MULTIPLE-EFFECT EVAPORATION REGIME IN HIGH-PRESSURE BOILERS

Abstract

In the 1930’s, a multiple-effect evaporation technique was proposed for generating clean steam that was based on the use of boiler drums partitioned into sections for processing boiler water with different salt concentration [1]. In a sectioned drum, the “clean” section served for generating steam with a salt concentration lower than that in the steam generated in the “salt collection” section; the steam flowing from the two sections was mixed together to produce a steam with rated characteristics required for normal operation of superheaters and turbines. The method in question is quite satisfactory for low-pressure boilers in which requirements placed on the quality of steam are too stringent and can be provided by use of simple water treatment techniques. Numerous data have been reported in the literature that were mainly focused on the advantages of drum boilers with multiple-effect evaporation for generating standard-quality steam. It was shown in [2] that in steam generators using steam washing techniques that gained wide acceptance in the 1950’s, the continued application of multiple-effect evaporation added little (if any) to the quality of steam. Therefore in view of the ever-increasing requirements placed on the quality of feed water supplied at higher pressures (which caused the increase in impurities, especially silicic acid, in steam), the advisability of using multiple-effect evaporation in high-pressure boilers was questioned in a number of works [3–7]. At present, even in the use of incompletely desalinized water for feeding high-pressure (HP) boilers at thermal power plants (TPPs), no problem arises with the quality of steam produced either by multiple- or single-effect evaporation (domestic TP-100-, VPG-250-, and TPE-214-type boilers, as well as boilers available from foreign manufacturers, or medium-pressure boilers currently in service in Russia). As is known, the performance of an evaporating heating surface is to a significant extent determined by the rate at which inside fouling deposits build up. With this in mind, it is important to consider the water-and-salt regime (WSR) for the boiler water fed to drum boilers with multiple-effect evaporation. The annual rate of faults in the water-wall tubes of drum boilers (bulging, corrosion) is rather high, whereas these faults seldom (if ever) occur in high-pressure and supercritical-pressure (SCP) direct-flow boilers. The reason for this is that, with the feed water of the same quality, the boiler water for the cooling water wall of drum boilers contains soluble impurities in concentrations higher than that for direct-flow boilers (by a factor of 100 against the feed water at a 1% blow-down), which creates favorable conditions for the buildup of deposits and corrosion. Water-soluble impurities (Ca, Mg, Na, SiO2 salts, etc.) are removed from the drum boiler via continuous blow-down. Therefore requirements placed on the quality of boiler water and, consequently, feed water for drum boilers in terms of soluble impurities can be less stringent than for direct-flow boilers: in the latter, all the impurities are confined within the water-steam circuit. As to iron oxides, the situation is somewhat different; most iron oxides deposit inside the water-wall tubes and are not removed readily by blow-down. With the increase in operating pressure, requirements placed on the quality of boiler water and, consequently, feed water become more stringent — with a view to preventing corrosion and fouling of the wa