CO2 Pressure Flow ControlOptimization

GIL supplies C02 from their ammonia plant to various customersinside and outside the company. To meet the customers' changingdemands and to balance them with the CO2 output of the ammoniaplant, an innovative control system was installed.

Wayne E. ArmstrongGIL, Inc., Courtright, Ontario, Canada

The microprocessor based controlsystem described in this paper wasplaced in service in July, 1986 andhas resulted in optimum C02control/exportâtion in the CILAmmonia I plant at Courtright,Ontario.

The Ammonia I plant is an M.W.Kellogg design ammonia producingplant built in 1966 with a designproduction rate of 1175 Te/day.

The system design maintains thehigh degree of reliability requiredfor C02 back pressure control andprovides for export optimization ofC02 within the works, as well as toexternal consumers.

The system has proven veryreliable and has been accepted bythe operating personnel.

CIL Lambton WorksCourtright, Ontario, CanadaAUTHOR: Wayne E. ArmstrongProcess Control Technologist

Through the use of thisprogrammable stand-alone control,CIL Lambton Works was able tooptimize C02 export and become thelargest exporter of food/medicalgrade product in Canada.

The operating tolerances wereestablished by the Ammonia Ioperating supervision while theexport requirements and limits werenegotiated by the Marketing/SalesDivision with two C02 producerssituated on CIL property. Itshould be noted that the C02 plantsare operated with their ownmanpower.

The Process Control Section waspresented with information from theabove disciplines and asked todesign, install and commission acontrol system that would satisfyall requirements.

Until this time the Ammonia Icontrol room contained a smallamount of electronic datacollection equipment while allclosed loop analog control systemswere pneumatic.

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The system was required toaccurately control C02 backpressure while trying to minimizeC02 venting by trying to increaseexport. This goal was obtained byusing a number of electronicstand-alone controllers. They canbe operated discreetly or are ableto be switched to operate from aProgram Director and minimizeoperator interface.

The control panel mountedoperator's display combine analogLED displays and LED digitalvalues. This allows the operatorquick reference to systemcontrolability while eliminatingthe requirement of multiplying thescale by a factor to arrive at theexact engineering units.

INTRODUCTION

Maintaining a back pressure onthe C02 separator of approximately40 KpA is required to stabilize theC02 in the gas exiting the C02absorber, reducing fluctuations inmethanator temperatures. Becausethe C02 is not totally consumedin-plant, the back pressurecontroller will vent the excess gasback to the atmosphere via a highvent.

Many ammonia producing plantshave an associated Urea producingplant that uses the C02 gas in theproduction of Urea. Accuracy andreliability of the control systemis required to stabilize thesuction pressure at, in the CILWorks, two Urea plants. In theUrea process, the most crucialprocess variable, the third stagesuction pressure is adjusted by abattery limit manually operatedvalve. Any pressure loss at thecompressor suction could result ina low pressure trip.

To maximize Urea production, itis obvious that a stable supply ofC02 gas is of prime importance.Internal consumption of C02 by theUrea plants equals approximately320 Te/day.

Secondary uses for C02 afterpurification and. liquification arefound in food preparation, medicalprofession and industrialapplications. In CIL's case, theexcess C02 produced is processed byother companies, Liquid CarbonicInc and Canada Liquid Air, locatedwithin our site boundaries.

The market demand for C02 forrefrigeration and carbonationprocesses as well as industry hasbeen increasing steadily withapplications in:

a) freezing of food,especially poultry, fish andvegetables.

b) the bottling industry inthe soft drink and beer market.

c) industrial applicationssuch as plastic molding, rubberflashing and foundry applications.

In the early to mid-1980's,many sources of C02 were no longeravailable as a result of closure ordownscaling many refineries withthe decrease in off-shore oilprices.

The loss of C02 sources and theincreasing demand forced many C02producing plants to upgrade orincrease their existing facilitieswhere the C02 source was stable.

The above conditions resultedin a requirement on CIL LambtonWorks to effectively optimize C02export to customers at a levelnever before considered, whilemaintaining its internalrequirements effectively.

Up to this time, Ammonia I wasa dedicated pneumatic controlsystem plant with a minimum numberof other thaip basic feedbackcontrol loops, i.e. cascade, Splitrange or steam drum level control.A large number of control roominstruments were the same type theplant had started up with and theoperations personnel had becomevery familiar with some of theiroperator initiated functions, i.e.multiplication factors, mechanicaloffsets, and finding problems, etc.

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The C02 back pressure controlwas a field, mounted instrument thatmodulated a sixteen inch butterflyvalve with two two inch holesdrilled in the disc to preventabsolute shut off. The sensingpoint was downstream of the Ureadraw-off point and was susceptibleto head pressure. Closeexamination of the indicatingcontroller indicated calibrationoffset and valve examinationrevealed excessive hysterisis. Upuntil the optimization requirement,the control system was operable andmaintained a back pressure thatoperations personnel had learned towork with.

On the high pressure side ofthe vent valve the Lambton WorksUrea plant and Liquid Carbonic, ourfirst customer, took suction fromthe C02 separator. FIGURE 2.

Liquid Carbonic operated a twostage reciprocating compressor witha maximum capacity of 220 Te/daywhile the Urea plants had a maximumdraw of 320 Te/day.

Because the combined C02 drawequalled 540 Te/day approximately750 Te/day went to vent duringmaximum usage. The system upset,at time of a step change of C02draw was not considered significantwith respect to vented gas.

The only protection existingfor a C02 pressure loss, was a lowpressure switch on the C02 blowerfrom Ammonia I to Liquid Carbonic.This switch would trip the blowerin the event of low C02 pressuredue to a shortage for any reason.

This system would be completelyupgraded to accommodate Marketing'snew requirements. These newrequirements consisted of sellingC02 to competitors, Liquid CarbonicInc who had tripled theirproduction facilities and who had afirst right-of-refusal and CanadaLiquid Air, a new customer withapproximately one half LiquidCarbonic's capacity.

Note. Liquid Carbonic Inc. will b«abbreviated to LCI.Canada Liquid Air will beabbreviated to CLÂ.

System FunctionWhat The System Must Do.

The control/optimization systemwas required to perform thefollowing functions which arelisted on a priority basis.

1) CQZ.j3a.ck Pressure Control.Maintain the C02 separator backpressure at 40 KpA during periodsof maximum venting as well asminimum venting.

2) gnsurg Ipfrernftl CongumpUopWith Minimum Disturbance.After satisfying criteria numberone it must provide minimum systemdisturbance during a start-up orshut-down of a Urea producing, C02consuming CIL plant.

3) Measure Export Flows.The flow measurement must be madein Te/day so that both supplier andconsumers can be assured of truemass flow in the event of differentpressure and temperature conditionsdue to individual blower dischargedifferences.

4) Control Export Flows.Flow control of export flows wasrequired in order to meetcontractual agreements and be ableto adjust customer flows in theevent increased or decreasedavailability or requirement.

5) Allow Man/Control SystemInterface.Under normal conditions the controlsystem will be operated in fullautomatic cqntrol, but at no timecan the operator be prohibiteü fromintervening as a consciousdecision. The system willannunciate at any time the systemis not in full automatic control.

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6) Ensure C02 Ffrcess Bef PTC

Vent the excess C02 to atmospherebut not allow customer incrementingafter venting quantity is minimal.

7) Control C02 Drawoff Rate.Limit the incremental rate of C02drawoff so as to not affectcriteria number one or number two.

8) Recognize LCI Requirement.Recognize and supply the C02required to a maximum contractuallyset first tier value (455 Te/day)to the first right-of -refusal C02producer.

9) Recognize CIA Requirement.Recognize and supply theincremental C02 required to amaximum contractually set firsttier value (330 Te/day) to thesecond right-of-refusal C02producer .

10) Recognize LCI AdditionalRequirement.Recognize and supply theincremental C02 required to thefirst right-of-refusal customerafter criteria number eight andnumber nine are met.

11) Recognized CLA AdditionalRequirement .Recognize and supply theincremental C02 required to thesecond right-of-refusal customerafter criteria number eight, nine,and ten are met.

12) Reduce C02 Export IfRequired.In the event of a CO 2 shortage dueto production reduction orincreased internal use, reduce C02export in the reverse order ofthose criteria previously stated.

13) Shed Load To CIA.Should the system be manuallyover-ridden and the C02 supply bediminished, the secondright-of-refusal customer must beprohibited from any C02consumption.

14) Shed Load To LCI.Subsequent to criteria numberthirteen, the firstright-of-refusal customer must beprohibited from any C02 consumptiondue to diminished supply.

15) Allow On-Line Maintenance.The control system must be easilymaintained with a minimum number ofcomponents and be able to bemaintained withoutproduction/export loss.

Design Principles FIGURE 3.How The System Has Designed.

1) CQ2 Back Pressure.The electronic pressure transmitterwas mounted on the C02 separatorvessel with a self-draining legline removing the head pressureproblem and sensing the pressurebefore any draw off occurred.

The transmitter was sourcedfrom and transmitted to a boardmounted electronic PID controller.The controller had a membrane typekeypad for set-point entry,automatic-manual switching andoutput control. Operator selectionallowed viewing the set-point andprocess variable values eitherdigitally or in an analog display.

The final control element wasreplaced with two new butterflycontrol valves, a sixteen inchvalve in parallel with a four inchvalve operating in a split rangecontrol mode.

During periods of high C02venting both valves would controlthe back pressure by having thefour inch valve fully open andthrottling the sixteen inch valve.As venting was reduced the sixteeninch valve would close and the fourinch valve would throttle the flowto control back pressure. Thisarrangement increased systemrangeability greatly.

Both control valves were handwheel equipped to allow manualfield control should systemmaintenance be required.

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2) Enffure Interna.!With Minimus Disturbance .A change in internal consumption ofC02 is in a step change fashionwhich is initiated by the startingor stopping of the C02reciprocating compressor.

Stopping of the compressorwould introduce an excess of C02 inthe system which would increase theback pressure until the vent valvescould assume their new controlposition.

Starting of the compressor atthe Urea plant required advancenotification of the Ammonia IControl Room so that vent valveposition could be checked on apanel mounted digital indicator.This would ensure adequate C02availability to minimize systempressure instability.

3) Measure Export Flows.In order to meet their exact plantinlet requirements, each CO 2purchaser had built within theAmmonia I battery limits their ownblower houses complete with inlet,outlet separators and after coolersas well as surge control.

These different processconditions and contractualagreements required the C02 bemeasured and calculated to massflow for a comparison standard.

Both companies had requestedunits of Tonnes /day of raw gas asthe unit of measurement.

The possibility of moisturecarryover and introducing apressure drop in the twelve inchand sixteen inch discharge linesmade a pitot-tube style devicebetter suited for this applicationthan an orifice plate installation.

Again, in this application allleg lines had to be self drainingand the possibility of liquidchannelling along the bottom of thepipe required the pitot tube beinstalled forty-five degrees off ofvertical .

The mass flow measurement wasaccomplished by using twoelectronic differential-pressuretransmitters, one for low flow andone for high flow, an electronicpressure transmitter and anelectronic temperature transmitterwhich were sourced from andtransmitted to their own flowcomputer. The mass flow wascalculated using this formulas

Q =

STANDARDFIGURE 4.

= CONSTANT= DIFF PRESS= PRESS= TEMP.

The flow computer thenre-transmits the mass flow signalto a board mounted electronic PIDcontroller.

Both export installations weremade identical and sourced from acommon supply to ensure equality ofmeasurement to customers.

4) Control Export Flows.Export flow control wasaccomplished by taking the massflow transmitted signals to theirrespective electronic PID flowindicating controllers mounted sideby side adjacent the back pressurecontrollers. These controllers arecapable of .manual-automatic-remoteset point operation which will bediscussed later.

These controllers are of thesame make and design as thepressure controller.

The two flow controllers sentan output signal to theirrespective cpntrol valves locatedin the sixteen inch suction pipingto each blower house. Controlvalveo smaller than sixteen inchescould have been selected for betterresponse time but any additionalpressure drop was avoided by goingto line size.

In order to increase responsetime of this big valve, the twoflow controllers were programmed tolimit their output signal and toprohibit "reset wind-up".

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Both control valves wereequipped with hand wheels formanual overide andsolenoid-operated valves to bediscussed later.

A large part of the designconsiderations is incorporated intoone panel mounted instrument thatis called a "Program Director". Itis a microprocessor based, panelmounted device which monitors manyfunctions and output signals bothdigital and analog, to ensuremaximum system performance.

No operator adjustment of thisinstrument is allowed a securitylock out feature of the device.

The device monitors/outputs thefollowing values;

DIGITAL

Pressure Controller Auto-ManualStatus (Input)LCI Flow Controller Auto-ManualStatus (Input)CLA Flow Controller Auto-ManualStatus (Input)Output To Annunciator (Output)

ANALOG

Pressure ControllerProcess VariablePressure ControllerValvesMass Flow To LCIMass Flow To CLAMass Flow Set PointControllerMass Flow Set PointController

Set Point -(Input)

Output To(Input)(Input)(Input)

To LCI Flow(Output)

To CLA Flow(Output)

Full automatic operation of thecontrol system is accomplishedwhen:

1) Pressure Controller Is InAutomatic.

2) LCI Flow Controller Is InRemote Set Point.

3) CLA Flow Controller Is InRemote Set Point.

All following scenarios arepresented with the system in fullautomatic control.

5) Allow Man/Control SystemInterface.

As a result of operatordecision the control system can beoperated in a number of conditionsfrom total automatic to totalmanual control. In order to informthe operations personnel that C02control optimization is notoccurring, if the Program Directordoes not see the pressure and flowcontrollers or any combinationthereof in full automatic mode, itwill output to an annunciator paneland stay in alarm until the systemis returned to full automatic. FIG. 5.

During initial start-up andcommissioning this feature was ofbenefit to assist operationspersonnel of system status quickly.

6) Ensure CQ2 Excess Before

Before C02 export can begin orincrease, there must be an excessof gas available, an inferentialmeans of measuring, this is by ventvalve position. The ProgramDirector ensures the vent valve(s)are open by a value entered in itsprogram before allowing incrementalexport. Should the vent valves notbe open far enough, example, threepercent, the remote set points sentto LCI and CLA flow controllerswould be frozen at their last valueto avoid C02 shortage and thus apressure reduction.

7) Control CQ2 ftrawoff Rate.The blowers that removed C02 fromthe CIL system to provide it tooffsite customers were capable ofextracting approximately 200 Te/dayduring operation and it was notedthat this step change of C02draw-off could seriously bump ourcontrol system.

It was decided that the systemflow controllers would be employedas a rate-of-change of incrementlimiter through the ProgramDirector» The system wasconfigured to allow an increase inexport rate of 100 Te/day/min atany time. Should customer draw tryto exceed this rate, the blowerhouse flow control valves wouldclose because the process variablewas increasing faster than the setpoint.

The rate-of-change controlallowed the pressure control valvesto respond and maintain stability.

8) Reco<%nize,,Customer Number,One Requirements.An automatic, an operator freemethod of determining that thecustomer is capable of increasingproduct take was recognized as arequirement and a means to do thiswas designed.

As stated previously, theProgram Director monitors thecustomer mass flow and transmits aset point to the flow controllers.These two signals were compared andan increase in product demand wasseen by the following means.

Exampleif flow to customer =

X Te/day, adjust set point toX +. ten Te/day.

- if (SET POINT - FLOW)decreases to less than ten then thecustomer is taking more product.

- the SET POINT would becontinuously incremented by FLOW +ten Te/day until the differenceremains static, at this point, thecustomer is requesting no moreproduct.

Only after LCI is no longerrequiring flow or a contracturaltier has been reached, does theProgram Director poll CLA.

FIGURE 6.

9) Recognize CIA Requirement.The same technique was used torecognize an increased CLA draw offwith the exception that should LCIincrease his draw off while CLA wasincreasing draw off, the ProgramDirector would make the followingdecisions:

a) let CLA draw off increase ifLCI is at his first tiercontractural level and CLA is not.

b) freeze CLA draw off andincrease LCI draw off if below hisfirst tier contract level.

c) freeze CLA draw off andincrease LCI draw off if bothcustomers are at or in excess oftheir contractual first tier.

It should be noted at this timethe scan time for the completeprogram is one second.

10) Recognize LCI AdditionalRequirement.In the event of LCI being at orabove their first tier valve andCLA being in a saturated orno-increase state and LCI requiringmore flow, the control system wouldrecognize their requirement andincrement the set point at a rateof 100 Te/day/min until the processvariable increment halted.

In the event of CLA being belowtheir first tier and requiringadditional C02 during anincrementing of LCI while abovetheir first tier, LCI set pointwould be frozen and incrementing toCLA would begin.

11) Recognize CLA AdditionalRequirements.The same procedure as describedabove applies in this case, withthe exception that incrementingwill only be allowed if excess gasis available as inferred from thevent valve position.

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12) Reduce C02 Export IfRequired.Since the system could recognize anincreased product draw off by thecustomers and control the change inrate of draw off automatically, itmust be capable of recognizing aC02 shortage and reducing draw offaccordingly.

A reduction in C02 availabilitycould be a result of reducedammonia production, a Urea plantstart-up or a start-up of acustomer plant while theircompetitor is drawing in excess oftheir contractual first tieramount.

The first occurrence notedwould be the vent valve positionbeing fully closed, which aspreviously stated would freeze theset points to the flow controllers.

As a result of loss of C02availability, the C02 back pressurewould begin to drop below setpoint. The Program Directorcalculates a negative valve forpressure controller (setpoint-process variable) and when aprogrammed valve, minus 3 KpA forexample is reached, the flow setpoints to the flow controllers willdecrement at 50 Te/day/min in apre-established order discussedlater.

The Program Director willcontinue to reduce the export flowsuntil the calculated (setpoint-process variable) is lessthan the programmed value.

At this time the vent valveswill be closed, thus freezing theset points at their last value,maintaining system stability:

In the event of continuouslydiminishing supply, the system willreduce export in this order untilzero export or system stability isattained:

a) reduce CLA to contractualfirst tier.

b) reduce LCI to contractualfirst tier.

c) reduce CLA to zero export.d) reduce LCI to zero export.

13) Shed Total Load To CLA.As long as manual intervention orequipment failure was possible,consideration of a non-defeatablefail safe trip system wasnecessary. In the CLA blower houseupstream of the 16" flow controlvalve, a pressure switch calibratedfor a lower than normal pressurewas installed. Actuation of thisswitch would de-energize a solenoidvalve and vent air from the flowcontrol valve closing it.

An associated alarm wouldannunciate and the valve could onlybe reset from the Ammonia I controlroom.

To eliminate the possibility ofmistakenly isolating the pressureswitch, no isolation or instrumentvalve was placed in its sensingline.

It should be noted, this totalload shedding measure is a lastattempt to maintain systemstability and Ammonia I operation.

14) Shed Total Load To LCI.A similar load shedding system,similar to above only, calibratedto trip at a slightly lowerpressure (1.37 KpA) was installedin the LCI blower house. Itsoperation and function are asdescribed above.

15) Allow Qn-Line Maintenance.Maintenance, calibration and slightprogram alterations had to beconsidered, so careful design ofcontrollers, components and valveswas required.

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First, all control valves wereof the same manufacture (one tofour inchs, three to sixteeninches) and were all identicallyhand wheel equipped. All valvesoperated on a spring-diaphragmprinciple and no air pressurehigher than 100 KpA was required.

The Program Director can beremoved from the system by placingthe flow controllers in localcontrol, allowing remoteconfiguration changes, which upuntil now has not been required.

All the transmitters and flowcomputers can be removed fromservice by placing their associatedcontrollers in manual operationmode.

Finally, all the controllerscan be removed from service andaltered by placing the associatedcontrol valves on hand wheeloperation.

System Construction AndCommissioning.

The control system wasinstalled during a plant shutdownand the check out was completed inapproximately twenty-four hours.

The controllers and ProgramDirector were installed on thecontrol panel in advance of thestart-up and through the use ofsimulators, all operating shiftsreceived training on the system.

Commissioning of individualloops went very well butdifficulties in getting the blowerhouses operating as designed,prevented the control system frombeing placed in its final operatingstate at time of start-up.

When final commissioning wastaking place, it was found thatsome parameters entered from designhad to be altered to accommodatethe operating system.

Parameters such assa) vent valve position to

initiate set-point freeze.b) the pressure controller

(set-point-process variable) valuerequired to begin flowdecrementing, were altered severaltimes on-line through the ProgramDirector to obtain optimum resultsand stability.

The Program Director alsocontains the values of contractualtiers and the number one customeraddress location. All valves andcustomer preference can be enteredinteractively on-line through theDirector key pad and immediately beapplicable.

Operating ExperienceWhat Would Be Changed?

1) The introduction of thefirst electronic control system ina 20 year old pneumatic controlroom was only one of the systemfeatures that operations personnelhad to adjust to in Ammonia I.

2) Until this time the smallfixed quantity of C02 exported wasa small, somewhat insignificantpart of the process, that if itshutdown for any reason it would beattended to later.

3) In the past the control ofthe C02 separator back pressure wasfield checked once or twice pershift on a gauge that was a'little' out-of-calibration but hadno major importance as long as itwas around 35 KpA.

4) For many years, the C02high vent was always venting C02gas that looked like steam andincoming shifts had regarded thevent gas as an indicator of planthealthiness, this would changedrastically.

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How It Changed And OperatorAcceptance.

1) The Ammonia I plant was allpneumatically controlled and assuch had no continuous electricalback-up power or UninteruptablePower Supply (UPS). A small UPSsystem was installed tocontinuously power all theinstruments. The UPS was connectedto an alarm annunciator, to informoperations personnel if a loss ofprimary feed to the UPS was notavailable. Operations had tofamiliarize themselves with itslocation and function as well asthe required corrective measures.

The electronic controllers weregrouped side-by-side in a PressureController-Program Director-FlowController arrangement to avoid amix and match distribution on thepanel. The controllers presentedmore information consisting ofdigital, analog and discreet LED'sfor process status than operationshad previously seen.

These features took the longestto adjust to because you didn'thave to 'tap' the controller toattain an accurate reading or youdidn't have to allow for the offsetbetween the red and black pointerthat had been there since youbecome a board operator.

One feature that wasimmediately adjusted to was theengineering unit display, no longerrequiring the (factor X reading)exercise to log flow rates.

2) After the economics ofincreased C02 sales was presentedand field training given, theexercise of starting the blowersbecame more important. Anotherindicator of company commitment wasthe building, blower and controlsystem upgrade that was completedin both blower houses. At thistime should a blower house alarmannunciate the response is veryquick and the alarm condition isimmediately rectified.

3) The C02 back pressure isnow checked as part of the controlpanel readings and the control ismuch better. During periods ofmaximum export the board operatorwill alter the back pressureslightly to raise blower suctionpressure and thus blower dischargepressure. The blower dischargepressure is part of the controlsystem at the C02 plants whichaffects compressor loading.

The one complaint noted, wasthat with a set point of 40 KpA,the senior board operator felt thecontrol could be better.Examination found the set-point tobe 40.00 KpA while the processvariable varied from 39.55 to 41.45KpA. The altering of the digitsfrom 39.55 KpA to 41.45 KpA led himto believe the control was erratic.A piece of black tape over the lasttwo digits put his concerns torest.

4) At periods of low C02consumption, the high vent appearsas it had for years, but duringperiods of maximum C02 draw off,there is no C02 going to vent.This is still alarming to somesenior operators but the no-ventindication does means maximum saleson a product that for years hadbeen considered of littleimportance.

CONCLUSIONS

The experience of CIL with the C02control/export optimization systemhas shown that incorporation of amicro-processor based electroniccontroller into a pneumatic Ammoniaproduction plant can stabilizeinternal requirements whilemaximizing C02'. export to aselection of customers.

This maximization of flowscombined with the efforts ofoperating personnel has made CILLambton Works the largest food andmedical grade C02 exporter inCanada.

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VENT

16"

TO LIQUID CARBONIC INC.220 Te/doy

320 Te/doy 16"

16"

TO UREA #1 PLANT

TO UREA #2 PLANT

SEPARATOR

Figure 1. Original CO2 control/distribution.

3 PSIGFALLING

TO LIQUIDCARBONIC

ORIGINAL CQ,

BLOWER CONTROL

VENT(REMAINDER)

CANADA LIQUID AIR(330Te/doy MAX)

LIQUID CARBONIC(690 Te/doy MAX)

UREA H-.(̂320 Te/doy)

UREA I

Figure 2. Existing CO2 distribution.

VENTK

16"

sox

VAtVE P09T1ON

IS* OPEN 4' OPOf

*"OPEN

ie* aosoj v OPEN

CLOSED1E* CLOSED 4* CLOSED

TO LCI

TO CLA

TO UREA #1 PLANT

TO UREA #2 PLANT

CQ

SEPARATOR

Figure 3. New CO2 back pressure control system.

•TO FIC

/ \FT FT

\16"

NXJ

/^16"

FQI 0/P C'= CONSTANTh = DIFF PRESSUREP = PRESSURET = TEMPERATURE

Figure 4. Mass flow measurement to customer.Automatic switching internal to FQI took placebetween FT-LO and FT-HI in flow computer.

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WC AUTO-MANUAL STATUS

AC-LCI REMOTE LOCAL STATUS

FIC-CLA REMOTE LOCAL STATUS

PIC (SET POINT-PROCESS VAL.)

PIC OUTPUT TO VENT VALVE

MASS aow TO La

MASS FLOW TO CLA

ANNUNCIATOR

PROGRAM

DIRECTOR

XC

CONTROLLER

Figure 5. Program director input/output.

SET POINT = PROCESS VARIABLE + 10 Te/day

IF PROCESS VARIABLE > SET POINT - 10 Te/dayTHE CUSTOMER IS REQUIRING MORE PRODUCT

CONTINUE INCREASING SET POINT UNTIL NO CHANGES OCCUR

Figure 6. Recognize customer requirements.

Wayne E. Armstrong

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