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BACKGROUND

In high pressure boilers, it is necessary to monitor thereturn condensate before it goes back to the boiler toensure that it is not contaminated beyond acceptablelimits. In smaller boilers, the return condensate canhave a fairly high dissolved solids concentration,which can be measured with a standard conductivitymeter. In the case of large boilers, such as those usedto generate electricity, the allowable level of solids isin the low parts-per-million range. The conductivitycaused by the dissolved solids is often masked by thelarger conductivities of water treatment chemicals thathave been added to protect the boiler. In a case likethis, the Larson Lane Condensate Monitor ModelCH16D is used to separate the conductivity causedby the dissolved solids from the conductivity causedby the water treatment chemicals.

PROCESS

In high purity boilers, the Total Dissolved Solids (TDS)level of the water ought to be limited to a maximum of1 to 2 ppm and sometimes less than 0.1 ppm. Inorder to do this, it is necessary to start with deionizedwater, which will have a conductivity in the range of

0.055-0.1 S/cm (S/cm). Various water treatment

chemicals are then added to the deionized water toprevent deposits from forming, to regulate the pH, andin other ways to improve boiler life and performance.

When the water is heated to steam, most of thesolids remain in the boiler with a few of them beingcarried over into the steam. In time, some steam willbe lost and require make-up water to be added. Thiswill eventually cause a build-up of undesirable dis-solved solids in the water such as chlorides or sul-fates, gases, such as carbon dioxide or oxygen, andother harmful materials.

A simple conductivity measurement of this waterwould give the overall conductivity of the water,

which would be the total conductivity of the goodstuff, such as the water treatment chemicals, andthe bad stuff, such as the chlorides, sulfates, etc. Ina smaller boiler, the total conductivity could bebetween 1 and 20 S/cm (or in some plants evenhigher), and it will be necessary to detect contamina-tion in the order of 0.1 to 2 ppm, which would be 0.2to 5 S/cm. This is often not a very satisfactorymethod of measurement.

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MEDIUM PURITY CONDENSATE MONITORING

Appl ication Data SheetADS 4900-80/rev.B

August 2004Power Industry

INLETNeutral Salts

Na++Cl- 2.2 S/ppm

2Na++Cl- 2.1S/ppm

Alkaline

NH4+OH- 6.6 S/ppm

(Ammonia in Water)

Acid

H2++CO3-2 5S/ppm

(Carbon Dioxide in Water)

OUTLETChanges to Acid

H++Cl- 11.7 S/ppm

H2++SO4-2 8.8 S/ppm

Changes to Water

H++OH- 0.055 S/ppm

Remains Unchanged2H++CO3

-2 5S/ppm

FIGURE 1

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Emerson Process Management

Rosemount Analytical Inc.2400 Barranca Parkway

Irvine, CA 92606 USA

Tel: (949) 757-8500

Fax: (949) 474-7250

http://www.raihome.com

Rosemount Analytical Inc. 2004

One of the simplest methods of detecting smallamounts of contamination in return condensate isthrough the use of a cation conductivity measurement.In this type of measurement, the condensate must firstbe cooled to under 122F (50C) preferably under86F (30C) to prolong resin life. The sample pressuremust be reduced to under 5 psi in most systemsbefore the condensate enters the cation column.

When the water passes through the cation resin bed,the resin exchanges any positive ions in the solutionfor hydrogen ions. If a salt, such as NaCl, passesthrough the resin, the Na+ ion is held in the bed, andan H+ ion is released. In so doing, 1 ppm of NaCl salt(which has a conductivity of approximately 2.2 S/cm)is converted to 1 ppm of HCl acid (which has a con-ductivity of approximately 11.7 S/cm). In a like man-ner, other salts are converted to their correspondingacids, which have 3 to 6 times the conductivity of thecorresponding salt. This is shown in Figure 1.

Chemicals that are alkaline in nature, including mostwater treatment chemicals, also have their positive ionportion bound in the resin column with a correspond-ing release of hydrogen ion. Thus, a chemical such as

ammonia (which would be NH4OH in water), whichhas a conductivity of approximately 6.6 S/cm for 1ppm, has the HN4

+ion bound up in the resin bed andan H+ ion released making HOH, or water, with con-ductivity of 0.055 S/cm. In this way, most water treat-ment chemicals become bound in the column andrelease water, reducing the conductivity contributed bythese chemicals to close to 0.055 S/cm.

Chemicals that are acidic, such as H2CO3 (which isformed when CO2 dissolves in water), would passthrough the resin unchanged.

When the return condensate passes through thecation column, the conductivity of the bad stuff, such

as an acid, would be unchanged, while that of dis-solved salts would increase 3 to 6 times. The conduc-tivity of the good stuff, such as water treatmentchemicals, will generally decrease to near zero. Thus,the cation conductivity coming out of the resin columnis caused by the undesirable materials in the water. Inthis type of a system, we could detect as little as 10-20ppb of dissolved solids in condensate that has a totalconductivity of 5 S/cm.

INSTRUMENTATION

To make this measurement, the customer will need tosupply temperature and pressure reducing equipmentto bring the return condensate to a minimum of 122F(50C) and 5 psi. The cation column would be theModel CH16D, which is a standard system, or theModel CH16DE, which has a resin that changes colorto indicate when it needs replacement. Either of theseinclude a flow meter and final flow adjustment valve.

An Endurance Model 404 conductivity flow cell with aModel 1055 cation conductivity analyzer is used tomeasure the outlet of the column as long as the con-ductivity is under 10 S/cm. Since this water is comingfrom a cation column, it is acidic and requires aninstrument with a temperature compensator designedto track ultrapure cation water. In some cases, thecustomer may also want to measure the condensatewater before it goes into the column, which will give anindication of the condition of the cation column and thewater treatment system. In this case a Model 404 flowcell, with a cell mounting kit (PN 060-10747739), anda Model 1055 dual input conductivity analyzer wouldbe used instead of two of the Model 1055s, because

this analyzer offers the flexibility of one analyzer tomeasure two cell inputs.

See Application Note ADS 4900-82 for ModelCH16RB related article. Endurance is a trademark of Emerson Process Management, Liquid Division.

Model 1055 Dual Input Analyzer

Temperature correction for high purity

water, cation conductivity, linear tempera-

ture coefficient.

Two 4-20 mA outputs

Three fully-programmable alarms.

Choice of enclosures for pipe, surface, and panel mounting.

NEMA 4X (IP65) enclosure.

Model 404 Flow-Through

Contacting Conductivity Sensor

Small hold-up volume to ensure fast

response to changing conductivity.

PVC construction capable up to 60C.

Optional 316SST construction up to 100C.

Housing protects sample from air contamination.