Smelt-water explosions

Various theories were suggested to explain the explosions (e.g., free metallic sodium, hydrogen generation), but all were later proved to be 

Chemical reactions were shown to play a minor role in a smelt-water explosion, although gas samples from kraft smell-water incidents showed that hydrogen evolution could be correlated with Na2S content. There was general agreement that the explosion mechanism was physical in nature. 

A few attempts were made to measure the overpressures from explosions. Pressures were not high, but the rise time to the pressure maximum was short (ca. 1-2 msec) and the shock quite localized. The highest quoted overpressure was about 20 bar a few centimeters from an explosion (Nelson, 1973). 

Apart from the laboratory studies, statistical surveys of actual recovery bmler explosions have shown that such incidents are relatively rare and, in the United States occur, on the average, about once every 100 years of boiler operation. All explosions have been traced to events which allowed water to enter the furnace and contact the smelt, e.g., broken water tubes or dilute liquor feed. A listing of the presumed causes of all known smelt-water explosions is given in Table XII. 

Few in-depth studies have been made of actual furnace smelt-water explosions and, therefore, it is difficult to delineate expected overpressures and impulses. One case history is presented to indicate in a qualitative fashion the type of damage in a large explosion. 

---

Laboratory investigations into the mechanism of smelt-water explosive boiling events have been primarily of value in delineating the effect of smelt composition on the sensitivity of the salt in producing RPTs. For example, pure molten sodium carbonate has never led to explosive boiling. Addition of either (or both) sodium chloride or sodium sulfide lead to smelts which are more prone to explosive boiling. Investigators experimented with many additives both to the smelt and to the water in an attempt to obtain less sensitivity. Most had little or no effect. 

The superheated-liquid model introduced earlier to explain LNG-water RPTs was not considered applicable for smelt-water explosions since the very large temperature difference between the smelt and water would, it... 

As noted earlier, no viable theories of the smelt-water explosion had been widely accepted during the early period of investigation. Nelson in... 

The smelt-water explosion problem is of primary interest in kraft recovery furnaces where, from operational error or an equipment failure. 

The research results clearly indicated that the smelt-water explosion yielded a localized, high-energy shock wave which moved at velocities of over 700 m/sec. The maximum pressure rise was achieved in about 1 msec. These facts were related to the results found in real boiler explosions in descriptive terms as there were 3-4 inch depressions 3 to 6 feet... 

Lougher et al. (1968) reviewed the situation, presented some additional data, and developed recommendations for further work. They examined existing theories to explain smelt-water explosions and rejected all. They too noted that sodium aluminate in the smelt reduced the probability of an incident and also stated that CaCOj and Fe203 were effective. Most other additives (and over 90 were studied) either led to more violent explosions or were ineffective. No correlation of the smelt additive results was given. Also, water with various additives such as surfactants, starch, sucrose, glycerine, and hydroxymethyl cellulose still exploded when contacted with a sensitized smelt. 

Krause et al. (1973) carried out the last detailed U.S. experimental investigation of the smelt-water explosion phenomenon. A large number of experiments were conducted with variations in the smelt composition. The scale was quite small with 0.03-1 g quantities of water injected at high velocity (20-30 m/sec) onto the surface of the smelt. A few tests were also made with small drops (0.8-0.3 g) of water on the end of a ceramic tube that was dropped into the smelt. Some information concerning pres-... 

A model based on inertial restraints was developed, but it has not been widely accepted since it does not explain many of the experimental facts for smelt-water explosions. 

Taylor and Gardner (1974) and, more recently, Grace and Taylor (1979) have made detailed statistical analyses of the reported smelt-water explosions in modem recovery boilers operating in the United States and Canada. There have been 77 incidents in the period from January 1958 through July 1979. Of these 77 explosions, 55 occurred in the United States and 22 in Canada. The time period covered 5120 boiler-years of operation in the United States and 1070 boiler-years in Canada. 

The following was excerpted from a damage report of a smelt-water explosion. 

In a somewhat different type of furnace that suffered from a combustion-gas explosion, the floor beams, which were similar in size, were deflected at most 2-3 cm. A structural analysis of this explosion led to the conclusion that peak pressures were in the order of 30-40 kN/m (5-6 psi). Comparing the two damage descriptions, it is obvious that the smelt-water explosion generated pressures well in excess of 40 kN/m on the floor. 

While the mechanism proposed by Nelson explained many of the characteristics of a smelt-water explosion, it had one very serious drawback, i.e., the smelt temperature was significantly higher than the expected superheat-limit temperature of water (1100-1200 K compared to 577 K). For LNG-water, it was shown earlier in Section III that if the water temperature were much higher than the superheat-limit temperature of the LNG, explosions were then rarely noted. For such cases, the filmboiling mode was too stable and collapse of this vapor film was unlikely. 

Krause, H. H., Simon, R., and Levy, A. (1973). Smelt-Water Explosions, Final reports to Fourdrinier Kraft Board Institute, Inc. Biittelle Laboratories, Columbus, Ohio. Lemmon, A. W. (1980). Explosions of molten aluminum and water. In Light Metals 1980 (E. McMinn, ed.), p. 817. (Proceedings of Technical Sessions Sponsored by TMS Light Metals Committee at 190tb AIME Armuat Meeting.)... 

Lougher, E. H., Blue, G., Goddard, S., Gurev, H. S., Miller, J. F., Putnam, A. A., and Simon, R. (1968). Feasibility Study on Smelt-Water Explosions, Summary report to Fourdrinier Kraft Board Institute, Inc. Battelle Memorial Institute, Columbus,... 

Nelson, H. W. (1973). A new theory to explain physical explosions. Tappi 56(3), 123. Nelson, H. W. (1978). Method of preventing explosions using a smelt water explosion inhibitor. U.S. Patent 4,106,978. 

Shick, P. E. (1980). Ck>ncentration-gradient trigger mechanism for smelt-water explosions. 

Pap., Am. Pap. Inst. Annu. Recovery Boiler Comm. Meet., 1980, Chicago, Illinois. Shick, P. E., and Grace, T. M. (1982). Review of Smelt-Water Explosions," Prijj. 3473-2. Inst. Pap. Chem., Appleton, Wisconsin. 

---