ASTM D2186-05(2009)

Limiting the concentration of deposit-forming impurities in steam is of significance to protect both steam generators and steam turbines from damage or degradation of performance, or both.

Steam entering superheaters and reheaters of steam generators always contains some impurities. If the concentration of impurities is sufficiently low, the impurities are dissolved in superheated steam and are carried out of the steam generator. However, if the steam contains a sufficient amount of any substance to exceed its solubility limit in steam, the substance is likely to form a deposit on the heat-transfer surface. Because heat transfer in superheaters and reheaters in fossil-fueled steam generators is controlled principally by the low heat-transfer coefficient on the gas side, the formation of steam-side deposits will have little effect on the overall heat-transfer rate. However, steam-side deposits will increase the operating temperature of the heat-transfer surface. Such temperature increases can lead to swelling and ultimately to rupture of the tubing. Also, aggressive materials can concentrate under solid deposits of porous materials, such as magnetite (Fe₃O₄), and can cause serious corrosion of the tubing.

As steam flows through turbines, its temperature and pressure decrease rapidly. Because the ability of steam to dissolve impurities decreases with decreasing temperature and pressure, impurities in steam may exceed their solubility limit and form deposits on the turbine. Such deposits reduce steam flow area, particularly in the high-pressure portion of the turbine where flow passages are small, and the roughness of deposits and their effect on blade contours result in losses of turbine efficiency. All of these effects lead to reduction of the plant maximum capacity, which appreciably reduces the financial return on the capital investment in the power plant. Furthermore, aggressive materials, such as sodium hydroxide (NaOH) and sodium chloride (NaCl), may condense and deposit on turbine surfaces. Such deposits occasionally contribute to failure due to cracking of highly stressed turbine blades and rotors. Repairs and outages are extremely costly.

By monitoring the concentration of deposit-forming impurities in steam, a power plant operator can take steps necessary to limit the impurities to tolerable concentrations and thus minimize or eliminate losses due to excessive deposits.

1.1 These test methods cover the determination of the amount of deposit-forming impurities in steam. Determinations are made on condensed steam samples in all test methods. Test Methods A, B, and C give a measure of the amount of total deposit-forming material present; Test Method D deals with special constituents that may be present. Special precautions and equipment, calculation procedures, and ranges of applicability are described. The following test methods are included:

1.2 Test Method A is applicable for determining total dissolved and suspended solids in concentrations normally not less than 0.4 mg/L (ppm). It is applicable only to long-time steady-state conditions and is not applicable for transients.

1.3 Test Method B will measure minimum impurity concentrations varying from 3 mg/L (ppm) down to at least 0.005 mg/L (ppm), depending on the means for removing dissolved gases from the steam condensate. The means for removing dissolved gases also affects the storage capacity of steam condensate in the system and, thus, affects the response of the system to transients.

1.4 Because of the high sensitivity of methods for measuring sodium in steam condensate, Test Method C provides the most sensitive measure of impurity content for samples in which sodium is an appreciable percentage of the impurities present. Concentrations as low as 4.0 μg/L (ppb) can be detected by inductively coupled plasma atomic emission spectroscopy, 0.2 μg/L (ppb) by atomic absorption spectrophotometry, 0.1 μg/L (ppb) by graphite furnace atomic absorption spectroscopy, and as low as 0.5 μg/L (ppb) by sodium ion electrode. The apparatus can be designed with low volume, and, therefore, Test Method C is the most responsive to transient conditions.

1.5 Test Method D covers the determination of silica and metals in steam, which are not included in Test Methods B and C and are not individually determined using Test Method A.

1.6 This standard does not purport to address the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

6.1 The gravimetric test method is recommended for applications for which an average value of impurities over a period of several days or weeks is desired. It is particularly useful for samples in which a large percentage of the impurities are insoluble, do not contain sodium, or do not contribute appreciably to the electrical conductivity of the samples, because the other methods are not satisfactory for these conditions. Examples of such impurities are metals and metal oxides. It is not applicable when short-time trends are of interest or when immediate results are desired. The test method is useful for the determination of concentrations of impurities of 0.25 mg/L (ppm) or greater when a previously collected sample is used and for impurities concentrations of 0.1 mg/L (ppm) or greater when continuous sampling is used. Concentrations less than 0.1 mg/L (ppm) can be determined if a continuously flowing sample is evaporated for an extremely long period of time.

13.1 Ion-Exchange Degasser—An ion-exchange degasser consists of an ion-exchange resin that exchanges hydrogen ions for all cations in the sample, thereby eliminating all basic dissolved gases, including volatile amines. By converting mineral salts to their acid forms, it also increases the specific conductance of the impurities. As a result, the linear relationship between conductivity and impurity content is extended to a much lower level, depending on the carbon dioxide content. The test method is very useful for measuring low concentrations of impurities, such as condenser cooling water leakage, in steam condensate, and it is especially useful, for indicating small or intermittent changes in impurity content from some normal value. The test method is not satisfactory for the determination of impurities in steam condensate samples that contain acidic gases, such as carbon dioxide, large percentages of insoluble matter, or substances that ionize weakly. The sensitivity and accuracy of the method are decreased for samples in which hydroxides represent an appreciable percentage of the impurities, because hydroxides, which contribute to the formation of deposits, are converted to water by the ion-exchange resin. This characteristic is particularly significant when steam is generated at sufficiently high pressure to cause appreciable vaporization of sodium hydroxide from the boiler water.

13.2 Mechanical and Ion-Exchange Degasser—By combining mechanical and ion-exchange degassing of steam or condensed steam, or both, effective elimination of both acidic and basic dissolved gases is attained. This arrangement has the same advantages and limitations as the ion-exchange degasser alone, except that it will remove acidic gases, and the greater sensitivity afforded by measuring the conductance at atmospheric boiling water temperature extends the linear relationship between conductivity and the ionized impurity content down to about at least 0.005 mg/L (ppm). Although the relationship becomes somewhat nonlinear, the conductance is sensitive to concentration changes down to at least 0.005 mg/L (ppm).

20.1 The principal advantages of the sodium tracer test method are the freedom from interferences, the ability to measure extremely small concentrations of impurities, and the rapid response to transient conditions because of the absence of large stagnant sample volumes, such as reboil chambers. Either of two procedures may be employed for the sodium determination, as follows: Precise control of sample temperature is not required for the flame photometry test method. If the impurities are principally sodium compounds, impurity concentrations as low as 0.6 μg/L (ppb) may be detected by the flame photometry method and as low as 0.5 μg/L (ppb) by the sodium ion electrode test method. The sodium tracer test method is not recommended for samples having large percentages of impurities that do not contain sodium.

27.1 Silica and various metals are impurities that are occasionally found in steam and have definite tendencies to form deposits. Since these substances are not isolated when using Test Method A and are not detected when using Test Methods B and C, it is advisable to determine their concentrations separately when they are present in significant quantities.