StandardTest Method

Cooling Water Test UnitIncorporating Heat Transfer Surfaces

This NACE International standard represents a consensus of those individual memberswho have reviewed this document, its scope, and provisions. Its acceptance does not inany respect preclude anyone, whether he has adopted the standard or not, frommanufacturing, marketing, purchasing, or using products, processes, or procedures not inconformance with this standard. Nothing contained in this NACE International standard isto be construed as granting any right, by implication or otherwise, to manufacture, sell, oruse in connection with any method, apparatus, or product covered by Letters Patent, oras indemnifying or protecting anyone against liability for infringement of Letters Patent.This standard represents minimum requirements and should in no way be interpreted asa restriction on the use of better procedures or materials. Neither is this standardintended to apply in all cases relating to the subject. Unpredictable circumstances maynegate the usefulness of this standard in specific instances. NACE Internationalassumes no responsibility for the interpretation or use of this standard by other partiesand accepts responsibility for only those official NACE International interpretations issuedby NACE International in accordance with its governing procedures and policies whichpreclude the issuance of interpretations by individual volunteers.

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Reaffirmed 2001-03-15Approved March 1986

Reaffirmed September 1988Revised March 1994NACE International

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ISBN 1-57590-125-0 2001, NACE International

NACE Standard TM0286-2001Item No. 21219

TM0286-2001

NACE International i

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Foreword

This NACE standard test method describes a multiple-tube test heat exchanger that canbe used by process industry facilities to monitor corrosion and fouling control in coolingtower water systems. Methods for collecting test data to determine average foulingfactors and corrosion rates that can be used in predicting the expected service life of aplant exchanger are presented.

This standard was originally prepared in 1986 by NACE Task Group T-7A-10, acomponent of Unit Committee T-7A on Cooling Water, and was a revision of NACEPublication 5C165, “Standard Heat Exchanger for Cooling Water Tests.” It wasreaffirmed with editorial changes by T-7A in September 1988. This standard was revisedin 1994 by NACE Task Group T-3T-4, a component of Unit Committee T-3T on On-LineMonitoring Technology. It was reaffirmed in 2001 by Specific Technology Group (STG) 11on Water Treatment and is issued by NACE International under the auspices of STG 11.

In NACE standards, the terms shall, must, should, and may are used in accordance with thedefinitions of these terms in the NACE Publications Style Manual, 4th ed., Paragraph7.4.1.9. Shall and must are used to state mandatory requirements. Should is used to statesomething considered good and is recommended but is not mandatory. May is used tostate something considered optional.

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NACE InternationalStandard

Test Method

Cooling Water Test UnitIncorporating Heat Transfer Surfaces

Contents

1. General.......................................................................................................................... 12. Multiple-Tube Test Exchangers .................................................................................... 13. Controls and Data Collection......................................................................................... 44. Corrosion Data .............................................................................................................. 55. Fouling Data .................................................................................................................. 5References.......................................................................................................................... 6FiguresFigure 1: Test Exchanger Installation ................................................................................ 1Figure 2: Test Heat Exchanger – Design A ....................................................................... 2Figure 3: Test Heat Exchanger – Design B ....................................................................... 3

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Section 1: General

1.1 Cooling tower water systems are critical elements inthe operation of most process plants. Recirculation of thewater used in cooling tower systems reduces fresh-waterrequirements and makes it economically feasible to adjustthe water composition in order to control corrosion andfouling rates. The effectiveness of corrosion and foulingcontrol programs in cooling water systems can bemonitored by test heat exchangers mounted in a bypass.Conditions on heat transfer surfaces can then bedetermined at any time, without removing plant equipmentfrom service.

1.2 The test unit described in this standard should be usedwhenever cooling tower systems are critical to the process.Data from the test unit can be used to:

(a) predict service life of heat exchangers,

(b) predict heat transfer coefficients,

(c) determine fouling rate,

(d) determine relative performance of various treatmentprograms,

(e) optimize costs of treatment programs,

(f) minimize unplanned shutdowns resulting from foulingor corrosion, and

(g) determine effect of alloy selection on fouling andcorrosion rates.

1.3 This standard considers tube-side water only. It isrecognized that many exchangers have shell-side water;however, proper modeling of shell-side conditions is muchmore complex.

1.4 This standard covers test exchangers heated by high-purity steam only. Other heating fluids (hot water, processfluids, etc.) may be used, provided they are noncorrosiveand nonfouling. If the steam purity is low or variable, stepsto monitor steam purity are needed. Use of a heating fluidother than saturated steam requires modification of the heattransfer calculations in Section 5.

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Section 2: Multiple-Tube Test Exchangers

2.1 General

2.1.1 Multiple-tube test exchangers with tube-sidewater allow calculation of the overall average heattransfer coefficient and fouling factor. They also allowestimation of the actual rate of corrosion penetrationthrough the tube wall. The corrosion rate is derivedfrom measurement of actual corrosion penetration overvarious time periods. Use of multiple tubes allows forthe use of duplicate samples and collection of dataover varying time periods.

2.1.2 A typical installation is shown schematically inFigure 1. Operating conditions can be established toapproximate those of a given exchanger or, ifacceleration is desired, to approximate a condition thatis predictably more severe than that of plantequipment. The drawbacks of this test unit are that it isrelatively expensive, data generation is slow, and some

operating attention is required. These limitationsshould be evaluated with respect to the value of thedata generated.

2.1.2.1 Design A—This unit, shown in Figure 2,uses O-ring seals and has 12 tubes, similar to thedesign developed by NACE Task Group T-5C-1 in1965.

2.1.2.2 Design B—This unit, shown in Figure 3, isan evolutionary modification of Design A, usingcompression-type fittings with polytetrafluoro-ethylene (PTFE) ferrules. This modification shouldreduce the tube-side to shell-side leakage that issometimes encountered with the O-ring assemblyused in Design A. The tubes are more widelyspaced, resulting in a larger pipe shell and fewer(10) tubes.

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FIGURE 1Test exchanger installation: TI—Temperature indicator, PI—Pressure indicator.

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FIG

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FIG

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2.2 Selection of Operating Conditions

2.2.1 Both corrosion and fouling rates in watersystems depend on transfer rates of the corrosiveagent (e.g., dissolved oxygen) and foulants (e.g.,hardness salts) to the metal surface. These transferrates in turn depend on the temperature and turbulenceof the water at and near the metal surface. Corrosionand fouling rates may be considered comparable ifgeometry, turbulence, and surface temperatureconditions are comparable.

2.2.2 To model a specific exchanger adequately, it isnecessary for the exchanger tubes to have the samemetallurgy as that of the exchanger being modeled,and to estimate metal surface temperature and watervelocity (turbulence) in the area of interest.Comparable conditions can then be established in thetest unit.

2.3 Exchanger Assembly and Operation

2.3.1 If a pretreatment step is desired for the systemtested, tubes in the exchanger can be precleaned andprepassivated prior to testing.1 All such treatmentsshould be conducted so as to simulate the degree ofcleaning and pretreatment that will be achieved on aplant exchanger. Including some tubes that have beenprecleaned/prepassivated, and some that have not,allows quantitative evaluation of the benefits of thesetreatments.

2.3.2 The exchanger shell and heads should becovered with a minimum of 25 mm (1.0 in.) of insulatingmaterial.

2.3.3 After tube installation and piping assembly,valves should be opened to start water flow through theexchanger at the desired flow rate, throttling the waterflow at the exchanger outlet. Steam flow (usingsaturated steam only) should then be started, withprovisions made to vent all air from the steam side ofthe exchanger. The steam flow should be adjusted toachieve the desired condition (steam pressure onexchanger shell or temperature rise in water). Steamshould be throttled at the exchanger inlet.

2.3.4 For reliable results, operation at reasonablyconstant conditions is essential. The following datashould be collected at least once per 8-hour shift:

(1) Water flow rate

(2) Water temperature into unit

(3) Water temperature out of unit

(4) Steam pressure

(5) Condensate temperature

(6) Condensate flow rate

Automatic control and data acquisition systems arehighly desirable.

2.3.5 Heat transfer data can be calculated directlyfrom the data listed above, using the method outlinedin Paragraph 5.2.1.2.

2.3.6 Tubes should be removed on a staggeredschedule, with the first tubes being removed after 2 to 4weeks of exposure. Later removals may be made at 4-to 12-week intervals. Adequate records are essential,so that each tube can be positively identified and itsperiod of exposure known. If tubes are to be sectionedor cut for study, the identity of parts should bepreserved.

2.3.7 Carbon steel tubes that have been removed forevaluation should be split longitudinally and inspectedvisually for deposit types and locations. Samples ofdeposits should be taken for chemical andmicrobiological analyses. Special precautions areneeded to ensure reliable results from microbiologicalanalysis; persons qualified in this area should beconsulted. It may also be desirable to measure theweight and volume of deposit per unit area of tube.The tubes should then be cleaned (by suitable cleaningsolvent2 or gentle abrasive blasting) and carefullyexamined for corrosion. Depth of corroded areasshould be measured by micrometer or calibratedmicroscope. Type, distribution, and maximum depth ofcorrosion should be recorded. A plot of maximumcorrosion depth vs time of exposure is useful forestimating long-term corrosion rates. Tubes that aresectioned for further study should be photographedbefore and after cleaning.

2.3.8 Alloy steel and nonferrous tubes should also besplit, examined as described above, and carefullyexamined for metallurgical damage specific for thatmetallurgy (e.g., dezincification of brass).

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Section 3: Controls and Data Collection

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3.1 Because these test units are dynamic in nature, it isessential that critical variables (water velocity and heatinput) be maintained at the selected values. Water flow ratecan be controlled by conventional means, provided that the

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sensing and control elements are not overly sensitive tosolids deposition. Simple automatic devices have beenused successfully.

3.2 Heat input can be controlled by establishing constantheat flux or constant hot side temperature. Bothapproaches have been used successfully and both givesimilar results after the unit has reached steady-stateconditions.

3.3 Data can be recorded manually or instrumentally. Ifautomated data acquisition is used, readings should betaken at least once per shift. (Once per day is consideredan absolute minimum.) The operating personnel should beaware of the importance of maintaining constant control atthe desired values. Automatic data acquisition methods areuseful for these systems and provide a permanent record of

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data. Alarm systems can be used to alert operatingpersonnel of important deviations from the controlconditions. Continuous computerized data processing canalso be used, eliminating manual calculations.

3.4 It is essential that deviations from control conditions bedetected and recorded promptly. As an example,interruptions of water flow rate with continuing heat inputcan lead to a high rate of fouling. If such an event goesunrecorded, misleading high fouling factors are indicated.Interlocks that shut off heat input automatically when waterflow drops below a critical value are highly desirable.

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Section 4: Corrosion Data

4.1 The heated tubes used in the test systems described inthis test method can be weighed before and after exposureand an average corrosion rate calculated. Type,distribution, and maximum depth of localized corrosionshould be recorded. However, it is well established thatcorrosion in near-neutral water systems is almost alwayslocalized in nature and nonlinear with respect to time.Therefore, it is usually more relevant to measure corrosionin terms of maximum depth of penetration than in averageweight-loss terms. The use of the multiple-tube exchangerconcept allows correlation of corrosion depth with time, aswell as extrapolation to establish a long-term rate of

penetration. Such an analysis provides a more reliablebasis for predicting the expected service life of a plantexchanger.

4.2 Corrosion data should also be collected from coupons,nipples, and linear polarization-type devices. Resultsobtained from these methods provide useful informationabout the general corrosiveness of the water, and their useis recommended in conjunction with the method coveredhere. Note that these tests do not include heat transfer “hotwall” effects.

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Section 5: Fouling Data

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5.1 Fouling (increased resistance to fluid flow and heattransfer) in cooling water systems can result from thefollowing:

(1) Settling of suspended solids

(2) Precipitation of salts from solution

(3) Microbiological processes

(4) Process contamination

(5) Accumulation of corrosion products

(6) Reaction products from water-treatment additives

5.2 The following method of measuring total fouling isbased on measurement of thermal variations and does notdifferentiate type or cause of fouling. Accurate temperaturemeasurement is essential because small errors intemperature measurement generate large errors in results,especially when the test conditions selected are low heatflux and high water velocity.

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5.2.1 Average Fouling—This method provides anoverall measure of average fouling in the total test unit.The average fouling factor is calculated as follows:

5.2.1.1 Determine heat transfer coefficient (UC) bycalculation, as described, for example, by Kern.3

Many companies have standard computerizedmethods that are applicable. An alternativemethod involves calculating an actual heat transfercoefficient (see Paragraph 5.2.1.2) at startup whilethe exchanger is clean. If this method is chosen,the readings must be taken as soon as a steadystate is achieved. Delay of more than a fewminutes may introduce a significant error.

5.2.1.2 Determine an actual coefficient (UA) bycalculation using Equations (1), (2), and (3):

UA = Q∆

(1)

Q = M(Tout – Tin)CP (2)

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∆T = ( )Tout – Tin

Ln( )Tst – Tin

( )Tst – Tout

(3)

Where:UA = Actual coefficient of heat transfer,

W/m2 - °C (BTU/h - ft2 - °F)M = Water flow rate, kg/h (lb/h)Tout = Temperature of water out of exchanger,

°C (°F)Tin = Temperature of water into exchanger,

°C (°F)Tst = Temperature of condensing steam, °C (°F)A = Heat transfer area, m2 (ft2)Q = Total energy transfer rate, W (BTU/h)∆T = Log mean temperature difference, °C (°F)Cp = Specific heat. For water, Cp = 1 cal/g - °C

(1 BTU/lb - °F)

5.2.1.3 Calculate the average fouling factor usingEquation (4):

R* = 1U

– 1U (4)

Where:R* = Fouling factor, m2 - °C/W (h - ft2 - °F/BTU)UC = Clean coefficient of heat transfer,

W/m2 - °C (BTU/h - ft2 - °F)

(NOTE: This is the average total fouling on bothsides of the heat transfer surface. With a high-purity steam source, it is reasonable to assumethat steam/condensate fouling is very low, and,therefore, R* is approximately equal to water-sidefouling.)

5.3 When some of the tubes are removed from themultiple-tube test exchanger and replaced with unfoulednew tubes, the average fouling factor, calculated asdescribed in Paragraphs 5.2.1.1, 5.2.1.2, and 5.2.1.3,decreases. However, existing data indicate that this effectshould become undetectable after about two weeks, as thenew tubes become fouled to a steady-state level.

5.4 The fouling factor data calculated by this method applyonly to the specific conditions of the test. Plant exchangersoperating at different conditions of water velocity, surfacetemperature, water chemistry, or microbiological activitydevelop different steady-state fouling levels. Datadeveloped by the method described here provide base datafrom which fouling in other conditions can be estimated.

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References

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1. NACE Standard RP0189 (latest revision), “On-LineMonitoring of Cooling Waters” (Houston, TX: NACEInternational).

2. ASTM Standard G 1 (latest revision), “RecommendedPractice for Preparing, Cleaning, and Evaluating CorrosionTest Specimens” (West Conshohocken, PA: ASTM).

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3. Donald Q. Kern, Process Heat Transfer (New York, NY:McGraw-Hill Book Co., 1950), pp. 154-158, 835.