scope:

This standard describes a simple test method that employs GBB equipment to remove boiler waterside deposits on a piece of tubing removed from a represen- tative area of a boiler. The test specimens are cut from a sample tube, weighed before and after the cleaning process, and the amount of deposit per surface area is estimated by measuring the weight loss of the tube sample test piece after deposit removal via GBB and dividing by the surface area of the test piece. The DWD value that is obtained by this method is typically expressed in mg/cm² or g/ft² units, and is defined as the weight of the deposit per boiler tube surface area.(footnote 2) Currently, DWD values are commonly expressed in g/ft² units. Procedures for test specimen processing, dimen- sional analysis techniques, sources of potential interfer- ences, and a sample DWD calculation worksheet are included.

Measurement of deposit accumulation obtained by this test method should not be the sole source of information used to decide on the necessity of chemically cleaning a boiler unit. Although producing an accurate DWD value can be an important factor to aid in evaluating boiler cleanliness (and there are references in the literature that utilize DWD data to assist in establishing chemical cleaning guidelines as a function of boiler pressure), other details should be considered. For example, the chemical composition and relative thickness of the specific waterside scale formed on the heat transfer surface are key parameters that must be taken into account in the process of making a decision to clean a boiler system to avoid tube failure. In addition, specific boiler design, heat flux patterns, and operating conditions have significant influence on the amount of deposit loading that can be sustained prior to the occurrence of overheating and other deposit-related failure processes.

Caution should be used in the interpretation of high DWD values obtained from tubes subject to extreme temperature conditions (beyond the oxidation limit of the steel). This is because the DWD value produced may be unusually high due to the presence of excessive mag- netite scale via in situ oxidation. As such, deposit-loading estimates of superheater tubes, reheater tubes, or water- bearing tubes subject to excessive heat flux may actually reflect the presence of heavy, in situ oxides rather than transported and deposited water-formed scale constit- uents. No attempt to differentiate between water-formed scale and in situ oxidation products is made; the overall deposit weight per surface area is estimated with this test method. Other techniques (such as X-ray diffraction, microchemical analysis of deposit layers, metallography, etc.) may be needed to ascertain the relative distribution of deposit constituents and the influence of severe oxidation (excessive magnetite) on the DWD value.

Footnote 2 - g/ft²is equivalent to 1.075 mg/cm².