Measurement of Impurities in Water and Steam – Power Plant

The number of impurities present in water and steam can be found out approximately by carrying the following measurements.

Conductivity Meters

  • Electrolytic conductivity is the capacity of a solution to pass an electric current. Conductivity measurements are done for determining the salt content present in the feed water, condensate, and steam.
  • The number of impurities present in water improves conductivity, which means the rate of impurity and conductivity are in direct proportion to each other.
  • The electrical conductivity of pure water is minimum.
  • Conductivity measurement helps to determine the amount of total dissolved solids present in water.
  • The conductivity of a solution is measured in micromhos per centimeter or micro-siemens per centimeter at 25°C,
  • The acceptable limit is approximately 0.3 μ-mhos/cm³.
  • Conductivity is the reciprocal of resistivity of a centimeter cube of water between two plates 1 cm² in the area and placed 1 cm apart.
  • Conductivity increases with increasing impurities present in the solution.
  • The conductivity cell is used to measure conductivity.
  • The relationship for resistance, conductivity, and cell constant is shown below

Resistance = (Resistivity x Length of Sample) / (The cross-sectional area of plates)

Resistance = ρ*L/a


Conductivity= 1/ Resistivity = 1/ρ = (L/Ra) mho/cm = K


R = Cell resistance in ohms.

ρ= Resistivity in ohms cm²/cm

 L= Sample length in cm

 a = Area in cm²

K= Conductivity in mhos – cm/cm²

L/a = Cell constant in 1/cm = C

The term C = is a cell constant for a given set of electrode dimensions.

In contrast to pH measurement, the value depends on the hydrogen ion concentration alone.

For a given sample, conductivity is defined as a function of the concentration of all ions present in it.

Typical ranges of conductivity meters are listed below.

LevelCell constant CRange’s mhos/cm
Very low concentration0.0060-0.6
Low concentration0.060-10
Medium concentration1200-1000
  • For one particular kind of salt, conductivity is a measure of the salt content. Generally, in average cases, the composition of the feed water is constant hence the sufficient correct result is obtained.
  • As the conductivity meter is principally used for monitoring the salt content it is calibrated in mg- Nacl per liter instead of micromhos per centimeter. (1 mg Nacl/liter = 1.9 µmhos/cm at 26°C)
  • The accuracy of conductivity measurements depends to a considerable extent on the shape and arrangement of electrodes.
  • Detecting Elements for Low Concentrations.
  • The electrodes are arranged concentrically.
  • The external enveloping electrode is connected to the assembly fitting and earthed via the piping or tank.
  • The measurement is not affected by wall effects.
  • The internal electrode is insulated and surrounds a resistance thermometer for temperature correction.
  • The assembly for very low concentrations has a somewhat thicker internal electrode compared to that for low concentration assembly.
Conductivity Meter for Low Concentration
  • To avoid polarization effects, one applies an ac voltage in the measurement circuit as shown in the Figure below
  • This assembly is usually mounted in a flow chamber but immersion mounting is feasible.
  • For pressures exceeding 10 kg/cm² a pressure, a reducer must be provided.
Circuit for Conductivity Meter

For medium concentration assembly, a plastic sleeve consists of four annular platinum-plated electrodes as shown in the Figure below

Conductivity Meter for Medium Concentration
  • The two shielding electrodes are electrically connected they regulate the electric field.
  • The resistance thermometer allows temperature correction within a protecting tube alongside the electrode sleeve.
  • The corresponding electric circuit is shown in the Figure below.
Conductivity Meter Bridge Circuit

This type of assembly, suitable for pressures up to 1 kg/cm² is usually mounted in a flow chamber.

Measures to be considered:

  • The conductivity is influenced by the number of salts and by the content of dissolved gasses such as NH and CO₂ and hence is to be eliminated from the sample.
  • Generally, the elimination of NH3 is done by passing the sample through a filter filled with strong acidic cation-exchange resin.
  • The CO₂ is then eliminated by passing nitrogen through the sample.

Hydrogen ion Concentration (pH)

  • Since, pure water is tasteless, odorless, and neutral.
  • But natural and commercial waters are not pure.
  • Because of ionization, water is considered the most universal solvent.
  • Either OH- or H+ ions will predominate, causing either an alkaline or acidic condition.
  • pH scale devised to measure the intensity of acidity or alkalinity of a solution. An exact definition of the pH number is that it is the logarithm of the reciprocal of the hydrogen ion concentration (grams per liter).

pH = log10 (1/H) = – log10 (H+)

  • The pH value is a measure of the strength of an acid and base, to the neutral character of a solution.
  • The law of mass action applies to all chemical reaction states for the dissociation of water, H+ x OH= 10-14 K (at 25°C).
  • Just as the temperature is measured in degrees, so is acidity or alkalinity measured in pH values from 0 to 14.
  • pH 1 is strongly acidic, pH 14 is strongly basic, and pH 7 is a neutral solution.
  • pH 5 is 10 times as acidic as PH 6.
  • pH 4 is 10 times as acidic as pH 5 and 100 times as acidic as pH 6.

For instance.

  • If H+ is 10-9 then OH is 10-5 grams ions per liter, (H+ x OH = 10-14=K) and pH 9 designates the solution which, of course, is alkaline as OH ions predominate.

Analysis of Dissolved Oxygen

  • The dissolved oxygen present in feed water is a corrosion accelerator.
  • The unit used to mention dissolved oxygen is ppm.
  • Analysis to quantify dissolved oxygen can be done either by laboratory method or by online analysis.
  • The chemical titration method is normally used to find out the quantity of dissolved oxygen from the sample collected and immediately presented for the test.
  • The validity of measurement will be under suspicion as the delay causes the absorption of oxygen from the atmosphere into the sample.
  • Special portable kits are used to carry out the testing on the spot. It is always better to use analyzers that can test online.
  • Two such analyzers are discussed briefly here.

Wallace and Tiernan Dissolved Oxygen Analyzer

A simplified block diagram of the Wallace and Tiernan dissolved oxygen analyzer is shown below

Wallace and Tiernan Dissolved Oxygen Analyzer
  • The concentration of dissolved oxygen in boiler water is measured by electrochemical analysis in the analyzer.
  • The electrochemical analysis is continuous and automatic.
  • The analyzer can be divided into two units called the sampling cell unit and the transmitter indicator unit.
  • The sampling unit consists of a reference electrode, platinum measuring electrode, and calibration cell for measuring and controlling sample flow, pressure, temperature, and conductivity.
  • The transmitter-indicator unit consists of electronic circuits required for measurement, transmission, and calibration.
  • The indicating meter or recorder is normally calibrated in terms of ppb or parts per billion.
  • Ranges may be from 0 to 25 and 0 to 50 ppb with multiplication factors of 1 and 10. That means, in addition to 0 to 25 and 0 to 50 ppb ranges another set of ranges 0 to 250 and 0 to 500 ppb are made available.
  • The nominal output voltage in the range of 0-5 mV can be converted into standard signals of either 4-20 mA or 1 to 5 V DC.


  • The Kathrometer consists of four major parts: Cooler, Scrubber, Hydrogen Producer, and Wheatstone bridge circuit.
  • When the water sample is cooled and scrubbed by hydrogen, the dissolved oxygen present in the water gets displaced to form a mixture of hydrogen and oxygen.
  • The Wheatstone bridge circuit is used to measure the amount of dissolved oxygen. The heated wires of the bridge circuit are exposed to hydrogen in two opposite arms and to a hydrogen-oxygen mixture in the other two arms.
  • The unequal cooling of the heated filaments makes the bridge imbalanced.
  • The unbalanced voltage can be calibrated to indicate the dissolved oxygen.

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