Sodium heated steam boilers

Figure 10.1 Types of phosphate structures, (a) Where x = 12 to 14, the structure represents sodium polyphosphate, a phosphate typically used in HW heating and industrial steam boiler formulations. The structure is ill defined and described as glassy rather than crystalline. Where x = 2, it represents sodium tripolyphosphate, (b) This is the structure where effectively, x = 0, and represents trisodium phosphate (sodium orthophosphate), which is commonly supplied in either crystalline or anhydrous powder form and used as an alkalinity booster, boiler boil-out cleaner, and metal surfaces passivator.

In some a)kali works, to assist the solvent power of tho water employed in tho Iixiviation of black ash, it is usual to heat it to about 90° to 100°. This end may ho conveniently attained by passing the water through a coil cf pipes Contained in a steam boiler. The practice of heating the water, though it undoubtedly aids the process of Iixiviation, is nevertheless very objectionable, principally because sulphide of calcium is dissolved and decomposed by hot water, giving rise to the formation of sulphide of sodium, and the hotter the water tho greater Will this proportion of sulphida be, or rather of the donble sulphide of sodium and iron contained in the vat liquor. 

My first encounter with the operational challenges of ensuring pressure vessel safety came at Dounreay in northern Scotland, at the prototype fast reactor (PFR), in 1981. PFR was a highly unusual power station since the reactor was cooled by liqnid sodium. PFR had three identical sodium-heated boiler circuits, each containing a boiler, a steam drum, a superheater and a reheater (Fig. 7.8). 

The quahty of feed water required depends on boiler operating pressure, design, heat transfer rates, and steam use. Most boiler systems have sodium zeohte softened or demineralized makeup water. Feed-water hardness usually ranges from 0.01 to 2.0 ppm, but even water of this purity does not provide deposit-free operation. Therefore, good internal boiler water treatment programs are necessary. 

In addition to the loss of the more volatile constituents such as sulfur, phosphorus, chlorine, and oxides of sodium and potassium there was a loss of iron oxides owing to the formation of metallic iron, and also of silica. Silica loss was caused by its reduction to volatile silicon monoxide, which was subsequently reoxidized and removed in the gas stream as silica fume. With coke the silica loss was 5% at a steam oxygen ratio of 1.3, but this increased rapidly at a ratio of about 1.10 vol./vol. to more than 15%. Typical losses of sodium oxide, potassium oxide, and sulfur were 5, 5, and 40% respectively at 1.3 vol./vol., increasing to 40, 40, and 80% at a ratio of 1.10 vol./vol. These materials were deposited in varying degrees in the gas offtake and in the sumps of the waste heat boiler and final cooler. The deposits in the gas offtake were enriched with alkali metal oxides and had a silica concentration of more than 50%. Most of the material was removed in the bottom of the waste heat... 

The tests discussed thus far, while important in defining the effects expected from an alkali metal-water reaction, have involved large quantities of water and comparatively little alkali metal. In the event of water leakage into sodium in a heat-transfer system, the sodium would be in excess. To investigate the effects of a sodium-water leak in heat transfer systems, a small test system was operated at KAPL. The system consisted of a water boiler using one double-walled tube with flowing sodium on the tube side, boiler water on the shell side, and mercury as the third fluid. In each test run a hole was fabricated in the outside tube. After the boiler reached equilibrium conditions, steam pressure of 100 pounds per square inch and 475 to 500 F. sodium temperature, the inner tube was completely parted by a tensile load applied to a preformed peripheral notch. 

The most severe corrosive environments are encountered in the areas where corrosive impurities concentrate. These impurities can concentrate on heat transfer surfaces such as in boiler and steam generator tubes where the surface temperature is above the saturation temperature of the boding water, and on the surfaces in superheated and saturated steam where the impurities concentrate by precipitation from a steam solution and deposition [4,5,11,14]. Figure 6 shows the gradient of concentration of sodium hydroxide at the surface of a boiler tube. 

The secondary circuit was then used to heat the steam for the turbines. One major problem was the NaK/steam heat exchanger. In the gas-cooled reactors, the hot gas flowed through pipes which ran through the boilers. This had obvious problems for a sodium-cooled system. Water and sodium/potassium react together very violently even at room temperature, and one of the products is hydrogen, which can form an explosive mixture with air, so any arrangement where the two could mix as a result of a leak had to be avoided. A different system had to be devised ... 

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