measuring cation conductivity remains one of the most sensitive, simple, and reliable tools for detecting small amounts of contamination in feedwater and steam.

Actually, “cation conductivity” is a misnomer. A more precise name would be “cation-exchanged conductivity,” because it is a measure of the conductivity of a solution after it has been passed through a strong acid cation exchange resin.

When a condenser tube springs a leak, chloride, sulfate, carbonate, and many other anions contaminate condensate and feedwater.

The simplest, most reliable way to detect such contamination in real time has been cation conductivity.

superheated steam turbines have an extremely low tolerance for solids contamination in the steam.

Magnetite is dissolved and yields an acid solution containing both ferrous (Fe²+) and ferric (Fe³+) chlorides (ferric chlorides are very corrosive to steel and copper)

Fe3O4 + 8HCl FeCl2 + 2FeCl3 + 4H2O

magnetite

hydrochloric

acid

ferrous chloride

ferric chloride water

CONDENSER TUBE LEAK OR BRINE HEATER TUBE LEAK – EARLY DETECTION – QUICK ACTION

WHAT IS CATION CONDUCTIVITY

The online measurement of conductivity after water has passed through a column of strongly acidic cation-exchange resin is used to indicate the presence of potentially corrosive ionic contaminants. The technique may be referred to as “cation conductivity” or “acid conductivity” in some documentation.

The cation-exchange resin removes ammonium, sodium, and other cations—leaving an acidic solution of the anions that were present. The conductivity of this solution is highly responsive to the presence of strong-acid anions, because of the very high equivalent conductivity of the hydrogen ion. Thus, conductivity after cation exchange is extremely effective for rapidly indicating the onset of condenser leakage, particularly at seawater-cooled sites, and also for detecting contaminated makeup water

The cation exchanger removes the background conductivity caused by ammonia. With no contamination, cation conductivity equals the conductivity of pure water. With salt contamination, conductivity increases as the column effluent is now a dilute mineral acid.

LIMITS OF BOILER/FEED WATER/STEAM PARAMETERS DURING NORMAL OPERATION OF HIGH PRESSURE BOILERS

The maximum recommended concentration limits of

Total Dissolved Solids (TDS) total alkalinity

total dissolved solids

according to ABMA - American Boiler Manufacturers Association - are given in the table below:

Boiler Operating Pressure (psig)(bar)

Total Dissolved Solids - TDS -(ppm)

Total Alkalinity(ppm)

Total Suspended Solids (ppm)

0 - 50 0 - 3.5 2500 500

50 - 3003.5 - 20 3500 700 15

300 - 45020 - 30 3000 600 10

450 - 60030 - 40 2500 500 8

600 - 75040 - 50 1000 200 3

750 - 90050 - 60 750 150 2

900 - 100060 - 70 625 125 1

Maximum Allowable Impurities in Boiler Water

Chemical SymbolMaximum

Concentration (ppm)

Sodium sulphite NA2SO3 1.0

Sodium chloride NaCl 10.0

Sodium phosphate NA3PO4 25.0

Sodium sulphate NA2SO4 25.0

Chemical SymbolMaximum

Concentration (ppm)

Silica oxide SiO2 0.20

INTRUMENTS AVAILABLE FOR CONTINUOUS MONITORING

1. TURBINE CONDENSATE AFTER PUMP 1ST STAGE – CATION CONDUCTIVITY

2. FEED WATER AFTER FEED PUMPS - CATION CONDUCTIVITY/ PH

3. FEED WATER BEFORE ECONOMISER - CATION CONDUCTIVITY

4. BRINE HEATER CONDENSATE AFTER PUMP DISCHARGE - CATION CONDUCTIVITY

5. STEAM DRUM – WATER – TOTAL CONDUCTIVITY/PH

STEAM – CONDUCTIVITY/SILICA

6. LIVE STEAM – CATION CONDUCTIVTY

EFFECTS OF BOILER CONTAMINATION WITH SEA WATER ON BOILER TUBES AND TURBINE

Chloride and sulfate are known to contribute to corrosion fatigue in turbine blades, particularly those in the final rows of the low-pressure section. It is chloride and sulfate contamination that cation conductivity is really intended to detect. Early detection of this contamination can allow a plant time to correct contamination before it causes deposits and corrosion on the turbine (Figure 1).

EARLY INDICATIONS OF BOILER WATER CONTAMINATION IN CONTROL ROOM

ACTIONS OF SHIFT SUPERVISOR AND CONTROL ROOM OPERATOR

ACTIONS OF LABARATORY CHEMIST