ASME CRTD-103:2014 pdf free download CONSENSUS ON BEST TUBE SAMPLING PRACTICES FOR BOILERS & NONNUCLEAR STEAM GENERATORS
Circulation Problems section). Deposition on waterside surfaces results in a barrier which
insulates the tube and leads to higher metal temperatures. High tube metal temperatures can leadto overheating of the tube microstructure.
Deposits, particularly metal oxide deposits, can also have significant porosity which hasbeen found to contribute to the underdeposit concentrating mechanism. Higher heat fluxes resultin greater rates of evaporation (steam generation) per unit area. During operation, water diffusesthrough porous deposits and evaporates at the tube/deposit interface. This results in evaporativeconcentration of salts and other dissolved materials. In drum type and other subcritical boilersand steam generators, this evaporative concentration of salts and other materials at the tubesurface can result in under deposit corrosion (UDC).
For boilers, the best means of assessing the area(s) of highest heat flux is to perform thermalimaging studies of the furnaces.For HRSG units, arrays of thermocouples have been installed todetect areas of sudden temperature changes. These data also may be used to locate areas with thehighest gas temperatures. Flue gas velocity profiles also can indicate those areas in HRSG unitsthat are expected to have the greatest heat flux (at a constant gas temperature the highest gasflow corresponds to the highest heat flux).Sometimes temperature data from boilers or HRSGunits are evaluated using computational fluid dynamic(CFD) analyses to target specific areasprone to high heat flux. Most facilities have not performed these types of temperature or CFDevaluation.Sometimes, plant personnel have observed the furnace well enough during operationto know the area of highest heat transfer (e.g., areas experiencing flame impingement duringoperation).
High Metal Temperature. – In the preceding list, a distinction was made between areaswith the highest heat transfer and areas with the highest temperature. In drum type units, theseareas usually will be the same.However, in once through supercritical (OTSC) boiler designsthey could be first pass tubes, second pass tubes, or both first and second pass tubes. In one typeof design the first pass is a center or division wall between two furnaces and the second passtubes are in separate (outer) walls. In other OTSC boilers, first and second pass tubes often areliterally adjacent tubes in the same wall and have the same heat flux on the fireside surface, butthe fluid travels from the first pass to the second pass and so the fluid and the tube itself will behotter in the second pass. Since there is no phase change at supercritical pressures, when fluidsare heated the temperature of the fluid continues to rise as it flows through the first and secondpass waterwall tubes.
In supercritical boiler designs, the fluid’s thermal conductivity decreases significantly as thetemperature increases(See Figure 5 in Reference 8).For example, for water starting at 4000 psig(27.6 MPa) and 590°F(310°C), the relative thermal conductivity drops to 82% of the originalvalue at 680°F (360°C) and to 32% at 770°F (410°C) (calculated from data available in
Reference 12).As stated by Weick “The low thermal conductivity of the water filling the poresis responsible for the insulating effect of the magnetite layer” (8). Therefore, a deposit layer in asecond pass tube will be more insulating than the same deposit layer in a first pass tube as aresult of the higher fluid temperatures. Deposits in first pass tubes usually will have a higherproportion of corrosion products transported from the preboiler cycle. Samples of both first passand second pass tubes should be collected and the deposit weights and compositions analyzed forone first pass tube sample and one second pass tube sample.
First Pass Flow. – The preceding discussion explained that deposits in first pass tubes in OTSC boilers will be dominated by feedwater corrosion products. However, this general principle of deposition of feedwater contaminants on first pass tubes also can apply for some types of drum type boilers. There are basically two locations for this type of deposit: high- temperature economizer tubes and traditional generation bank type tubes.