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

And

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

Where

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.

Level	Cell constant C	Range’s mhos/cm
Very low concentration	0.006	0-0.6 0-4
Low concentration	0.06	0-10 0-200
Medium concentration	1	200-1000 1000-5000

• 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.

• 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.

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

• 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.

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

• 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.

Kathrometer

• 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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