An important factor in the efficient operation of a modern boiler system is the ability to accurately measure the level of carryover or steam purity. However, because of non-uniform distribution of contaminants in steam, this can be difficult to accomplish. Fortunately, techniques are available that can provide the desired accuracy and precision when they are properly followed.
Obtaining a representative steam sample has proven to be a highly complex problem. Steam leaving the boiler can exist in all three states: gas (steam), liquid (water), and solid (carryover). Each will exhibit a different density and, in many cases, the distribution of each leaving the boiler will vary from place to place and from time to time. Therefore, it is essential that the proper sampling techniques be used for any steam purity testing. Poor Sampling techniques can lead to inaccurate data and possibly erroneous interpretations of the data.
Watertech of America Inc. recommends the installation of sampling nozzles as specified by ASME or ASTM (Method D 1066). Construction and placement must conform to these standards and the rate of sampling must be isokinetic (i.e. the velocity of the steam entering the nozzle must be the same velocity as the velocity of the steam in a header).
There are many methods of analyzing a steam sample for its solids content. Some are outdated, others are currently being used and still others are in the experimental state.
A few are discussed below:
Involves collecting a steam sample, evaporating it to dryness and then weighing the solids. Obviously, considering the level of solids anticipated even with the grossest carryover, the accuracy of this method is extremely poor. It is also much too slow to be practical.
For routine measurement, conductivity is frequently used since it increases with an increase in dissolved solids content. Unfortunately, conductivity also measures dissolved gases such as ammonia and carbon dioxide as well as materials intentionally added to the steam system, such as neutralizing amines. Equipment has been introduced to reduce the effect of the gases that contribute conductivity but not dissolved solids. One such device is the Powell-McChesney degassing condenser which condenses steam and measures conductivity at the boiling point, before the gases can dissolve in the condensed steam.Another modification that uses conductivity is the Larson-Lane analyzer. With this instrument, steam is condensed and passed through a cation exchange resin operating in the hydrogen cycle. This removes all cations, amines, and ammonia, replacing them with hydrogen. The effluent is boiled to remove carbon dioxide and the conductivity is measured. Since the conductivity of hydrogen is several times higher than an equal quantity of metallic ions, a larger, more accurate reading is obtained. While accuracy of 2-5ppb total carryover has been reported for this instrument, experience shows that the correlation of conductivity to total carryover below 1 ppm is highly suspect.Conductivity measurement for steam purity (owing to the ease of the method) is an excellent tool for relative effects. For instance, if conductivity of the steam consistently measures below 5mmhos, an increase of 10mmhos indicates a carryover increase, although exactly to what extent cannot be accurately determined.
Other more exotic methods such as radioactive tracer techniques are being used but very infrequently.