Do transmitter and impulse lines need freeze protection?If heat tracing is used, are high temperatures prevented that could alter fluid composition in impulse lines or transmitter (e.g. vaporization, reactions, or formation of tars and polymers)?For plugging services, can impulse lines be purged or eliminated?

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Pressure and DP Measurement Checklist

Measurements

For purged impulse lines is purge flow and pressure high enough?For purged impulse lines is purge flow and pressure adjustable and indicated?For purged impulse lines is liquid purge needed to eliminate transients from compressibility of purge during fast static pressure disturbances and to prevent solids build-up at bubbler tip?Can impulse lines be eliminated by direct mount of transmitter or use of capillary system?Should an extended diaphragm be used to minimize fouling of a direct mounted level DP? Does diaphragm area need to be increased to increase threshold sensitivity? Does diaphragm area need to be decreased to increase speed of response?Is capillary length minimized to increase speed of response?Are the capillary systems at the same temperature (e.g. sun versus shade)? For DP measurement with low static pressure, can DP be computed from dual transmitters?Can a smart transmitter be used to detect plugged impulse lines?Can wireless transmitters be used to provide portability for process troubleshooting?Do electrical connections and enclosure meet electrical area classification and codes in plant?

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Pressure and DP Measurement Checklist

Measurements

The following checklist is not intended to cover all the specification requirements but some of the major application details to be addressed for non-contacting and guide wave radar. The following list assumes the materials of construction have been properly specified and that the sensor will work safely and reliably with acceptable accuracy for the maximum possible temperature. For more information see the Chemical Processing July 2011 article “Making the most of Radar”http://www.chemicalprocessing.com/articles/2011/make-the-most-of-radar.html and the Control magazine February 2012 Control Talk Column “Radar Love”http://www.controlglobal.com/articles/2012/level-measurement-radar-love.html For a detailed understanding see Chapters 5 in the ISA book Essentials of Modern Measurements and Final Elements in the Process Industrieshttp://www.isa.org/Template.cfm?Section=Books3&template=/Ecommerce/ProductDisplay.cfm&ProductID=10764Is the dielectric constant of the liquid too low for even guided wave radar?Is software available to improve signal strength and ignore false echos?If foam is present, do you want to detect surface of foam or surface of liquid?Is a stilling well needed to reduce turbulence and foam?Will the return signal be affected by gaps/holes in the stilling well?’Will tank bottom reflect signals causing false returns?Is the non-contacting beam or guided wave radar probe located away from vessel center, agitator, coils, and inlet streams?Is the path open enough for non-contacting radar?

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Radar Level Measurement Checklist

Measurements

Is the nozzle large enough for the cone (horn) antenna preferred for non-contacting radar?Will the nozzle neck be too long interfering with the horn antenna?For tall tanks and low dielectric, is the antenna large enough to handle the range and dielectric?Is the antenna size matched to stilling well size?Is high frequency radar needed for the non-contacting beam to be narrow enough for a tall tank and to avoid vessel internals?Is high frequency radar needed for recessed antenna or full port valve in nozzle?Is there too much vapor, foam, or condensation for high frequency radar?Will highest level including foam and swell be sufficiently below the radar antenna?Is the fluid too viscous, sticky, abrasive, or corrosive for guided wave radar?Is the dielectric constant so low guided wave radar is needed?Is the signal to noise ratio so low guided wave radar is needed?Is the surface so slanted a reflected signal to a non-contact device is unlikely requiring the use of guided wave radar? Is the minimum clearance between guided-wave probe and vessel internals > 4 inches?Is the stilling well diameter > 4 inches for guided wave radar?Do coatings and deposits require the use of a single lead guided wave probe?Do obstructing objects require the use of coaxial guided wave probes? Does a low dielectric constant require the use of coaxial guided wave probes?Does a viscous non-coating fluid require twin guided wave probes?

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Measurements

Is the vessel so tall, flexible guided wave probes are needed for level measurement range?Does the guided wave radar probe need to be anchored to vessel bottom to reduce sway?Does a DP need to be used for low level measurement due to erratic surface when vessel is nearly empty (e.g. voids and vortexes)?Any need for separate lightning arresters on top of the tank?Is tank properly grounded to minimize noise and transformer effect?If an electronic calibration simulation is prepared for installation, will it match actual conditions?Does the electronic housing allow removal of components for repairs while in service?Do electrical connections and enclosure meet electrical area classification and codes in plant?

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Measurements

Sensor Accuracy, Range, Output, and Size

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Criteria Thermocouple Platinum RTD Thermistor

Repeatability (oC) 1 - 8 0.02 - 0.5 0.1 - 1

Drift (oC/year) 1 - 20 0.01 - 0.1 0.01 - 0.1

Sensitivity (oC) 0.05 0.001 0.0001

Temperature Range (oC) −200 - 2000 −200 – 850(1) −100 - 300

Signal Output (volts) 0 - 0.06 1 - 6 1 - 3

Power (watts at 100 ohm) 1.6 x 10-7 4 x 10-2 8 x 10-1

Minimum Diameter (mm) 0.4 2 0.4

(1) RTD sensor sheath insulation errors can be significant for temperatures above 400oC

Operations may change a setpoint to account for the offset from drift. However the drift is unpredictable and the replacement of the sensor causes a error in operating point.

Measurements

Bare Element Speed of Response

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Bare Sensing Element Type Time Constant(seconds)

Thermocouple 1/8 inch sheathed and grounded 0.3

Thermocouple 1/4 inch sheathed and grounded 1.7

Thermocouple 1/4 inch sheathed and insulated 4.5

Single Element RTD 1/8 inch 1.2

Single Element RTD 1/4 inch 5.5

Dual Element RTD 1/4 inch 8.0

Measurements

Thermowell Speed of Response

41

Process Fluid Type

Fluid Velocity(feet per second)

Annular Clearance(inches)

Annular FillType

Time Constants(seconds)

Gas 5 0.04 Air 107 and 49

Gas 50 0.04 Air 93 and 14

Gas 150 0.04 Air 92 and 8

Gas 150 0.04 Oil 22 and 7

Gas 150 0.02 Air 52 and 9

Gas 150 0.005 Air 17 and 8

Liquid 0.01 0.01 Air 62 and 17

Liquid 0.1 0.01 Air 32 and 10

Liquid 1 0.01 Air 26 and 4


Liquid 10 0.01 Oil 7 and 2



Measurements

Thermocouple Advantages - Real or Perceived?

• There are many stated advantages for thermocouples, but if you examine them closely you realize they are not as important as perceived for industrial processes.

• Thermocouples are more rugged than RTDs. However, the use of good thermowell or protection tube design and installation methods makes an RTD sturdy enough for even high-velocity stream and nuclear applications.

• Thermocouples appear to be less expensive until you include cost of extension lead wire and cost of process variability from less sensor sensitivity and repeatability.

42

Measurements

Thermowell Assembly and Mounting

43

Measurements

Thermowell Conductivity & Length Effect on Error

44

Measurements

Length and Velocity Effects on Vibration Failure

45

Measurements

Fluid Heat Transfer & Mixing Effect on Error

46

Higher velocity increasesheat transfer coefficient

Thermowell tip should be incenter of pipe to reduce cross sectional temperature profile

error from wall and mixingwith enough length to reduce

conduction error

Measurements

Heat Exchanger and Static Mixer Installation

47

Heat Exchangeror Static Mixer

25 pipediameters

elbow

drain

flush

TE 1-1

An optimum location provides good velocity, sufficient mixing, and minimal time delayTransportation delay is volume between outlet of equipment and sensor divided by flow

Measurements

Desuperheater Installation

• For Desuperheaters, the distance from the outlet to the thermowell depends upon the performance of the Desuperheater, process conditions, and the steam velocity.

• To give a feel for the situation there are some simple rules of thumb for the straight piping length (SPL) to the first elbow and the total sensor length (TSL). Actual SPL and TSL values depend on quantity of water required with respect to the steam flow rate, temperature differential between water and steam, water temperature, pipe diameter, steam velocity, model, type, etc. and are computed by software programs.

• SPL (feet) = Inlet steam velocity (ft/s) * 0.1 (seconds residence time)• TSL (feet) = Inlet steam velocity (ft/s) * 0.2 (seconds residence time)• Typical values for the inlet steam velocity, upstream of the Desuperheater range

from 25–350 ft/s (7.6 to 107 m/sec). • Below 25 ft/s there is not enough motive force to keep water suspended in steam.• Velocities higher than 350 ft/s cause the Desuperheater to vibrate and damage the

unit to the point where it breaks apart

48

Measurements

The following checklist is not intended to cover all the specification requirements but some of the major application details to be addressed for resistance temperature detectors (RTD) and thermocouples (TC). The following list assumes the materials of construction have been properly specified and that the sensor will work safely and reliably with acceptable accuracy for the maximum possible temperature. For more info see Chapters 1-2 in the ISA book Advanced Temperature Measurement and Controlhttp://www.isa.org/Template.cfm?Section=Find_Books1&template=/Ecommerce/ProductDisplay.cfm&ProductID=10880Is the distance from the equipment outlet (e.g. heat exchanger exit) and sensor at least 25 pipe diameters for single phase to promote mixing (recombination of outlet streams) Is the transportation delay (distance/velocity) from the equipment outlet (e.g. heat exchanger exit) to the sensor less than 3 seconds?Does the distance from the desuperheater outlet to the first elbow provide a residence time (distance/velocity) that is greater than 0.1 sec?Does the distance from the desuperheater outlet to the sensor provide a residence time (distance/velocity) that is greater than 0.2 sec?Is a RTD used for temperatures below 400 degC to improve threshold sensitivity, drift, and repeatability by more than a factor of ten compared to TC if vibration is not excessive?For RTDs operating at temperatures above 400 degC, is length minimized and sheath diameter maximized reduce insulation error?For RTDs operating at temperatures above 600 degC, is sensing element hermitically sealed and dehydrated to prevent increase in platinum resistance from oxygen and hydrogen dissociation

49

Temperature Measurement Checklist

Measurements

For TCs above 600 degC is decalibration error from changes in composition of TC minimized by choice of sheath and TC type?For TCs above 900 degC is sheath material compatible with TC type? For TCs above the temperature limit of sheaths, is the ceramic material with best conductivity and design used to minimize measurement lag time?For TCs above the temperature limit of sheaths with gaseous contaminants or reducing conditions, is primary (outer) and secondary (inner) protection tubes designed to prevent contamination of TC element and still provide a reasonably fast response?In furnaces and kilns does location and design minimize radiation and velocity errors?Is the immersion length long enough to minimize heat conduction error (e.g. L/D > 5)?Is the immersion length short enough to prevent vibration failure (e.g. L/D < 20)?Is the velocity fast enough to minimize coatings (e.g. > 5 fps)Is the velocity fast enough to provide a fast response (e.g. > 0.5 fps)For pipes, is the tip near the centerline?For vessels, does the tip extend sufficiently past the baffles (e.g. L/D > 5)?For columns, does the tip extend sufficiently into tray or packing (e.g. L/D > 5)?For TC, is it more important to minimize noise by using ungrounded junction or to minimize sensor element lag time by grounded junction?

50


Measurements

To increase RTD reliability, are dual RTD elements used except when vibration failure is more likely due to smaller gauge?To increase TC reliability, does the sensor have dual isolated junctions?For maximum reliability, are 3 separate thermowells with middle signal selection used?Does sensor fit tightly in thermowell to minimize measurement lag from air gap (e.g. annular clearance < 0.01 inch)?Is an oil fill used that will not form tars or sludge at high temperature in thermowell with tip pointed down to minimize measurement lag?Is premium TC extension wire used to minimize measurement uncertainty?Is 4 wire RTD lead wire used to minimize measurement uncertainty?Are head mounted temperature transmitters used for accessible locations to eliminate extension wire and lead wire errors and reduce noise?Are wireless head mounted transmitters used to provide portability of measurement for process control improvement and to reduce wiring installation and maintenance costs?Are proper linearization tables used in transmitter?Do electrical connections and enclosure meet electrical area classification and codes in plant?

51


Measurements

52

¿ Food is burning in the operators’ kitchen¾ The only loop mode configured is manual½ An operator puts his fist through the screen¼ You trip over a pile of used pH electrodes » The technicians ask: “what is a positioner?”º The technicians stick electrodes up your nose¹ The environmental engineer is wearing a mask¸ The plant manager leaves the country· Lawyers pull the plugs on the consoles¶ The president is on the phone holding for you

Top Ten Signs of a Rough pH Startup

Measurements

53

pH Hydrogen Ion Concentration Hydroxyl Ion Concentration0 1.0 0.000000000000011 ` 0.1 0.00000000000012 0.01 0.0000000000013 0.001 0.000000000014 0.0001 0.00000000015 0.00001 0.0000000016 0.000001 0.000000017 0.0000001 0.00000018 0.00000001 0.0000019 0.000000001 0.0000110 0.0000000001 0.000111 0.00000000001 0.00112 0.000000000001 0.0113 0.0000000000001 0.114 0.00000000000001 1.0

aH = 10−pH

pH = - log (aH)

aH = γ ∗ cH

cH ∗ cOH = 10−pKw

aH = hydrogen ion activity (gm-moles per liter)cH = hydrogen ion concentration (gm-moles per liter)cOH = hydroxyl ion concentration (gm-moles per liter)γ = activity coefficient (1 for dilute solutions)pH = negative base 10 power of hydrogen ion activitypKw = negative base 10 power of the water dissociation constant (14.0 at 25oC)

Hydrogen and Hydroxyl Ion Concentrations in a Water Solution at 25oC

Extraordinary Sensitivity and Rangeability

Measurements

54

Mea

sure

d pH

pH 5

pH 6

pH 6.5

pH 7

pH 7.5

pH 9 pH 10pH 8

Effect of Water Dissociation (pKw) on Solution pH

Measurements

55

Effect of Sensor Drift on Reagent Calculations

pH

Reagent to Feed Flow Ratio

4

10

6

8

FeedforwardReagent Error

FeedforwardpH Error

Sensor Drift

pH Set PointInfluent pH

The error in a pH feedforward calculationincreases for a given sensor error as theslope of the curve decreases. This resultCombined with an increased likelihood ofErrors at low and high pH means feedforwardCould do more harm than good when goingfrom the curve’s extremes to the neutral region.

Flow feedforward (ratio controlof reagent to influent flow) workswell for vessel pH control if there are reliable flow measurements with sufficient rangeability

Feedforward control always requires pH feedback correction unless the set point is on the flat part of the curve, use Coriolis mass flow meters and have constant influent and reagent concentrations

Measurements

56

Double Junction Combination pH Electrode

High acid or base concentrations can affect glass gel layer and reference junction potentialIncrease in noise or decrease in span or efficiency is indicative of glass electrode problemShift or drift in pH measurement is normally associated with reference electrode problem

W W

WW

W

W

W

Em

R10R9 R8R7

R6

R5

R4

R3

R2

R1

Er

E5

E4

E3

E2

E1

outer gellayer

inner gellayer

secondjunction

primary junction

solution ground

Process Fluid

silver-silver chlorideinternal electrode

silver-silver chlorideinternal electrode

potassium chloride (KCl) electrolyte in salt bridge between junctions

7 pH buffer

Ii

Process ions try to migrate into porous reference junctionwhile electrolyte ionstry to migrate out

Nernst Equationassumes insideand outside gellayers identical

Measurementbecomes slow from a loss ofactive sites ora thin coatingof outer gel

WW

W

Measurements

57

High Today Low Tomorrow

AB A

B A

BpH

time

Most calibration adjustments chase the short term errors shownbelow that arise from concentration gradients from imperfectmixing, ion migration into reference junction, temperature shifts, different glass surface conditions, and fluid streaming potentials.With just two electrodes, there are more questions than answers.

Measurements

58

• Inherently ignores single measurement failure of any type including the most insidious PV failure at set point

• Inherently ignores slowest electrode• Reduces noise and spikes particularly for steep curves• Offers online diagnostics on electrode problems

– Slow response indicates coated or aged measurement electrode – Decreased span (efficiency) indicates aged or dehydrated glass electrode– Drift or bias indicates coated, plugged, or contaminated reference electrode

or high liquid junction potential– Noise indicates dehydrated measurement electrode, streaming potentials,

velocity effects, ground potentials, or EMI• Facilitates online calibration of a measurement

For more Information on Middle Signal Selection see Feb 5, 2010 posthttp://www.modelingandcontrol.com/2010/02/exceptional_opportunities_in_p_11.html

Middle Signal SelectionAdvantages

Measurements

59

Life Depends Upon Process Conditions

25 C 50 C 75 C 100 CProcess Temperature

Months

>100% increase in life from new glass designsfor high temperatures

High acid or base concentrations (operation at the extremes of the titration curve) decrease life for a given temperature. A deterioration in measurement accuracy (e.g. electrode efficiency) and response time often accompanies a reduction in life.

Measurements

60

New High Temperature Glass Stays Fast

Glass electrodes get slow as they age. High temperatures cause premature aging

Measurements

61

New Design Eliminates Drift after Sterilization

New Old #1 Old #2

Measurements

62

Horizontal Piping Arrangements

AE

AE

AE

AEAEAE

5 to 9 fps to minimize coatings0.1 to 1 fps to minimize abrasion

20 to 80 degreesThe bubble inside the glass bulbcan be lodged in tip of a probe that is horizontal or pointed up or caught at the internal electrodeof a probe that is vertically down

pressure drop foreach branch mustbe equal to keep the velocities equal

Series arrangement preferred to minimize differences in solids, velocity, concentration, and temperature at each electrode!

10 OD10 OD

20 pipe diameters

20 pipe diameters

static mixer or pump

flush

drain

flush

drain

throttle valve toadjust velocity


Measurements

63

Vertical Piping ArrangementsA

E AE AE

AE

AE

AE

5 to 9 fpscoating abrasion

10 O

D10

OD

0.1 to 1 fps

holeorslot

Orientation of slot in shroudthrottle valve toadjust velocity


Series arrangement preferred to minimize differences in solids, velocity, concentration, and temperature at each electrode!

Measurements

64

• A spherical or hemi-spherical glass measurement and flowing junction reference offers maximum accuracy, but in practice maintenance prefers:– A refillable double junction reference to reduce the complexity of installation and

the need to adjust reference electrolyte flow rate – This electrode is often the best compromise between accuracy and maintainability.

– A solid reference to resist penetration and contamination by the process and eliminate the need to refill or replace reference particularly for high and nasty concentrations and pressure fluctuations – This electrode takes the longest time to equilibrate and is more prone to junction effects but could be right choice in applications where accuracy requirements are low and maintenance is high.

• Select best glass and reference electrolyte for process • Use smart digital transmitters with built-in diagnostics• Use middle signal selection of three pH measurements

– Inherent auto protection against a failure, drift, coating, loss in efficiency, and noise (February 5, 2010 entry on http://modelingandcontrol.com/author/Greg-McMillan//)

• Allocate time for equilibration of the reference electrode• Use “in place” standardization based on a sample with the same temperature

and composition as the process. If this is not practical, the middle value of three measurements can be used as a reference. The fraction and frequency of the correction should be chosen to avoid chasing previous calibrations

• Insure a constant process fluid velocity at the highest practical value to help keep the electrodes clean and responsive

Options for Maximum Accuracy

Measurements

65

Symptoms and Causes

• Slow measurement– Coated glass, aged glass, dehydrated glass, thick glass, high temperature, low pH, low velocity

• Noisy measurement– Dehydrated glass, pure water, low water, low temperature, poor mixing, high velocity, EMI

• pH decreases with temperature– pKa or pKw decrease with temperature (missing solution temperature compensation)– Horizontal shift to right of Isopotential point due to measurement electrode type or problem

• pH increases with temperature– Nernst MV decrease with temperature (missing electrode temperature compensation)– Horizontal shift to left of Isopotential point electrode type or problem

• pH increases with salt or solvent concentration– Decrease in activity of hydrogen ion from increase in ionic strength or decrease in water content

• Constant 7 pH measurement– Broken glass, broken wires, protective caps still on electrodes

• Decrease in measurement range (decrease in electrode efficiency)– Dehydrated glass or aged glass

• Daily drift in measurement (change in electrode offset)– Coated reference junction

• Hourly drift in measurement– Slow equilibration of reference electrode

• Off-scale pH measurement– Poisoned reference electrode or high solvent concentration

Measurements

66

Wireless pH Transmitters Eliminate Ground Spikes

Wired pH ground noise spike

Temperature compensated wireless pH controlling at 6.9 pH set point

Incredibly tight pH control via 0.001 pH wireless resolution setting still reduced the number of communications by 60%

Measurements

67

Wireless Bioreactor Adaptive pH Control

Measurements

68

Calibration History in Probe

Measurements

pH Measurement ChecklistThe following checklist is not intended to cover all the specification requirements but some of the major application details to be addressed for automation component. The following list assumes the materials of construction have been properly specified and that the sensor will work safely and reliably with acceptable accuracy for the maximum possible temperature and pressure. For a detailed understanding see Chapter 6 in the ISA book Essentials of Modern Measurements and Final Elements in the Process Industrieshttp://www.isa.org/Template.cfm?Section=Books3&template=/Ecommerce/ProductDisplay.cfm&ProductID=10764Do O-rings and gaskets meet worst case corrosive and temperature operating conditions?Is the best glass used for the worst case temperature, pH, and chemicals that can attack glass (e.g. general purpose, high pH, high temperature, sterilizable, and HF resistant)?For pH < 1 or > 12 would conductivity or density give a better concentration measurement?Is the best reference design and fill used for the accuracy and speed requirement and worst case temperature, low water or pure water solutions that have low conductivity, salts and chemicals concentrations that change junction potential, plug junction, and poison reference

Flowing junction offers the most constant reference potential, has the fastest junction equilibration, and eliminates plugging-poisoning, but requires pressurized reservoir Aperture junction’s tiny hole has lowest junction potential but is susceptible to plugging Double and triple junction references can slow down internal contamination rateReplaceable junction can fix electrode before plugging -poisoning is problemLarge surface solid reference can essentially eliminate plugging, contamination , and poisoning but the junction potential may be large and slow to equilibrate

69

Measurements

Is the chemical attack, premature aging from high temperature, or dehydration (non-aqueous solvents or low water concentrations) so severe that automated retractable insertion needed?Is the solution conductivity so low (e.g. condensate, boiler feedwater, deionized water) a special assembly is needed to provide low sample flow, diffuser, and electrolyte reservoir ? Can a VP connector be used to quickly locally disconnect electrode cable eliminating the need to disconnect transmitter and retract cable through conduit or flex to prevent twisting of cable?Is a smart electrode with stored calibration record available for selected electrode?Is a smart transmitter to detect glass and reference problems available?Is solution pH temperature compensation needed besides Nernst temperature compensation?Is a solution ground needed for impedance diagnostics and ground potential elimination? Can a wireless transmitter be used to get latest features and enable portability of measurement to test the best electrode and location (least deadtime and least noise-bubbles)?Is the electrode installed with tip pointing down at a 30-60o angle to prevent bubble inside tip?Are electrodes always wetted even for batching and during shutdown of continuous operations?Is middle signal selection needed to eliminate response to single failure and noise?Is stream velocity and electrode protective shroud design the best for process conditions?

Velocity 5-10 fps and exposure of glass to flow helps prevent coatingsVelocity 0.1-1 fps and shroud reducing flow impingement helps decrease abrasion

Does the electrode tip extend into the center line of pipe and past baffles in vessel?

70

pH Measurement Checklist

Measurements

Is electrode location free from flashing (e.g. not on pump suction or valve discharge)?Are electrodes sufficiently downstream from pump or static mixer to reduce concentration and pressure fluctuations but not so far as to increase deadtime by more than 3 sec?Are insertion electrodes in series used to ensure same velocity and composition?Are electrode and transmitter connections always dry?Do electrical connections and enclosure meet electrical area classification and codes in plant?

71

pH Measurement Checklist

Measurements

Deadly and Sticky Situations

dead band

Deadband

Stick-Slip is worse near closed position

Signal(%)

0

Stroke(%) Digital positioner

will force valve shut at 0% signal

Pneumatic positionerrequires a negative % signal to close valve

The dead band and stick-slip is greatest near the closed position

Deadband is 5% - 50%without a positioner !

Plugging and laminar flow can occur for low Cv requirements and throttling near the seat

Consider going to reagent dilution. If this is not possible checkout out a laminar flow valve for an extremely low Cv

and pulse width modulation for low lifts

Final Control Elements

73

pH Control Valve Rangeability and Resolution

pH

Reagent FlowInfluent Flow

6

8

Influent pHB

A

Control BandSet point

BEr = 100% ∗ Fimax ∗ −−−−

Frmax

Frmax = A ∗ Fimax

BEr = −−−−

A

Ss = 0.5 ∗ Er

Where:A = distance to center of reagent error band on abscissa from influent pHB = width of allowable reagent error band on abscissa for control band Er = allowable reagent error (%)Frmax = maximum reagent valve capacity (kg per minute)Fimax = maximum influent flow (kg per minute)Ss = allowable stick-slip (resolution limit) (%)


Direct Connection Piston Actuator

Less backlash but wear of piston O-ring seal from piston pitch is concern


Significant backlash from link pin points 1 and 2

Link-Arm Connection Piston Actuator


Stick-slip from rack and gear teeth - particularly bad for worn teeth

Rack & Pinion Connection Piston Actuator


Lots of backlash from slot

Scotch Yoke Connection Piston Actuator


Port A

Port B

Supply

ZZZZ

ZZZ

Control Signal

Digital Valve Controller

Must be functionally tested

before commissioning!

SV

Terminal Box

Diaphragm Actuator with Solenoid Valves


Port A

Port B

Supply


SV

SV

VolumeTank



Piston

W

CheckValve

AirSupply

Terminal Box

Piston Actuator with Solenoid Valves


Size of Step Determines What you See

T im e (s e c o n d s )

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0

(% )

3 5

4 0

4 5

5 0

5 5

6 0

6 5

0 .5 % S te p s 1 % S te p s 2 % S te p s 5 % S te p s 1 0 % S te p s

(% )

4 0

4 5

5 0

5 5

6 0

6 5

7 0

1 % S te p s 2 % S te p s0 .5 % S te p s 5 % S te p s 1 0 % S te p s

4 " S e g m e n te d B a ll V a lv e s w ith M e ta l S e a ls ,D ia p h ra g m A c tu a to rs a n d S ta n d a rd P o s it io n e rs

F is h e r V 1 5 0 H D /1 0 5 2 (3 3 )/3 6 1 0 J

N e le s R 2 1 /Q P 3 C /N P 7 2 3

In p u t S ig n a lA c tu a to r P o s it io nF lo w R a te (F ilte re d )

Maintenance test of 25% or 50% steps will not detect dead band - all valves look good for 10% or larger steps


Effect of Step Size Due to Sensitivity Limit


Response to Small Steps (No Sensitivity Limit) Response to Small Steps (No Sensitivity Limit)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 1 2 3 4 5 6 7 8 9 10

St

roke

(%)

Time (sec)


0

5

10

15

20

25

30

35

40

45

50

0 1 2 3 4 5 6 7 8 9 10

Time (sec)

Stro

ke (%

)Response to Large Steps (Small Actuator Volume)Response to Large Steps (Small Actuator Volume)


Installed Characteristic (Linear Trim)Installed Characteristic (Linear Trim)

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Inherent CharacteristicInstalled Characteristic 1Installed Characteristic 2Installed Characteristic 3Installed Characteristic 4

Valve pressure drop ratio (ΔPR)for installed characteristic:

Characteristic 1: ΔPR = 0.5 Characteristic 2: ΔPR = 0.25Characteristic 3: ΔPR = 0.125Characteristic 4: ΔPR = 0.0625


Installed Characteristic (Equal Percentage Trim)Installed Characteristic (Equal Percentage Trim)

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Inherent CharacteristicInstalled Characteristic 1Installed Characteristic 2Installed Characteristic 3Installed Characteristic 4




Installed Characteristic (Modified Parabolic Trim)Installed Characteristic (Modified Parabolic Trim)




Limit Cycle in Flow Loop from Valve Stick-Slip

Controller Output (%)Saw Tooth Oscillation

Process Variable (kpph)Square Wave Oscillation


Limit Cycle in Level Loop from Valve Deadband

Manipulated Flow (kpph)Clipped Oscillation

Controller Output (%)Rounded Oscillation

Level (%)


Real Rangeability Real Rangeability Minimum fractional flow coefficient for a linear trim and stick-slip:

Minimum fractional flow coefficient for an equal percentage trim and stick-slip:

Minimum controllable fractional flow for installed characteristic and stick-slip:

Cxmin = minimum flow coefficient expressed as a fraction of maximum (dimensionless)ΔPr = valve pressure drop ratio (dimensionless) Qxmin = minimum flow expressed as a fraction of the maximum (dimensionless)Rv = rangeability of control valve (dimensionless) R = range of the equal percentage characteristic (e.g. 50)Xvmin = maximum valve stroke (%)Sv = stick-slip near closed position (%)

maxmin

v

vx X

SC =

]1[

minmax

−

= v

v

X

S

x RC

2min

minmin

)1( xRR

xx

CPP

CQ∗Δ−+Δ

=

min

1

xv Q

R =


Best Practices to Improve Valve Performance

• Actuator, valve, and positioner package from a control valve manufacturer• Digital positioner tuned for valve package and application• Diaphragm actuators where application permits (large valves and high

pressure drops may require piston actuators)• Sliding stem (globe) valves where size and fluid permit (large flows and

slurries may require rotary valves)• Next best is Vee-ball or contoured butterfly with rotary digital positioner

• Low stem packing friction • Low sealing and seating friction of the closure components• Booster(s) on positioner output(s) for large valves on fast loops (e.g.,

compressor anti-surge control)• Valve sizing for a throttle range that provides good linearity [4]:

o 5% to 75% (sliding stem globe), o 10o to 60o (Vee-ball)o 25o to 45o (conventional butterfly)o 5o to 65o (contoured and toothed butterfly)

• Online diagnostics and step response tests for small changes in signal • Dynamic reset limiting using digital positioner feedback [2]


Volume Booster with Integral Bypass(Furnace Pressure and Surge Control)

Signal from Positioner

Air Supply fromFilter-Regulator

Air Loadingto Actuator

Adjustable BypassNeedle Valve


Port A

Port B

Supply

ZZZZ

ZZZ

Control Signal




1:1

Bypass

VolumeBooster

Open bypass justenough to ensurea non-oscillatory fast response

Air Supply

High CapacityFilter Regulator

Increase air line size

Increase connection size

Terminal Box

Booster and Positioner Setup(Furnace Pressure and Surge Control)


Control Valve Misconceptions

• There are a lot of misconceptions from sales pitches that lack an understanding of the need for a valve to have minimum backlash and maximum resolution and sensitivity. Most of this stems (pun intended) from using step sizes that are way too large. Today, the smallest step change commonly cited is 0.5%. Maybe I should be happy because the step size was 25% until we had smart positioners that could tell us how much the valve shaft actually moved. What I really want are 0.1% steps over the entire throttle range to sort fact from fiction.

• Common misconceptions:– Rotary valves provide tighter control than sliding stem valves– Stated rangeability takes into account pressure drop, backlash, and stiction– “High Performance Valves” (tight shutoff valves) provide high performance– Piping valves and on-off actuators can be used for control valves– Piston actuators provide tighter control than diaphragm actuators– Step tests at 50% open tell the whole story

93


Rotary Valve Watch Outs

• Rotary valves tend not to have as large a throttle range where the gain (sensitivity) of the installed characteristic is acceptable. Rotary valves that are “High Performance Valves” and piping valves have markedly reduced throttle range. If you consider the effect of increased backlash and stick-slip of these valves especially near the seat, the actuator shaft used for positioner feedback may not be representative of actual internal flow element (disc, ball, or slotted plug) due to shaft windup, and a poor inherent flow characteristic, you understand these valves are not really control valves. Similarly, the scotch yoke, rack and pinion, and link arm on-off actuators have excessive backlash or insufficient resolution for throttling service. A diaphragm actuator has the best threshold sensitivity. Higher pressure diaphragm actuators have been developed extending their thrust and torque capability. The next best actuator is a double acting piston. All should have smart positioners with 2-stage or high gain pneumatic relays. High volume spool positioners used on dampers and piping valve posing as control valves have an extremely poor threshold sensitivity requiring step changes of 2%.

• To learn more about what really is important about control valves and variable speed drives read Chapter 7 on Final Element Fundamentals in the ISA book Essentials of Modern Measurements and Final Elements in the Process Industries, http://www.isa.org/Template.cfm?Section=Books3&template=/Ecommerce/ProductDisplay.cfm&ProductID=10764

• Article “Improve Control Loop Performance”, Chemical Processing, Oct, 2007• http://www.chemicalprocessing.com/articles/2007/200.html• “Key Design Components of Final Control Elements”, InTech, March-April, 2011• http://www.isa.org/InTechTemplate.cfm?Section=Control_Fundamentals1&template=/Content

Management/ContentDisplay.cfm&ContentID=8167994


The following checklist is not intended to cover all the specification requirements but some of the major application details to be addressed for automation component. The following list assumes the materials of construction have been properly specified and that the sensor will work safely and reliably with acceptable accuracy for the maximum possible temperature and pressure. For a detailed understanding see Chapters 7 and 8 in the ISA book Essentials of Modern Measurements and Final Elements in the Process Industrieshttp://www.isa.org/Template.cfm?Section=Books3&template=/Ecommerce/ProductDisplay.cfm&ProductID=10764The use of digital positioners has improved the precision of control valves to 0.1% if the positioner is tuned and the rest of the valve package does not pose limitations. The threshold sensitivity of pneumatic positioners ranged from 0.1% for two stage relay to 1% to spool type positioners. Pneumatic positioners did not sufficiently hold their calibration and did not provide a readback of valve positioner. Consequently, the low output limits of controllers needed to be -10% to ensure the valve was closed and problems with the dynamic response of control valves were hidden and test step sizes of 10% or larger were used. Digital positioners are the first step but not the only step for a good valve response. Except for pH control, a precision of 0.1% and deadband of 0.2% is normally sufficient since the amplitude of a limit cycle is within the noise band or control band. The sliding stem globe valve with low friction packing and diaphragm actuators can achieve a 0.1% precision. However, rotary valves are advantageous for large lines sizes and streams with solids. Rotary valves can achieve a precision of 0.2% if a diaphragm actuator is used, the connection between the actuator shaft and valve stem is splined instead of pinned, there are no linkages introducing backlash, the friction of sealing surfaces and shaft length and size does not cause shaft windup, and the valve stem is cast with the internal flow element.

95

Control Valve Checklist


Piping valves original designed as isolation valves posing as control valves are the biggest culprit. A precision of 4% and a deadband of 8% have been observed for butterfly valves and ball valves designed for tight shutoff. Furthermore, the positioner feedback was actuator shaft instead of flow element position. Tests results from the digital positioner indicated a precision and deadband 10 times smaller than actual. Flow measurements and travel gages on internal flow element revealed the true response capability of the valve. Thus, putting the best digital positioner on a poor valve and actuator design does not eliminate the problem and in fact may give a false assurance from invalid response trends and diagnostics. This problem occurs frequently because leakage but not precision and deadband are on the control valve specification and the digital positioner is thought to solve all problems with valve response.Does sizing software have the fluid physical properties for worse case operating conditions?Does location and valve type eliminate or reduce damage from flashing and erosion?Did you include swage effect from piping reducer?Did you compute and plot installed valve characteristic for worse case operating conditions?Is actuator sized to deliver 1.5x max torque or thrust required?Is actuator threshold sensitivity better than 0.1%?Is positioner threshold sensitivity better than 0.1%?Is smart positioner tuned for application (otherwise you have a dumb positioner)?Is total valve assembly deadband less than 0.4% over the entire throttle range?Is total valve assembly resolution better than 0.2% over the entire throttle range?Is installed characteristic slope > 0.5% max flow per % signal over the entire throttle range?Is installed characteristic slope < 2.0% max flow per % signal over the entire throttle range?

96

Control Valve Checklist


The following checklist is not intended to cover all the specification requirements but some of the major application details to be addressed for automation component. The following list assumes the prime mover (e.g. pump or fan) materials of construction have been properly specified and will work safely and reliably with acceptable pump discharge pressure for the maximum possible temperature and static head. For a detailed understanding see Chapter 7 in the ISA book Essentials of Modern Measurements and Final Elements in the Process Industrieshttp://www.isa.org/Template.cfm?Section=Books3&template=/Ecommerce/ProductDisplay.cfm&ProductID=10764Is Pulse Width Modulation (PWM) drive used to reduce torque pulsation (cogging) at low speeds?Is a totally enclosed fan cooled (TEFC) motor with constant speed fan or booster fan as necessary with class F insulation (inverter duty) and 1.15 service factor to prevent overheating? Is a totally enclosed water cooled (TWEC) motors for high temperatures to prevent overheating?Is a NEMA frame B motor used to prevent steep torque curve?Is pump sized to prevent operation on flat part of pump curve?Is a recycle valve to keep pump discharge pressure well above static head at low flow and low speed limit prevents reverse flow for highest possible destination pressure? (see article "Watch out for variable speed pumping") http://www.chemicalprocessing.com/articles/2008/069.htmlAre signal input cards > 12 bit to improve resolution limit of signal to 0.05% or better?For tachometer control, does the number of gear teeth for magnetic pickups and discs with holes or bands with mirrors on the shafts for optical pickups provide enough pulses per revolution?For tachometer control, is the speed control in the VFD to prevent violation of the cascade rule where the secondary flow loop should be 5 times faster than the primary (flow)?

97

Variable Frequency Drive Checklist


Contribution of Each PID Mode

• Proportional (P mode) - increase in gain increases P mode contribution– Provides an immediate reaction to magnitude of measurement change to minimize

peak error and integrated error for a disturbance– Too much gain action causes fast oscillations (close to ultimate period) and can make

noise and interactions worse– Provides an immediate reaction to magnitude of setpoint change for P action on Error

to minimize rise time (time to reach setpoint)– Too much gain causes falter in approach to setpoint

• Integral (I mode) - increase in reset time decreases I mode contribution– Provides a ramping reaction to error (SP-PV) to minimize integrated error if stable (since

error is hardly ever exactly zero, integral action is always ramping the controller output)– Too much integral action causes slow oscillations (slower than ultimate period)– Too much integral action causes an overshoot of setpoint (no sense of direction)

• Derivative (D mode) - increase in rate time increases D mode contribution– Provides an immediate reaction to rate of change of measurement change to minimize

peak error and integrated error for a disturbance– Too much rate action causes fast oscillations (faster than ultimate period) and can make

noise and interactions worse– Provides an immediate reaction to rate of change of setpoint change for D action on

Error to minimize rise time (time to reach setpoint)– Too much rate causes oscillation in approach to setpoint

PID Controllers

Contribution of Each PID Mode

ΔCO2 = ΔCO1

ΔSP

seconds/repeatΔCO1

Time(seconds)

Signal(%)

0

kick fromproportional

mode

bump fromfiltered

derivativemode

repeat from integralmode

Contribution of Each PID Mode for a Step Change in the Set PointStructure of PID on error (β=1 and γ=1)

PID Controllers

SPPVIVP

5248?

TC-100Reactor Temperature

steam valveopens

watervalveopens

50%set point

temperature

time

PV

Should steam or water valve be open ?

Reset Gives Operations What They Want

PID Controllers

Open Loop Time Constant (controller in manual)

%CO

Time(seconds)

Signal(%)

0 θo

Dead Time (Time Delay)

τo

Open Loop(process)

Time Constant (Time Lag)

%PVSP

Controller is in Manual

Open LoopError Eo (%)

0.63∗Eo

PID Controllers

Closed Loop Time Constant (controller in auto)

%CO

Time(seconds)

Signal(%)

0 θo

Dead Time (Time Delay)

τc

Closed Loop Time Constant

(Time Lag)Lambda (λ)

%PV

SP

Controller is in Automatic

ΔSP (%)

0.63∗ΔSP

PID Controllers

Conversion of Signals for PID Algorithm

SensingElement

ControlValveAOPIDSCLR

AI

SCLR

SCLR%

% %SUB

%PVSP

%

%CO OUT(e.u.)

ProcessEquipment

SmartTransmitterPV - Primary Variable

SV - Second Variable*TV - Third Variable*FV - Fourth Variable*

PV(e.u.)

PID

DCS

MV(e.u.)

The scaler block (SCLR) that convert between engineering units of application and % of scaleused in PID algorithm is embedded hidden part of the Proportional-Integral-Derivative block (PID)

Final Element

Measurement

* - additional HART variables

PV(e.u.)

To compute controller tuning settings, the process variable and controller outputmust be converted to % of scale and time units of deadtimes and time constantsmust be same as time units of reset time and rate time settings!

PID Controllers

• PID on Error Structure– Maximizes the kick and bump of the controller output for a setpoint change. – Overdrive (driving of output past resting point) is essential for getting slow loops,

such as vessel temperature and pH, to the optimum setpoint as fast as possible.– The setpoint change must be made with the PID in Auto mode.– “SP track PV” will generally maximize the setpoint change and hence the kick and

bump (retaining SP from last batch or startup minimizes kick and bump)• SP Feedforward

– For low controller gains (controller gain less than inverse of process gain), a setpoint feedforward is particularly useful. For this case, the setpoint feedforward gain is the inverse of the dimensionless process gain minus the controller gain.

– For slow self-regulating (e.g. continuous) processes and slow integrating (e.g. batch) processes, even if the controller gain is high, the additional overdrive can be beneficial for small setpoint changes that normally would not cause the PID output to hit a limit.

– If the setpoint and controller output are in engineering units the feedforward gain must be adjusted accordingly.

– The feedforward action is the process action, which is the opposite of the control action, taking into account valve action. In other words for a reverse control action, the feedforward action is direct provided the valve action is inc-open or the analog output block, I/P, or positioner reverses the signal for a inc-close.

Fed-Batch and Startup Time Reduction - 1

PID Controllers

• Full Throttle (Bang-Bang Control) - The controller output is stepped to it output limit to maximize the rate of approach to setpoint and when the projected PV equals the setpoint less a bias, the controller output is repositioned to the final resting value. The output is held at the resting value for one deadtime. For more details, check out the Control magazine article “Full Throttle Batch and Startup Response.” http://www.controlglobal.com/articles/2006/096.html– A deadtime (DT) block must be used to compute the rate of change so that new values of

the PV are seen immediately as a change in the rate of approach.– If the total loop deadtime (θo) is used in the DT block, the projected PV is simply the current

PV minus the output of the DT block (ΔPV) plus the current PV.– If the PV rate of change (ΔPV/Δt) is useful for other reasons (e.g. near integrator or true integrating

process tuning), then ΔPV/Δt = ΔPV/θo can be computed. – If the process changes during the setpoint response (e.g. reaction or evaporation), the

resting value can be captured from the last batch or startup – If the process changes are negligible during the setpoint response, the resting value can be

estimated as:– the PID output just before the setpoint change for an integrating (e.g. batch) process– the PID output just before the setpoint change plus the setpoint change divided by the process gain

for a self-regulating (e.g. continuous) process– For self-regulating processes such as flow with the loop deadtime (θo)

approaching or less than the largest process time constant (τp ), the logic is revised to step the PID output immediately to the resting value. The PID output is held at the resting value for the T98 process response time (T98 = θo + 4∗ τp ).


PID Controllers

Structure, SP Feedforward, & Bang-Bang Tests

Structure 3Rise Time = 8.5 min

Settling Time = 8.5 minOvershoot = 0%

Structure 1Rise Time = 1.6 min

Settling Time = 7.5 minOvershoot = 1.7%

Structure 1 + SP FFRise Time = 1.2 min


Structure 1 + Bang-BangRise Time = 0.5 min


PID Controllers

• Output Lead-Lag– A lead-lag on the controller output or in the digital positioner can kick the signal

though the valve deadband and stiction, get past split range points, and make faster transitions from heating to cooling and vice versa.

– A lead-lag can potentially provide a faster setpoint response with less overshoot when analyzers are used for closed loop control of integrating processes When combined with the enhanced PID algorithm (PIDPlus) described in:– Deminar #1 http://www.screencast.com/users/JimCahill/folders/Public/media/5acf2135-

38c9-422e-9eb9-33ee844825d3– White paper http://www.modelingandcontrol.com/DeltaV-v11-PID-Enhancements-for-

Wireless.pdf• Deadtime Compensation

– The simple addition of a delay block with the deadtime set equal to the total loop deadtime to the external reset signal for the positive feedback implementation of integral action described in Deminar #3 for the dynamic reset limit option http://www.screencast.com/users/JimCahill/folders/Public/media/f093eca1-958f-4d9c-96b7-9229e4a6b5ba .

– The controller reset time can be significantly reduced and the controller gain increased if the delay block deadtime is equal or slightly less than the process deadtime as studied in Advanced Application Note 3 http://www.modelingandcontrol.com/repository/AdvancedApplicationNote003.pdf


PID Controllers

Deadtime Compensator Configuration

Insert deadtime

block

Must enable dynamic reset limit !

PID Controllers

• Deadtime is eliminated from the loop. The smith predictor, which created a PV without deadtime, fools the controller into thinking there is no deadtime. However, for an unmeasured disturbance, the loop deadtime still causes a delay in terms of when the loop can see the disturbance and when the loop can enact a correction that arrives in the process at the same point as the disturbance. The ultimate limit to the peak error and integrated error for an unmeasured disturbance are still proportional to the deadtime, and deadtime squared, respectively.

• Control is faster for existing tuning settings. The addition of deadtime compensation actually slows down the response for the existing tuning settings. Setpoint metrics, such as rise time, and load response metrics, such as peak error, will be adversely affected. Assuming the PID was tuned for a smooth stable response, the controller must be retuned for a faster response. For a PID already tuned for maximum disturbance rejection, the gain can be increased by 250%. For deadtime dominant systems where the total loop deadtime is much greater than the largest loop time constant (hopefully the process time constant), the reset time must also be decreased or there will be severe undershoot. If you decrease the reset time to its optimum, undershoot and overshoot are about equal. For the test case where the total loop deadtime to primary process time constant ratio was 10:1, you could decrease the reset time by a factor of 10. Further study is needed as to whether the minimum reset time is a fraction of the underestimated deadtime plus the PID module execution time (0.5 sec) where the fraction depends upon the dead time to time constant ratio

Deadtime Myths Busted

For access to Deminar 10 ScreenCast Recording or SlideShare Presentation go tohttp://www.modelingandcontrol.com/2010/10/review_of_deminar_10_-_deadtim.html

PID Controllers

• Compensator works better for loops dominated by a large deadtime. The reduction in rise time is greatest and the sensitivity to per cent deadtime modeling error particularly for an overestimate of deadtime is least for the loop that was dominated by the process time constant. You could have a deadtime estimate that was 100% high before you would see a significant jagged response when the process time constant was much larger than the process deadtime. For a deadtime estimate that was 50% too low, some rounded oscillations developed for this loop. The loop simply degrades to the response that would occur from the high PID gain as the compensator deadtime is decreased to zero. While the magnitude of the error in deadtime seems small, you have to remember that for an industrial temperature control application, the loop deadtime and process time constant would be often at least 100 times larger. For a 400 second deadtime and 10,000 second process time constant, a compensator deadtime 200 seconds smaller or 400 seconds larger than actual would start to cause a problem. In contrast, the deadtime dominant loop developed a jagged response for a deadtime that was high or low by just 10%. I think this requirement is unreasonable in industrial processes. A small filter of 1 second on the input to the deadtime block in the BKCAL path may have helped.

• An underestimate of the deadtime leads to instability. In tuning calculations for a conventional PID, a smaller than actual deadtime can cause an excessively oscillatory response. Contrary to the effect of deadtime on tuning calculations, a compensator deadtime smaller than actual deadtime will only cause instability if the controller is tuned aggressively after the deadtime compensator is added.

• An overestimate of the deadtime leads to sluggish response and greater stability. In tuning calculations for a conventional PID, a larger than actual deadtime can cause an excessively slow response. Contrary to the effect of deadtime on tuning calculations, a compensator deadtime greater than actual deadtime will cause jagged irregular oscillations.

Deadtime Myths Busted

PID Controllers

• Feed Maximization– Model Predictive Control described in Application Note 1

http://www.modelingandcontrol.com/repository/AdvancedApplicationNote001.pdf– Override control (next slide) is used to maximize feeds to limits of operating constraints via

valve position control (e.g. maximum vent, overhead condenser, or jacket valve position with sufficient sensitivity per installed characteristic).

– Alternatively, the limiting valve can be set wide open and the feeds throttled for temperature or pressure control. For pressure control of gaseous reactants, this strategy can be quite effective.

– For temperature control of liquid reactants, the user needs to confirm that inverse response from the addition of cold reactants to an exothermic reactor and the lag from the concentration response does not cause temperature control problems.

– All of these methods require tuning and may not be particularly adept at dealing with fast disturbances unless some feedforward is added. Fortunately the prevalent disturbance that is a feed concentration change is often slow enough due to raw material storage volume to be corrected by temperature feedback.

• Profile Control– If you have a have batch measurement that should increase to a maximum at the batch end

point (e.g. maximum reaction temperature or product concentration), the slope of the batch profile of this measurement can be maximized to reduce batch cycle time. For application examples checkout “Direct Temperature Rate of Change Control Improves Reactor Yield” in a Funny Thing Happened on the Way to the Control Roomhttp://www.modelingandcontrol.com/FunnyThing/ and the Control magazine article “Unlocking the Secret Profiles of Batch Reactors”http://www.controlglobal.com/articles/2008/230.html .


PID Controllers

Secondary loop slowed down by a factor of 5

Secondary SP

Secondary CO

Primary PV

Secondary SP

Primary PV

Secondary CO

Effect of Slow Secondary Tuning (cascade control)

PID Controllers

ControlValveAOPIDPID

AI AI

FlowMeter Process

ProcessSensor

Secondary (Inner) Loop Feedback

Primary (Outer) Loop Feedback

ProcessSP

FlowSP Out

PV PV

RelayPID*

Position Loop Feedback

DCS Valve Positioner

Position (Valve Travel)

I/P

Drive Signal

* most positioners use proportional only

Triple Cascade Loop Block Diagram

PID Controllers

Feedforward Applications• Feedforward is the most common advanced control technique used - often the

feedforward signal is a flow or speed for ratio control that is corrected by a feedback process controller (Flow is the predominant process input that is manipulated to set production rate and to control process outputs (e.g. temperature and composition))

– Blend composition control - additive/feed (flow/flow) ratio– Column temperature control - distillate/feed, reflux/feed, stm/feed, and bttms/feed (flow/flow) ratio– Combustion temperature control - air/fuel (flow/flow) ratio – Drum level control - feedwater/steam (flow/flow) ratio– Extruder quality control - extruder/mixer (power/power) ratio – Heat exchanger temperature control - coolant/feed (flow/flow) ratio– Neutralizer pH control - reagent/feed (flow/flow) ratio– Reactor reaction rate control - catalyst/reactant (speed/flow) ratio– Reactor composition control - reactant/reactant (flow/flow) ratio– Sheet, web, and film line machine direction (MD) gage control - roller/pump (speed/speed) ratio– Slaker conductivity control - lime/liquor (speed/flow) ratio – Spin line fiber diameter gage control - winder/pump (speed/speed) ratio

• Feedforward is most effective if the loop deadtime is large, disturbance speed is fast and size is large, feedforward gain is well known, feedforward measurement and dynamic compensation are accurate

• Setpoint feedforward is most effective if the loop deadtime exceeds the process time constant and the process gain is well known

For more discussion of Feedforward see May 2008 Control Talkhttp://www.controlglobal.com/articles/2008/171.html

PID Controllers

Feedforward Implementation - 1• Feedforward gain can be computed from a material or energy balance ODE * &

explored for different setpoints and conditions from a plot of the controlled variable (e.g. composition, conductivity, pH, temperature, or gage) vs. ratio of manipulated variable to independent variable (e.g. feed) but is most often simply based on operating experience

– * http://www.modelingandcontrol.com/repository/AdvancedApplicationNote004.pdf– Plots are based on an assumed composition, pressure, temperature, and/or quality

– For concentration and pH control, the flow/flow ratio is valid if the changes in the composition of both the manipulated and feed flow are negligible.

– For column and reactor temperature control, the flow/flow ratio is valid if the changes in the composition and temperature of both the manipulated and feed flow are negligible.

– For reactor reaction rate control, the speed/flow is valid if changes in catalyst quality and void fraction and reactant composition are negligible.

– For heat exchanger control, the flow/flow ratio is valid if changes in temperatures of coolant and feed flow are negligible.

– For reactor temperature control, the flow/flow ratio is valid if changes in temperatures of coolant and feed flow are negligible.

– For slaker conductivity (effective alkali) control, the speed/flow ratio is valid if changes in lime quality and void fraction and liquor composition are negligible.

– For spin or sheet line gage control, the speed/speed ratio is valid only if changes in the pump pressure and the polymer melt quality are negligible.

• Dynamic compensation is used to insure the feedforward signal arrives at same point at same time in process as upset

– Compensation of a delay in the feedforward path > delay in upset path is not possible

PID Controllers

• Feedback correction is essential in industrial processes– While technically, the correction should be a multiplier for a change in slope and a bias for a change

in the intercept in a plot of the manipulated variable versus independent variable (independent from this loop but possibly set by another PID or MPC), a multiplier creates scaling problems for the user, consequently the correction of most feedforward signal is done via a bias.

– The bias correction must have sufficient positive and negative range for worst case.– Model predictive control (MPC) and PID loops get into a severe nonlinearity by creating a controlled

variable that is the ratio. It is important that the independent variable be multiplied by the ratio and the result be corrected by a feedback loop with the process variable (composition, conductivity, gage, temperature, or pH) as the controlled variable.

• Feedforward gain is a ratio for most load upsets.• Feedforward gain is the inverse of the process gain for setpoint feedforward.

– Process gain is the open loop gain seen by the PID (product of manipulated variable, process variable, and measurement variable gain) that is dimensionless.

• Feedforward action must be in the same direction as feedback action for upset. • Feedforward action is the opposite of the control action for setpoint feedforward.• Feedforward delay and lag adjusted to match any additional delay and lag,

respectively in path of upset so feedforward correction does not arrive too soon. • Feedforward lead is adjusted to compensate for any additional lag in the path of

the manipulated variable so the feedforward correction does not arrive too late.• The actual and desired feedforward ratio should be displayed along with the bias

correction by the process controller. This is often best done by the use of a ratio block and a bias/gain block instead of the internal PID feedforward calculation.

Feedforward Implementation - 2

PID Controllers

117

Linear Reagent Demand Control(PV is X axis of Titration Curve)

• Signal characterizer converts PV and SP from pH to % Reagent Demand– PV is abscissa of the titration curve scaled 0 to 100% reagent demand– Piecewise segment fit normally used to go from ordinate to abscissa of curve– Fieldbus block offers 21 custom space X,Y pairs (X is pH and Y is % demand)– Closer spacing of X,Y pairs in control region provides most needed compensation– If neural network or polynomial fit used, beware of bumps and wild extrapolation

• Special configuration is needed to provide operations with interface to:– See loop PV in pH and signal to final element– Enter loop SP in pH– Change mode to manual and change manual output

• Set point on steep part of curve shows biggest improvements from: – Reduction in limit cycle amplitude seen from pH nonlinearity– Decrease in limit cycle frequency from final element resolution (e.g. stick-slip)– Decrease in crossing of split range point– Reduced reaction to measurement noise– Shorter startup time (loop sees real distance to set point and is not detuned)– Simplified tuning (process gain no longer depends upon titration curve slope)– Restored process time constant (slower pH excursion from disturbance)

PID Controllers

Open Loop Backup Configuration

SP_Rate_DN and SP_RATE_UP used to insure fast getaway and slow approach

Open loop backup used for prevention of compressor surge and RCRA pH violation

Open Loop Backup Configuration

PID Controllers

Open Loop Backup Disturbance Response

Open Loop Backup

PID Controllers

Conductivity Kicker for Evaporator

PID Controllers

121

Top Ten Reasons Why an Automation Engineer Makes a Great Spouse or at Least a Wedding Gift

• (10) Reliable from day one• (9) Always on the job• (8) Low maintenance (minimal grooming, clothing, and entertainment

costs• (7) Many programmable features• (6) Stable• (5) Short settling time• (4) No frills or extraneous features• (3) Relies on feedback• (2) Good response to commands and amenable to real time optimization• (1) Readily tuned

122

Enhanced PID Algorithm Originally Developed for Wireless

PID integral mode is restructured to provide integral action to match the process response in the elapsed time (reset time set equal to process time constant)PID derivative mode is modified to compute a rate of change over the elapsed time from the last new measurement valuePID reset and rate action are only computed when there is a new valueIf transmitter damping is set to make noise amplitude less than communication trigger level, valve packing and battery life is dramatically improvedEnhancement compensates for measurement sample time suppressing oscillations and enabling a smooth recovery from a loss in communications further extending packing -battery life

+

+

+

+

Elapsed Time

Elapsed Time

TD

Kc

Kc

TD

http://www2.emersonprocess.com/siteadmincenter/PM%20DeltaV%20Documents/Whitepapers/WP_DeltaV%20PID%20Enhancements%20for%20Wireless.pdf

Link to Enhanced PID White Paper

PID Controllers

123

Loop Block Diagram (First Order Approximation)

τp2 θp1 τp1Kpθp2

τc1 τm2 θm2 τm1 θm1Kmθcτc2

Kc Ti Td

Valve Process

Controller Measurement

Kvτvθv

KLτLθL

Load Upset

Δ%PV

Δ%CO

ΔFv

ΔPV

PID

Delay Lag

DelaySecondary

DelayPrimaryDelay

Delay

Delay

Delay

LagSecondary

LagPrimary

Lag

LagLagLag

Lag

Gain

Gain

Gain

Gain

LocalSet Point

ΔL

First Order Approximation: θo ≅ θv + θp1 + θp2 + θm1 + θm2 + θc + τv + τp2 + τm1 + τm2 + τc1 + τc2(set by automation system design for flow, pressure, level, speed, surge, and static mixer pH control)

%

%

%

Delay <=> Dead TimeLag <=>Time Constant

For integrating processes: Ki = Kv ∗ (Kp / τp1) ∗ Km

100% / span

Hopefully τp1 is the largest lag in the loop

½ of Wireless Default Update Rate

PID Controllers

124

Time (seconds)

% Controlled Variable (%PV) or

% Controller Output (%CO)

Δ%CO

Δ%PV

θo

Ko = Δ%PV / Δ%CO

0.63∗Δ%PV

%CO

%PV

Self-regulating process open loopnegative feedback time constant

Self-regulating process gain (%/%)

Response to change in controller output with controller in manual

observed total loopdeadtime

ideally τp1τo

Maximum speedin 4 deadtimesis critical speed

Open Loop Response ofSelf-Regulating Process

Noise Band

For CSTR τo >> θo processresponse appears to ramp for 10 θo and is termed a “near-integrating” process

For plug flow reactor andthe manipulation of feed θo >> τo process responseis a transport delay and istermed “deadtime dominant”

PID Controllers

125

Time (seconds)θo

Ki = { [ %PV2 / Δt2 ] − [ %PV1 / Δt1 ] } / Δ%CO

Δ%CO

ramp rate isΔ%PV1 / Δt1

ramp rate isΔ%PV2 / Δt2

%CO

%PV

Integrating process gain (%/sec/%)

Response to change in controller output with controller in manual% Process Variable (%PV)

or% Controller Output (%CO)

observed total loopdeadtime


Open Loop Response ofIntegrating Process

Wireless Trigger Level > Noise

Wireless DefaultUpdate Rate

Wireless default update rate must be fastenough that excursion for maximum ramprate is less than wireless trigger level that is set just larger than measurement noise

PID Controllers

126

Response to change in controller output with controller in manual

Noise Band

Acceleration

Δ%PV

Δ%CO

1.72∗Δ%PV

Ko = Δ%PV / Δ%CO Runaway process gain (%/%)

% Process Variable (%PV) or

% Controller Output (%CO)

Time (seconds)observed total loopdeadtime runaway process open loop

positive feedback time constant

For safety reasons, tests are terminated after 4 deadtimes


Open Loop Response ofRunaway Process

Wireless presently notadvisable for runaway

τ’o must be τ’p1θo

Tests are terminatedbefore a noticeableacceleration leadingto characterization asan integrating process

PID Controllers

127

COtPVMaxKKo

oi %/)/%( ΔΔΔ==

τ

For “Near Integrating” gain approximation use maximum ramp rate divided by change in controller output

The above equation can be solved for the process time constant by taking the process gain to be 1.0 or for more sophistication as the

average ratio of the controlled variable to controller output

Tuning test can be done for a setpoint change if the PID gain is > 2 and the PID structure is

“PI on Error D on PV” so you see a step changein controller output from the proportional mode

Near Integrator Gain Approximation

The maximum ramp rate is found by passing filtered process variable (PV) through a deadtime (DT) block to create an old process variable. The deadtime block uses the total loop deadtime (θo) for the time interval (Δt ). The old process variable is subtracted from the new process variable and divided by the time interval to get the ramp rate. The maximum of a continuous train of ramp rates updated each module execution over A period of 3 or more deadtimes is selected to compute the near integrating process gain. For an inverseresponse or large secondary time constant, the computation may need to continue for 10 or more deadtimes.

PID Controllers

128

SRoo

oNI TT ∗

∗+∗

=τθ

θ4

3

The near integrating test time (3 deadtimes) as a fraction of the self-regulating test (time to steady state is taken as 98% response time TSR = T98 = θo + 4 το ) is:

If the process time constant is greater than 6 times the deadtime

oθτ ∗≥ 6o

Then the near integrating tuning test time is reduced by > 90%:

SRNI TT ∗≤ 1.0

For example:

sec100o =τ sec4o =θThe near integrator tuning time is reduced by 97%!

SRNI TT ∗≤ 03.0

Reduction in Identification Test Time

PID Controllers

129

4

PVSUB

First Principle Parameters = f (Ki)

CO0Initial Controller Output at time 0

∆COθo

Ko

Ki∆PV Switch

ODE (Ki)

∆PV

∆PV

Sum

PV0Initial Controlled Variable at time 0

Ko = PV0 / CO0 process gain approximationτo = Ko / Ki negative feedback time constant

τ’o = Ko / Ki positive feedback time constant

Methodology extends beyond loops to anyprocess variable that can be measured

and any variablethat can be changed

CO

τo

Ko τ’o

1

2

3

∆PV

For the manipulation of jacket temperature to control vessel temperature, the near integrator gain is

)(/)( opi MCAUK ∗∗=Since we generally know vessel volume (liquid mass), heat transfer area, and process heat capacity,

We can solve for overall heat transfer coefficient (least known parameter) to provide a usefulordinary differential equation (ODE) for a first principle model (1).

Rapid Process Modeling Opportunity

PID Controllers

130

ooo

ox EE ∗

+=

)( τθθ

ooo

oi EE ∗

+=

)(

2

τθθ

Peak error is proportional to the ratio of loop deadtime to 63% response time(Important to prevent SIS trips, relief device activation, surge prevention, and RCRA pH violations)

Integrated error is proportional to the ratio of loop deadtime squared to 63% response time(Important to minimize quantity of product off-spec and total energy and raw material use)

Loop Performance Ultimate Limit

Total loop deadtimethat is often set byautomation design

Largest lag in loopthat is ideally set by

large process volume

Wireless default update rate affects ultimate performance limitbecause ½ of default update rate is additional loop deadtime

PID Controllers

131

oco

x EKK

E ∗∗+

=)1(

1

oco

fxii E

KKtT

E ∗∗

++=

τΔ

Peak error decreases as the controller gain increases but is essentially the open loop error for systems when total deadtime >> process time constant

Integrated error decreases as the controller gain increases and reset time decreases but is essentially the open loop error multiplied by the reset time plus signal delays and lags for systems when total deadtime >> process time constant

Open loop error forfastest and largestload disturbance

ocffi

r SPKSPCOKSPT θ+

∗+=

)|,min(|( max ΔΔΔ

Rise time (time to reach a new setpoint) is inversely proportional to controller gain

Loop PerformancePractical Limit

PID Controllers

132

oo

oc K

Kθ

τ∗

∗= 4.0 oiT θ∗= 4 opT θτ ∗== 5.02d

For runaway processes:

For self-regulating processes:

oic K

Kθ∗

∗=15.0 oiT θ∗= 4 opT θτ ∗== 5.02d

oic K

Kθ∗

∗=16.0

oiT θ∗= 40 opT θτ =∗= 2d 2

For integrating processes:

oo

oc K

Kθ

τ∗

∗='6.0

oic K

Kθ∗

∗=14.0

Near integrator (τo >> θo):

oiT θ∗= 5.0

Near integrator (τ’o >> θo):

Deadtime dominant (τo << θo):

0d =To

c KK 14.0 ∗=

Fastest Controller Tuning(Reaction Curve Method*)

These tuning equations provide maximumdisturbance rejection but will cause

some overshoot of setpoint response

* - Ziegler Nichols method closed loop modifiedto be more robust and less oscillatory

1.0 for Enhanced PID if Wireless Default Update Rate > Process Response Time !

Wireless default update rate affects fastest controller tuningbecause ½ of default update rate is additional loop deadtime

PID Controllers

133

Effect of Wireless MeasurementUpdate Time and Interval on Performance

o

omS E

S τθ ∗∗=

5.0

ovwo

x ET

E ∗++

=63

θθθo

vwoi E

TE ∗

++=

63

2)( θθθowoT τθθ ++=63

),( STw Min θθθ Δ= wT TΔΔ ∗= 5.0θmax)/%(

5.0tPV

SmS ΔΔ

∗=θ

)/()/%( max ooi KEKtPV ∗=ΔΔo

oi

KKτ

=o

oE

tPVτ

=max)/%( ΔΔ

PID Controllers

134

Additional Deadtime fromValve Stick-Slip, Resolution, or Deadband

ox

oovv EK

KS∗

∗∗∗=

θθ 5.0

max)/%(5.0

tCOSv

v ΔΔ∗

=θ maxmax )/%()/%( tPVKtCO c ΔΔΔΔ ∗=

oo

x

KEK

tCO o

θ∗

∗=max)/%( ΔΔ

[ ]⎥⎦⎤

⎢⎣

⎡−∗

∗=

002.0),(max,min

mm

v

oo

oxc SN

SKKK

θτ

o

oE

tPVτ

=max)/%( ΔΔ

Increase in process gain from elimination of controller reaction to noise by wireless trigger level or PID threshold sensitivity setting decreases deadtime from valve stick-slip, resolution, or deadband

PID Controllers

135

ΔCV = change in controlled variable (change in process variable in % of scale)Δ%CO = change in controller output (%)Kc = controller gain (dimensionless)Ki = integrating process gain (%/sec/% or 1/sec)Kp = process gain (dimensionless) also known as open loop gainΔL = change in load (engineering units)ΔFv = change valve flow (engineering units)Δ%PV = change in process variable (%)ΔSP = change in setpoint (engineering units)SPff = setpoint feedforward (engineering units)Δt = change in time (sec)Δtx = execution or update time (sec)θo = total loop dead time (sec)τf = filter time constant or well mixed volume residence time (sec)τm = measurement time constant (sec) Τp2 = secondary (small) self-regulating process time constant (sec) τ’p1 = primary (large) runaway process time constant (sec) τp1 = primary (large) process time constant (sec) Ti = integral (reset) time setting (sec/repeat)Td = derivative (rate) time setting (sec)Tr = rise time for setpoint change (sec)to = oscillation period (sec)λ = Lambda (closed loop time constant or arrest time) (sec)λf = Lambda factor (ratio of closed to open loop time constant or arrest time)

Nomenclature(Process Dynamics & Performance)

PID Controllers

136

Ei = integrated error for unmeasured load disturbance (% sec)Ex = peak error for unmeasured load disturbance (%)Eo = open loop error (loop in manual) for unmeasured load disturbance (%)Ki = near integrator process gain (% per % per sec)Ko = open loop gain (product of valve, process, and measurement gains) (dimensionless)Kx = detuning factor for controller gain (dimensionless)Nm = measurement noise (%)Δ%CO/Δt = rate of change in PID % controller output (% per sec)Δ%PV/Δt = rate of change in PID % process variable (% per sec)ΔTw = wireless default update rate (update time interval) (sec)Sm = wireless measurement trigger level (threshold sensitivity) (%)Sv = valve stick-slip, resolution, or deadband (%)T63 = 63% process response time (sec)θo = original loop deadtime (sec)θΔt = additional deadtime from default update rate (sec)θs = additional deadtime from wireless trigger level (sec)θv = additional deadtime from valve (sec)θw = additional deadtime from wireless measurement (sec)τo = self-regulating open loop time constant (largest time constant in loop) (sec)τ’o = runaway open loop time constant (largest time constant in loop) (sec)

Nomenclature(Wireless Dynamics & Performance)

PID Controllers

Liquid Reactants (Jacket CTW) Liquid Product Optimization

137

TT1-4

TC1-3

TC1-4

AT 1-6

LY 1-8

FY 1-6

FT1-2

FC1-2

reactant A

reactant B

CAS

residencetime calc

ratiocalc

AC1-6

makeup

return

LY 1-8

FY 1-6

reactant A

reactant B

residencetime calc LT

1-8TT1-3

LC1-8

CAS

ratiocalc

product

ventFT1-1

FC1-1

FC 1-1CAS

ZC1-4OUT

ZC1-4

FC1-7

FT 1-7

PT1-5

PC1-5

FT 1-5

CTW

ZC1-4 is an enhanced PID VPC

Valve position controller (VPC) setpointis the maximum throttle position. TheVPC should turn off integral action toprevent interaction and limit cycles. Thecorrection for a valve position less thansetpoint should be slow to provide a slow approach to optimum. The correction fora valve position greater than setpoint mustbe fast to provide a fast getaway from thepoint of loss of control. Directional velocitylimits in AO with dynamic reset limit in anenhanced PID that tempers integral actioncan achieve these optimization objectives.

PID Controllers

Liquid Reactants (Jacket BFW) Liquid Product Optimization

138

AT 1-6

LY 1-8

FY 1-6

FT1-2

FC1-2

reactant A

reactant B

CAS

residencetime calc

CAS

ratiocalc

AC1-6

steam

PT1-4

TC1-3

PC1-4

LY 1-8

FY 1-6

reactant A

reactant B

residencetime calc LT

1-8TT1-3

LC1-8

ratiocalc

product

ventFT1-1

FC1-1

LT1-9

LC1-9

BFW

ZC1-4

ZC1-9

ZY 1-1

FC1-1CAS

ZY1-1OUT

low signalselector

FC1-7

FT 1-7

PT1-5

PC1-5

FT 1-5

FT 1-9

ZC1-4 & ZC-9 are enhanced PID VPC

PID Controllers

Examples of Optimization by Valve Position Control (VPC)

139

Optimization VPC PID PV VPC PID SP VPC PID Out

Minimize PrimeMover Energy

Reactor FeedFlow PID Out

Max Throttle Position Compressor or PumpPressure SP

Minimize BoilerFuel Cost

Steam Flow PID Out Max Throttle Position BoilerPressure SP

Minimize BoilerFuel Cost

Equipment Temperature PID Out

Max Throttle Position BoilerPressure SP

Minimize Chilleror CTW Energy

Equipment Temperature PID Out

Max Throttle Position Chiller or CTW Temperature SP

Minimize PurchasedReagent or Fuel Cost

Purchased Reagent or Fuel Flow PID Out

Min Throttle Position Waste ReagentOr Fuel Flow SP

Minimize Total Reagent Use

Final NeutralizationStage pH PID Out

Min Throttle Position First NeutralizationStage pH PID SP

Maximize Reactor Production Rate

Reactor or CondenserTemperature PID Out

Max Throttle Position Feed Flow or Reaction Temperature SP

Maximize Reactor Production Rate

Reactor VentPressure PID Out

Max Throttle Position Feed Flow or Reaction Temperature SP

Maximize Column Production Rate

Reboiler or CondenserFlow PID Out

Max Throttle Position Feed Flow or Column Pressure SP

Maximize Ratio orFeedforward Accuracy

Process FeedbackCorrection PID Out

50%(Zero Correction)

Flow Ratio or Feedforward Gain

PID Controllers

Key PID Features for Valve Position Control

140

Feature Function Advantage 1 Advantage 2

Direction Velocity Limits Limit VPC Action Speed Based on Direction

Prevent Running Out of Valve

Minimize Disruptionto Process

Dynamic ResetLimit

Limit VPC Action Speed to Process Response

Direction Velocity Limits Prevent Burst of Oscillations

Adaptive Tuning Automatically Identify and Schedule Tuning

Eliminate Manual Tuning Compensation of Nonlinearity

Feedforward Preemptively Set VPC Out for Upset

Prevent Running Out of Valve

Minimize Disruption

Enhanced PID Suspend Integral Action until PV Update

Eliminate Limit Cyclesfrom Stiction & Backlash

Minimize Oscillations from Interaction & Delay

Valve position control should minimize interactions with temperature loop, oscillations from valve backlash and stick-slip, and prevent saturation of temperature control valve for large disturbances

PID Controllers

Gas Reactants (Jacket BFW) Gas Product Optimization

141

AT 1-6

FT1-2

FC1-2

CAS

AC1-6

TC1-3

FY 1-6

gas reactant A

TT1-3b

TT1-3a

CAS

ratiocalc

product

FT1-1

FC1-1

steamBFW

gas reactant B

TT1-3c

steamBFW

steamBFW

TY 1-3

high signalselector

average bedtemperatures

Fluidized BedCatalytic Reactor

PT1-5

PC1-5

FT 1-5

Fast reaction, short residence time, and high heat release prevents inverse response in manipulation of reactantfeed rate for temperature control.

Temperature controller inherentlymaximizes reactant feed rate toamount permitted by the numberof BFW coils in service

PID Controllers

FC

FC

FC

FC

FC

LC

LC

TC

142

Material Balance – Type 1 (Direct)

DL

Q

F

V

B

Change in temperature controller output does not affect column until level controller output changes so high level PID gain or feedforward is needed

PID Controllers

The level control of reflux provides some internalreflux control by decreasing reflux for an decreasein vapor flow from cold weather or rain storm

FC

FC

FC

FC

FC

LC

LC

TC

143

Material Balance – Type 2 (Indirect)

DL

Q

F

V

B

PID Controllers

FC

FC

FC

FC

FC

LC

LC

TC

144

Material Balance – Type 3 (Indirect)

If the distillate flow (D) was on auto with a local set point instead of cascade with aremote set point from level, the materialbalance would be fixed and we would have separation control rather material balance control. Separation control is not recommended because it requires much larger adjustments to the V/F, energy perunit feed to control composition severelylimiting the range of control (size of upsetsand set point changes handled before low and high V causes weeping and flooding)

DL

Q

F

V

B

PID Controllers

FC

FC

FC

FC

FC

LC

LC

TC

145

Material Balance – Type 4 (Direct)

DL

Q

F

V

B

steam

level

Inverse Response

PID Controllers

+1%D/F

-1%D/F

146

Control Stage Location

PID Controllers

CONTROL STAGE TEMPERATURE, degF

98

98.2

98.4

98.6

98.8

99

99.2

99.4

99.6

99.8

100

85 90 95 100 105 110

CO

MPO

SITIO

N, w

t%

BOTTOM PRODUCT

DISTILLATE PRODUCT

147

Temperature Sensitivity

PID Controllers

148

Disturbance Analysis

PID Controllers

PID Controller Option Checklist

The following checklist is not intended to cover all the configuration requirements but some of the major application details to be addressed for PID controllers. If you don’t get the valve action and control action right, nothing else matters. The controller output will ramp off to an output limit. The valve action (inc-open and inc-close) can be set in many different places, such as the PID block, analog output (AO) block, splitter block, signal characterizer block, current to pneumatic (I/P) transducer, or the positioner. Make sure the valve signal is not reversed in more than one location for an inc-close (fail open) valve. Once the valve action is set properly, the control action is set to be the opposite of the process action. The control action is reverse and direct if a change in the PID output causes the PID process variable (PV) to increase or decrease, respectively. Verify with process engineer the valve action, process action, and resulting control action required. The setting of all options and parameters must be verified as applicable. Simulations representative of the dynamic behavior of the process and the field automation system along with the actual configuration to form a virtual plant is advisable for testing and confirmation plus training and opening the door to process control improvement (see Exceptional Opportunities in Process Control – Virtual Plants) http://modelingandcontrol.com/2010/01/exceptional_opportunities_in_p_8/Does measurement scale cover entire operating range including abnormal conditions?Is valve action correct (inc-open for fail close and inc-close for fail open)? Control action correct (direct for reverse process and reverse for direct process if valve action set)?

149

PID Controllers

PID Controller Option Checklist

Is PID Form ISA standard? Is output limits set to obey setpoint limits in cascade and remote cascade mode?Is back calculate correctly connected for bumpless transfer (PV for back calculate in secondary)?Is PID Structure correct for application (PI action on error, D action on PV for most loops)?Does setpoint track PV in manual unless setpoint must be inherently saved in PID?Do setpoint limits to match process, equipment, and valve constraints?Do output limits to match process, equipment, and valve constraints?Do anti-reset windup (ARW) limits to match output limits?Is execution time less than 10% of minimum reset time?Is signal filter less than 10% of minimum reset time?Is PID tuned with auto tuner or adaptive tuner?Is rate Time less than ½ deadtime (typically zero except for temperature loops)?Is dynamic reset limit enabled for cascade, AO velocity limits, and slow valve?Are AO velocity limits set for blending, valve position control, and surge control?Is integral deadband > limit cycle PV amplitude from deadband and resolution?

150

PID Controllers

Effective Use of Measurements, Valves, and PID Controllers · was inducted into the Control...

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