Field experiences on large MSF desalination plants designed to operate with two different top brine temperatures

Desalinution,66 (1987) 43-57 ElsevierSciencePublishersB.V.:Amsterdam-PPrintedin The Netherlands

43

FIELD EXPERIENCESON LARGE MSF DE4ALINATIONPLANTS DESIGNED TO OPERATEWITH TWODIFFERENT TOP BRINE TEMPERATURES

A. MASSARANI

Franc0 Tosi IndustrialeS.p.A.,Legnano (Italy) SUMMARY Some design typical aspects of MSF plants, foreseento be able to operate at differentbrine temperaturesin their hottest zone, are analized;the technical limits of a double conditionof operationare exeminedso as the actions needed to change the operatingmode of the plant. Applied design criteriaare then comparedwith operationresults of large plants we supplied,evidencingthe various problemsencounteredand illustrating their solution. Finally, a simple method to verify the foulingfactor and to prevent scale formation,used in the course of high temperatureoperationmode, is illustrat&L

DFXQJ CRITERIA It has been some years now that the possibilityor, better,the convenience has arisen for some users to requireMulti Stage Flash (MSF)plants designed for dual operatingtop brine temperaturewhich could allow the operationwith low temperatureadditives(90 OC) and which, at the same time, could easily be convertedto operationwith hi& temperatureadditives (il.0 to 11'5DC), a severer condition,though more rewardingfrom the productionand efficiency points of view. Furthermore,the neoessityof operatingin accordancewith the seasonal variationsof seawatertemperatureof approximately20 W has requireda plant design for a "flash range?' varying from 40 + 45 W to more than 85 OC and, therefore,in a variationrange quite unacceptablefor the plants of past generation. In taoklingthe new design problemswhich were arising,particularattention has been paid to the advisabilityof avoidingsolutionswhich might create

OOll-9164/87/$03.500 1987ElsevierSciencePublishersB.V.

44

constructional complexitiesthat would adverselyaffect operationand maintenance. This being so, we have attemptedto find a technicallyreliablesolution for industrialuse, where the positiveresults of past design and manufacturing experiencecould converge. Consequently,we have analyzedthe advisabilityto extend,generalize and possiblyadapt the already acquireddesign criteriato the new problems through an evolutionprocess rather than an entirelyinnovatoryone. simulation To this purpose,it has been of great importancethe computerized. techniqueof plant operationthrou& which we were able to study the influence of the dimensimal and geometricalparameterson plant behaviourand perform+ ante accordingto variationsin plant operatingmode, seawatertemperature and lOad. The data shown in table no. I are relevantto some importantand recent F. Tosi's achievementsfor which the above considereddesign techniqueshave been adopted and for which, in the light of comparedsimulationtest results, it has baen deemed sufficientto restrictthe extent of conversionwork only to variationof the area of brine orifices,sinoe such variationalone was largelysufficientto meet Client'srequirements. From the data of the above mentionedtable it is possibleto note that the increasedproductivitydue to high temperatureoperationgets up to 4C$ of the nominal designvalue for the Ras Lanuf installation, while it is about 2@

only

for the Sharjah and Dubd plants. Such differencedependsnot so much on technicaland technological problems and environmentalconditionsunder of MSF process,as on the circumstanoes which the plant must be operated. As a matter of fact, for what concerneSharjah and DuId installations, the desaltersare integralpart of a dual-purposeplant suitablefor fresh water and electricenergy productionand, althou& such plant has been made flexible by means of interconnections among more groups and of a desalterfeeding system both with extractionsteam from the turbineand with boilerpass-out steam, we thoughtproper to fix limits to the higher evaporatorproductivityin order to avoid an unjustifiedoversizingof boilerand turbine. Conversely,for what concernsthe Ras Lanuf installation, no externallimits have been put to the higher productivityof the desalinationunits, as such

45

MSF

De*alination

Type and stages

Brine recycle

RdS

Plant

arrangement

pump

Top brine tenperatwe, seawater

tenperature,

Seawater

salinity,

LanUf

Sharjah U.A.E.

Libia

OC 'C

Croz~ flow design single deck

Cross flow design tar0 tiers

Cross flow design tw tiers

1 x 100% driven by steam turbine

2 x 50% driven by electric motor

2 x 50% driven by electric motor

90 minimum design maximum

ppm

Dubai - Jebel Ali U.A.E.

115

16 23.8 26.7

16 23.8 26.7

90

110

88

110

20 30 38

20 30 38

20 30 38

20 30 38

42180

42180

42270

42270

Distillate output, t/h . with minimum *eaw.ter temperature . with design seawater temperature . with maximum seawater temperature

277.7 250.0 240.3

377.5 351.0 341.8

901.8 852.5 743.2

Gained . with . with . with

6.257 6.250 6.236

6.443 6.485 6.494

Brine recycle flow, t/h . with minimum seawater temperature . with design sebwate~ temperature , with maxi"wm seawater t‘?!"perature

3098 3098 3098

Seaurater inlet flow, t/h . at all seawater taIperatureS

Output Ratio, Kg/Kg minimum seawater temperature design seawater temperature maximum seawater temperature

42270

42270

1062.6 1020.8 927.8

1151.2 1092.0 949.2

1351.6 1338.0 1217.6

7.6R4 7.401 7.263

8.082 7.965 7.926

7.819 7.500 7.457

8.337 8.119 8.150

3076 3076 3076

9910 11060 11060

8760 9450 9450

12877 14530 14530

11000 12300 12300

,625

,625

6050

6050

8200

8200

Brine blow-down tenpePature. OC . with minimum Seawater temperature . with design seawater temperature . with maximum sea\vater temperature

33.20 39.20 41.30

37.20 43.30 44.50

31.50 41.10 47.76

31.79 41.42 48.28

30.99 40.71 47.27

31.21 41.21 49.06

Available flash range, 'C . with minimum seawater temperatllre . with design seawater tmnperatule . with maximum seawater tenperatwe

56.80 50.80 48.70

77.80 71.70 70.50

58.50 48.90 42.24

78.21 68.58 61.72

57.01 47.29 40.73

78.79 60.79 60.94

Table no. 1

46

plants are part of a complex cogenerationsystemwhere the desalinationheat consumptionappearsrather moderate if comparedwith the large capacity of boilersand steam supply networks,the sizing of which could practically disregard the higher steam consumptionat high temperatureoperation. It is now clear that before proceedingwith the designingof a MSF dualoperatingplant, it would be necessaryto study oaref'ully end to set the desiFed higher produotivitylimits for high temperatureoperation,so as to avoid unnecessarycapitalinvestementsor fail to meet Customer'sexpectations. Once all the externallimits are known or pre-established, not only in terms of fresh water productionat differenttemperatures,but also in terms of flexibility of the plant versus load (normallyfrom 5#

to lo@

of referenceoapac-

ity at each operatingcondition),the internallimits of each single component or part of the plant can be identifiedthroughthe above mentionedsimulation process. The main problemsin this respect can be outlinedes follows: Structuraldesign of the evaporator: Generally,the struoturaldesign of the evaporatordoes not set technicallimits to the best performancesachievable from the plant, as the stressesdue to flows and movementof fluids inside the evaporatorare nearly negligiblein comparisonwith the considerablestresses due to the remainingload conditions. Nevertheless,the design of a plant for a dual-operating conditionand the advisabilityto reach operatingtemperatures of 110 to 115 OC involvesdefinitelysevererconditionsthen those of the earlier generationplants designedfor a max. operatingtemperatureof 90 V.

As

regardsthe old plants designedfor high temperaturewith acid treatment, the changed geometryof the stages and the differenceof size due to the higher unit outputwhich can be nowadaysachieved,are factorswhich make any comparison unacceptable. The generalcriteriafollowedfor the structuraldesign of the plants shown in the above consideredtable no. 1 has been to make use of reinforcingstructures preferablyexternalto the evaporatorshell, in order to avoid, as much as possible,the necessityof internalreinforcements, suoh as struts or other elements,which may interferewith the process. The presenceinsidethe evapo; ator of stage partitionplates end of other importantfunctionaldevices,such as distillatetrays, tube supportingplates,weirs and deflectorshas been nec-

essarilytaken into account in view of their considerablereinforcementaction, As for the stress conditionsunder which the evaporatorhas been verified, we would point out the following: - design pressureand temperatureat static conditions - hydrostatictest pressure - absolutevacuum in the whole plant - max. values of differentialpressurebetweenadjacentstages - thermal stressesduring operationboth at high and low temperature - its own weight and weights of overlyingparts both during operationand hydrostatictest - reactionsat supportpoints - seismic loads. Referringto the relativeincidenceof the above oonsideredstresses,it is importantto observehow the differenttype of plant (on one or two tiers) could involveconsiderablechanges. Particularly,as regards the Sharjeh and Dubai installations where the plant has been constructedin a single piece with stagesarranged on two tiers, we consideredadvisableto examinethe possibleincidenceof the "bananaeffect", due to the differenttemperaturesbetweenthe upper and lower stages, on load distributionrelevantto the elevatedfoundation,as well as the extent of thermal stressesin the parts which are most affected by the phenomena. Finally,without gettinginto technicaldetails of stress analysisand finite elementscalculationwhich do not come under the scope of this paper, we wish to emphasizehow the incidenceof the dual-operating conditionon the geometri+ al featuresof the stages and/or their operatingdevicescould furtheraffect the evaporatorstructuraldesi@r,which is alreadyaggravatedby the fact that dimensioningfor abeolutevacuum conditioninside the plant overlapswith the requirementof checkingthe internalpressures due to high temperatureoperation. Demisters: The efficiencyof demistersis normallya functionof the mass velocity of the steam which passes

through them, accordingto a curve that

presentsan optimalvalue: therefore,it is necessaryto ascertainthat the efficiency of demistersand consequentlythe purity of productwater, satisfies the requirementsat all operatingconditions,possiblyresortingto a

stage

48 geometrywhich, by increasingthe disengagingdistancebetweenthe evaporating surfaceand the demister,would appreciablyreduce the saline mist entrained by the steam rising to the demister. Orificesand seals betweenadjacentstages: A good seal betweenadjacent stages both on the brine and the distillatesides,has always representedone of the major problemsencounteredin the design of MSF plants, expeciallylarge ones. Level controldevices in each stage with throttlingdevicesmovable from the outsidehave the disadvantageof poor reliability,greateroperatingcomplexity and higher installationand maintenancecosts. Conversely,fixed throttlingdevicescannot basicallyensure a "perfect"seal betweenthe stages under differentoperatingconditions. Therefore,the extent of steam leaks from one stage to the next, and the relevantthermodynamicefficiencylosses, have to be carefullyevaluatedin order to foreseetheir overalleffect on both plant performancesand dynamic stability,expeciallyat reduced loads. The calculationformulas,containedin the named simulationprogrsmmesand which relate to the geometriesusually adopted by F. Tosi, have revealedfor the above mentionedplants that the extent of steam leaks throughthe distilliite orificeswas not such as to justifya more complexconstruction. As regardsthe brine, the results of our aforecitedstudieshave led to a solutionwhere the presence of suitableorificeadjustabledevicesbolted to the stages dividerplates allowed settingthe plant to meet the requirementsof each of the two operatingmodes considered,so a6 to ensure a correct

ape+

ation within the limits of the seasonalseawatertemperatureand load variation range. The presenceof oval holes for fasteningsuch adjustabledevices,besides permittinga standardization of plant construction, would have also allowed to make minor adjustementsduring commissioningand final setting-upof the plant. Materialsand velocityin the heat exchangetubes: There are upper and lower velocitylimits in the heat exchangetubes beyondwhich corrosionunder deposit phenomenaand erosionand failure of passivationlayers could arise. The optimalrange of applicationvaries from one materialto another. Aluminum brass, one of the more commonlyused alloys in desaltermanufacture, owing to its relativelylow cost and good resistanoeto corrosion,has a rath-

49

er restrictedoptimalvelocityrange, if comparedwith other materials, Restrictingits use to the heat recoverysection only, and excludingthe stages which could exceed85 + 90 OC when operatingat high temperature,we have considered such materialadequatealso for dual-temperature desalters,as the presence of a continuouscleaningsystem could eliminatepossiblecorrosion under deposit problemsat reducedloads, even limitingthe max. brine velocity to approx. 1.85 m/s, i.e. within max. values already confirmedby experience. Evaporatingsurface: For plant productionincreasesof the order of 20 to 2%, as in the Sharjah and Dub& plants,we did not think it advisableto resort to an oversizingof the evaporatingsurface. For Ras Lanuf, on the contrary,a certain oversizingof the evaporating surface in relationto the requirementsfor operationat a lower temperature was necessary,due to the considerableincreasein plant production. However,the above mentionedchoiceshave been made not only on the basis of general considerationbut also throughan analyticalmethod,after having taken into consideration with the assistanceof the programmesalreadymentioned the influenceexerted by the evaporatingsurfaceand the brine transit time through the stages on both the non-equilibrium thermodynamiclosses and the overallplant performances. Continuouscleaningsystem: There are no particulardesign problemsarising from the dual-operating mode of desalters,except for the necessityof checking that the n

p values betweenthe inlet/outletwaterboxesof desalter

stages,at the lowest brine flows (in the event of load reduction),are still sufficientto ensure operationof the system. Additives dosing system:

The hazard of plant scalingdemandsa particular

attentionto additivesdosing system design, especiallywhen operationat high temperatureis foreseen. As well as consideringthe requirementsimposedby the various commercial productsavailable,the design criteria must considerof the utmost importance the reliabilityand accuracy of the system,which has to bs equippedwith suitable instrumentation for reservoirlevels and process flows monitoring.

50 Control

valves: It is common knowledgethat a possibleoversizingof these

devicesmay involvea series of operatingproblemsnot less importantthan the ones which may arise in case of undersizing. The range and limits of a satisfactoryoperabilityfor a controlvalve depend on many factors,such as kind of fluid,valve type, constructional featuresas well as characteristics of the circuitin which it is placed. Besides,valve dimensionsmay prove significantsince,under same conditions, smallervalves placed in circuitshaving similarhydrauliccharacteristics can better stand operatingconditionsoutsidethe optimaloperatingrange. As far as desalterproblemsare concerned,the controlvalves of major interest are the ones installedon the brine recirculatingsystem,and make-up, continuousblow-down,feed steam and distillatesystems. Though we are not going into much detail,we would like to mentionthat, as a preliminaryverificationrule, we have assumeda

Ap

value not exceeding

30 to 3% of the pressureupstreamof the butterflyvalve on water systems, when operatingunder partial load conditions. At this limit value, however,some circuitsappearedparticularlycritical, so that, right at the design stage, calibratedorifices,easily adjustable during commissioningand final settineup of the plant, were provideddownstream of the valves. As regards feed steam, even though no major cavitationproblemse&t because of the compressibility of the fluid, the considerableflow variations and particularlythe

A p

variationof the valve in the two differentape-

ating conditionsrequirea throu& checkingwith the valve manufacturer. Pumps and motors: Selectioncriteriaof these componentsalone would require a lengthy discussion. In order to adequatelymeet the flexibilityrequirements of plants which have to operateunder dual-designconditions,F. Tosi has given preference,especiallyfor large plants, to horizontaltype brine recirculating pumps, each sized for 5C$ of the maximum requiredrecirculatingflow, Steam turbinesfor recirculatingpump drive, if specifiedby the Customer,could easily be adoptedto the advantageof the recirculatingflow controlsystem, resultingin reductionof the energy dissipatedby the controlvalves.

51 FIELD WPERIENCES

Referringto the above consideredplants,we have liked to focus attention on the field experienceacquiredmainly on the Dubai installation, which are particularlysignificantfrom the point of view of large unit output,and of the long field assistancerenderedby cur personnelduring the guarantee period. During an eighteen-month period,reliabilitytest runs were conducted at 88 OC and

110 W,

operatingtests at reduced loads down to 5@ of the nominal

fresh water production,as well as performancetests, This has enabledus to verify with a fairly good degree of accuracyour design criteriaon the strengthof field operationresults,also in the light of possiblefoulingphenomena,seasonaltemperaturevariations,internal inspectionsat a stage of final s&tin&up and, later on, in the Course of the first scheduledmaintenance. As for problemsencounteredduring the initial operatingperiod,we have to report considerablevibrationsof the brine recirculatingand make-up control valves. Already presentat 10% flow, this phenomenontended to increaseto quite unacceptablevalues on reaching80 or 8% of the total flow, therebymaking any control of the plant load practicallyimpossible. Basically,the phenomenonwas due to cavitationproblemsgeneratedby excessive throttlingof the above mentionedvalves. This phenomenonwas furtheraggravatedby continuousoscillation,particularly at low loads, on account of the action exerted by vibrationson the control circuit devices,such as E/P convertersand positioners. Owing to the presence of fixed throttlingdevices downstreamof the valves, we decidedto change their originalcharacteristics, after having carefully recalculated the new orificedimensionson the basis of operatingdata, and also taking account of all limit operatingconditions,differentfrom already experiencedones. The modificationsthat we have introducedhave permittedus to operate under more favcurableconditionsfrom the fluid-dynamics point of view, eliminating the above reportedproblemstanks both to the now possibleconsiderable extent of valves openingand to the higher pressurethat was buildingup dcwn-

52 stream of the same. With the named alterationwe had to slim down some of the originaldesign margins,mainly due to the well-knownprudentialfactorscontainedin the usual formulasused to calculatepressuredrop in tubs bundles. Nevertheless, results of performancetest carried out after the above modificationshave indicated that the plant could still delivernot only the guaranteed performances but also a fresh water productionexceeding

1.1%

the contractvalue,

with a G.O.R. of about 8 Kg/Kg as againsta guaranteedvalue of 7.5 Kg/Kg. Furthermore,it should be noted that these values shall not be understood as absolutelimit values, since performancetests have indicatedthat fresh water productionand G.O.R. can be increasedfurther,withouthaving to reset the brine orificesadjustableplates, but simplymodifyingsome operating pi+ rameters,particularlythe brine flow and the top brine temperatureas shown by the data in table no. 2 . Test no. 5 conductedat a top brine temperatureof 93 "C has demonstrated that a fresh water productionaround 29,000 m3/day (corresponding to 111s

of

the contractvalue) can be achieved,with a G.O.R. of more than 8.6 Kg/Kg (equivalentto 11% of the guaranteedvalue), and a 0.37 ppm TDS content only in the productwater as againsta maximum guaranteedvalue of 10 ppm. The above margins,which have been confirmedby commercialoperation,can be attributednot only to the design criteriaadopted by us, but also and above all to the need for a sufficientlyflexibleplant capableto operate,except for some slight adjustements,also at a high temperature(110 "C). Another problemwith which we were faced was to developa simplified prccedure that would permit the user to detect in advance,with the only aid of installedtraditionalinstrumentation, possiblescalingphenomenabefore these might grow to such an extent as to prove detrimentalto plant efficiency. As a matter of fact, we had noticed measurementof G.O.R., obtainedas a direct relationshipbetweenproductwater flow and brine heater condensate flow, could be comparedwith clean plant referencevalues only througha series of fairly complex elaborations(similarto those used duringperformance test) and througha sufficientlyaccuratemeasurementof the steam enthalpy at brine heater. Therefore,on Customerrequest, we have at once attemptedto work out al-

No.

t/h

m3/day

Ratio,

Kg/Kg

purity,

ppm

difference

%

%

lo6 Cal/h

Cal/Kg

Cal/Kg

diPPerence

total output,

Output

Distillate

Gained

Distillate continuous

OC OC

t/h

Cal/Kg

Heat consumption,

drop,

enthalpy,

Condensate

Enthalpy

enthalpy,

steam

L.P.

t/h

Plow,

flow,

reticle

L.P. steam

Brine

flow,

inlet

Seawater

temperature,

temperature,

Seawater

Top brine

TIME

DATE

TEST

/

+

+

0.90

11.28

8.346

4.32

27340

74.608

553.68

93.59

647.27

134.750

12462

8210

1.18

26518

+

Table

0.33

6.53

7.9896

+

72.909

559.91

91.98

651.89

130.215

14497

8206

28025

+ 12.21 0.67

0.37

8.416

+ 6.93

11.32

8.349

6.40

27886

no. 2

+

+

77.061

95.85 551.17

552.21 77.144

647.02

95.03

14502 139.814

l&514 139.701 647.24

8200

8202

91.5 33.70

33.36

33.11

13.00

11.00

7/9/84

4

33.40

90.9

88.0

89.6

9.00 11.00

9.00

6/9/M

3

II

0.37

~15.16

8.637

+11.09

29117

80.735

550.03

97.35

647.38

146.782

14556

8230

33.83

93.1

11.45

9.45

8/Y/84

5

CALCULATION

PHASE

DATA

STATION TEST

11.00

-

9.00

5/Y/84

2

8

11.00

4/9/84

1

NO.

STEZAM POWER

UNIT

DUBAI DESALINATION

6

+

0.35

7.17

8.038

2.83

26951

75.508

553.14

93.90

647.04

10.00

7.500

26208

80.356

551.78

93.85

645.63

145.600

14530

30

88

14068 136.508

+

(*I

(*I

(*)

Guaranteed values

8200

(*)

values

reference

or

Contractual

8078

32.84

89.8

10.00

8.00

9/9/84

SHEET

E

54

ternative methods based on measurement of other parameters, for their possible utilization in high temperature operation. Particularly, we have examined the possibility of indirect measurement,of the overall heat exchange coefficient referred to both brine heater and reoovery stages, as a means of calculating the fouling factor. As far as the brine heater is concerned, the problem was very simple in that the overall heat exchange coefficient could be calculated through

the

following formula: T

G.C P

u

BH

.ln

= A

-T

1

' T

S

(1) -T

2

where:

c

= measured brine flow, Kg/h

A

=

C

heat exchange surface, m*

=

brine specific heat, Cal/Kg OC

=

measured condensation temperature, OC

=

measured brine inlet, outlet temperatures, W

P TS

T1 '

T2

which is directly derived from the theory of heat exchange in the presence of condensation only. Taking account of the size and characteristics of the brine heater, as well as of the theoretical formulas used for calculation of the heat exchange ooefficient with clean tubes, the fouling resistance

$

BH

(m2 h V/Cal)

could

be calculated according to the formula:

1 II

F 3H =

Q.

K3H'

lnRBH

where: K

7%

=

R _,I

=

Lxx

calculation constant

Ts - T1 Ts

-

T

2

1 .348

. F.

(015%

. T_/33

- 1.5)

12)

55

*M -

*i - *2 2

Much more complex, however, was the situation concerning the heat recovery section, as we had determined that by applying formulas similar to (2) to the individual stages, the measurement uncertainties would excessively affect the results, since a subtraction

between temperatures too close one to the other

had to be carried out within the ten

9".

At this point, we have decided to consider the possibility of establishing a correlation between fouling factor and extreme temperatures of the whole heat recovery section (T,, T2, Tj and Tq), namely:

*, , T2

= measured brine temperature at B.H. inlet, outlet,

*3

=

OC

measured evaporating brine temperature entering in last recovery stage, OC

*4

=

measured cooling brine temperature at heat recovery section inlet, OC

By simulating desalter operation under different operating conditions and by varying the fouling factor of heat recovery stages, we observed that the following correlation:

1-C RF REX

1

.G.lnR

(3)

c2

G.lnR l

where: C

1'

G

3

s

plant constants

L

measured brine flow, Kg/h, end

T3+0.75845T,-o.9375T4-0.8272*,-2.161

'4

.(1.458-0.555 -

R=

T3+0.75845T2-0.8272T4-0.9375*,-2.161

T

1

-0.0022

l

T,,

56 reflected, with an approximation oft

2$, the theoretical results on variation

of seawater temperature and plant load, under both operating conditions of 88~ and 110 V. As a confirmation of the results obtained, the above simplified formulas were verified on the plant in order to bring to light possible problems

assc-

oiated with sensitivity of method and accuracy of measurements. Since no problems and/or application difficulties were experienced during the observation period, we decided to perform high temperature operation tests with the only aid of industrial instrumentation installed on the plant, and keeping a watch on possible scale formation by the above described simplified method, as well as by alternative computerized and more complex methods. The procedure that we agreed upon with the Customer can be summarized

as

follows: 1)

Adjust the orifices in such a way as to meet the operating requirements

2)

Start-up the plant and allow a short stabilization period at 90 OC

3)

Gradually rise the top brine temperature to 110 V,

4)

Arrange for one-month reliability test runs at 110 OC and maximum plant

at 110 V

with stabilization

periods of at least 5 day-s at intermediate temperatures of 95, 100

W

output

5)

Shut-down and final inspection of the brine heater and of the hottest stages tube bundles

6)

Restore the orifices for operation at low temperature.

As a precautionary measure during the test run period, proportioning anti-scaling additives was brought to 7 ppm.

Throu&out

of

this period, all the

operating variables were kept under careful observation, and the fouling faotors and G.O.R. were calculated.

Since no indications of scaling were found

at all, and having ascertained that the actual plant performances were in line with the expected values, we decided to carry on with tests for two more days, at Customer's request, reducing dosing of anti-scaling additives to 5 ppm. Not even during this short period were any particular phenomena highlighted, nor was any appreciable variation noticed in the fouling factor, as it

could

also be observed when the plant was shut-down and the tube bundles inspected. As regards the simplified method of fouling factor determination, we would

57 emphasize that its constant application by plant operators, (which is

made

possible by the simple for;nulas used), besides giving the opport.dnity to oI+ tain quicker results, make operators ?etter conscious of scaling problems and operating parameters

, particularly under limit conditions.

ComLusI cm3 Modern simulation techniques permit an accurate study of plants under different operating conditions, so as to reasonably anticipate operational problems and provide solutions ri&t Even if of short duration, hi&

at the plant design stage. temperature operation has revealed entirely

satisfactory results, which have not only confirmed technical expectations and plant performances, but has also contribu-ted to refine operating personnel's skills for an immediate knowledge of possible scaling problems ad

a better

interpretation of control room measurements through application of simplified methods and formulas.