ASME CRTD-103:2014 pdf free download

ASME CRTD-103:2014 pdf free download.CONSENSus ON BEST TUBESAMPLING PRACTICES FOR BOILERS
&NONNUCLEAR STEAM GENERATORS.
Sampling Frequency
Tube samples should be collected on a schedule established by a risk assessment of potential failure in boilers operating ator above pressures of 604) psig (4.1 MPa). The goal overtime 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 refcrcnces (5, 6. 8). These rcferenccs may prove useful when developing a sampling plan for a specific facility. Also, contact with the equipment supplier is suggested to see if there arc recommendations regarding tube sampling frequency far 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 arc 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)
• I lighest 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, II).
• Wall-Fired Boilers with Roof or Nose Tubes. Over extended periods (two or more decades). these sloped tubes can accumulate sufficient deposits to cause lube failures despite fairly low heat fluxes. For roof tubes, the deposits tend to be greatest just above (within first 2-5 feet or 0.6-1.5 ni of) the turn from the vertical wall. In some boiler designs, the nose tubes can experience significant heat tiux and can accumulate heavy deposits and experience lube failures in much shorter service intervals.
• Wall-Fired Boilers with Lower Slope. Many wall-fired boilers have a lower slope well below the burners. Whilc excessive deposits are not common, at least a fcw units have been found to have localized deposits in these tubes.
• Cyclone-Fired Furnaces, It used to be customary to sample the re-entrant throat tubes. However, in recent years one manufacturer has recommended sampling screen tubes from the ccntcrlinc of the cycloncs directly in front of one of thc cyclonc furnaces (samples should alternate between the cyclones).
• Fltmidized Bed Boilers. Waterwall tubes should be sampled just above the region with the refractory. Also, if used. the steam generating tubes that are immersed in the Iluidized fuel (e.g.. an in-bed steam generating tube, or an external fluidized bed heat exchanger) should be sampled.
• Grate-Fired Boilers. Grate-fired boilers arc commonly used to burn various Ionns of biomass (bark, sawdust, bagassc) and other materials (tires. refuse, coal). Waterwall tube samples should be collected 2-8 feet (about 0.6-2.4 m) above the grate. The actual locations of highest heat flux can be in the side wall 1/3 distance from the ash discharge wall or from the center of the ash discharge wall itself. Heavy deposits have also been found around air ports for some grate-fired boilers.