OCR Text |
Show h1gh surtace temperatures can also render the tubes susceptible to creep da~ge . In highly rated boilers part1cular condit10ns give rise to the undesirable phenomenon of tube dry-out which 1s dependant on a number of factors including the heat flux and the pressure, flow and stea~ quality .J n t bet u be . As the local heat flux exceeds the critical value the tube metal temperature will show a pronounced rise. Figure 17 shows regions of critical heat flux, the shaded portion being an area of uncertainty. Normally. the high heat transfer coefficients associated with boiling hold the tube metal temperature close to saturation temperature. When dryout occurs, the heat transfer coefficients are significantly reduced to those of low velocity steam and the tube metal temperature responds by rising towards that of the flame. Dry-out 1s to be avoided as 1t can result in the concentration of traces of soluble salts present in the feed water. Such concentrations may develop in slight defects or porous oxides at the boiler tube inner surface leading to corrosion of the tub@., ~~~ ?j.8IJrQ 10. In the extreme, the corroded reglon of boiler tube may be unable to withstand the internal pressure ,leading to failure. The onset of this phenomenon can be recognised and remedial action taken before tube damage occurs . heat fluxes approaching the cr1t1cal value will warn of the possibility of dry out while significant excursions of tube temperature above the saturation temperature will confirm it . Areas of high heat flux (and hence high tube surface temperatures) occur for a nu~ber of reasons, the most co~on being localised slag fallIng (or re~oval) and direct flame 1mp1nge~ent, b condition which can see burning coal particles adherlng to the boiler tube surface. Lccalised high heat fluxes can cause eitber of the problems outlined above or indeed both. Boiler trials have shown areas of boiler wall ~h1ch regularly appear to clean preferent1ally to others and thus are more susceptible to high heat fluxes. This is a consequence of insuffic1ent data on the conditions of the boiler wall and the effectiveness of applied sootblow tecbniques. Heat flux monitoring devlces can provide the 1nforma:10n reqUired to avoid areas of localised high heat flux and 50 reduce the incidence of evapor~~pr ~~p~ ,~rro~.pn . |