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ASME CRTD-103-2014 pdf download

ROUTINE TUBE SAMPLING FOR CLEANLINESS ASSESSMENTS As a general rule, routine tube sampling is employed to assess the condition and cleanliness of tubes which are filled with steam/water mixtures during service. The primary objectives of routine tube sample collection are to measure the internal deposit weight per unit area and determine the deposit composition. This information is used to determine if a cleaning is required and can be used to assess the performance of the feedwater and boiler water treatment programs. Waste heat boilers or process coolers with a shell and tube design are usually excluded from routine tube sampling. Collection of tube samples often is impractical due to the steam generator geometry or materials of construction. In current practice, steam-cooled tubes generally are not sampled for routine cleanliness assessments. Steam tubes are sampled for the purposes of assessing the condition of the tubes and determining the remaining life. Such considerations are beyond the scope of this document. The procedures for sampling steam tubes in the event of a failure are covered by this document in the “Tube Sampling Following Tube Failures” section. The deposit weight per unit area can be referred to as the deposit weight density (DWD), deposit density, deposit loading and the specific deposit weight. This document refers to this measurement as the deposit weight. Standard methods for deposit removal and deposit weight measurement are presented in References 1 and 2. Some boiler manufacturers have provided suggestions for maximum acceptable deposit weights (3, 4). EPRI also has presented guidance on deposit weight limits for boilers and heat recovery steam generators (HRSG) as well as presenting other recommendations for determining the need to clean these units (5, 6). There is ongoing research in deposit weight limits (7). Another objective of tube sampling is to perform dimensional measurements to assess and track changes in wall thicknesses and the depths of any pits or depressions that are present. Information on recommended tests for routine tube samples is provided in the subsequent section “Routine Analyses”.
Sampling Frequency Tube samples should be collected on a schedule established by a risk assessment of potential failure in boilers operating at or above pressures of 600 psig (4.1 MPa). The goal over time is to have sufficient data to define the trend in deposit accumulation. Tube sampling recommendations for power boilers and HRSG units are cited in the references (5, 6, 8). These references may prove useful when developing a sampling plan for a specific facility. Also, contact with the equipment supplier is suggested to see if there are recommendations regarding tube sampling frequency for the unit. The consensus recommendation is that tube sampling be performed every one to five years and after any major boiler (or HRSG) water chemistry upset or tube failure. Tube sampling may not be an option for some specialty steam generators. Some plants prefer to chemically clean based on a fixed time period and do not routinely collect tube samples. However, a representative tube sample should still be collected in the outage prior to the scheduled cleaning outage to plan the cleaning process properly. If a tube has failed and is being removed for failure analysis, both the failed tube and an adjacent tube, which has not failed, should be collected for analysis. When a tube fails, the failure process often blows out much of the internal deposits. Tube Sample Location for Routine Evaluation Collection of one or more tube samples from the area(s) of highest heat transfer is commonly recommended. However, this area is not always the location with the greatest amount of accumulated deposit. Collection of tube samples from the areas of highest heat flux and deposit weights are suggested. All of the following conditions have been known to cause greater deposition to occur in a localized area: ? Highest heat flux (highest heat transfer) ? Highest metal temperature ? First pass flow ? Settling ? Circulation problems (low water circulation or flow disruptions) High Heat Flux. – Holmes and Mann cited a 1960 work by Mankina that studied the effect of heat flux on the rate of iron and copper oxide deposition (9). Oxide deposition increased with the square of the heat flux (9). More recent studies have shown different relationships between heat flux and deposition rates (10, 11).

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