Steam bubbles

Vp, = The minimum water holding capacity required to replace all of the steam bubbles in the risers, gal. 

Thermal shock In biphase systems, steam bubbles may become trapped in pools of condensate in a flooded main, branch, or tracer line, as well as in heat exchanger tubing and pumped condensate lines. Since condensate temperature is almost always below saturation, the steam will immediately collapse. 

Water hammer results from the collapse of this trapped steam. The localized sudden reduction in pressure caused by the collapse of the steam bubbles has a tendency to chip out pipe and tube interiors. Oxide layers that otherwise would resist further corrosion are removed, resulting in accelerated corrosion. 

BWRs do not operate with dissolved boron like a PWR but use pure, demineralized water with a continuous water quality control system. The reactivity is controlled by the large number of control rods (>100) containing burnable neutron poisons, and by varying the flow rate through the reactor for normal, fine control. Two recirculation loops using variable speed recirculation pumps inject water into the jet pumps inside of the reactor vessel to increase the flow rate by several times over that in the recirculation loops. The steam bubble formation reduces the moderator density and... 

In boiling convection, liquid motion is created by steam bubbles breaking loose from the surface. 

This represents a special case of high-level turbulence at a surface by the formation of steam and the possibility of the concentration of ions as water evaporates into the steam bubbles . For those metals and alloys in a particular environment that allow diffusion-controlled corrosion processes, rates will be very high except in the case where dissolved gases such as oxygen are the main cathodic reactant. Under these circumstances gases will be expelled into the steam and are not available for reaction. However, under conditions of sub-cooled forced circulation, when cool solution is continually approaching the hot metal surface, the dissolved oxygen... 

Butler and Ison S have suggested that variation in corrosion rate can be influenced by surface roughness, which allows a large number of nuclei for steam bubble formation. In these circumstances they have suggested that concentration of ions in solution next to the surface will be greater, and their observations on corrosion damage indicate that the steam bubbles may provide crevices or at least enhanced conditions for dissolution at the triple interface (solution/metal/steam). 

In a boiler, with the continued application of heat, steam under pressure is produced via a combination of steam bubble formation (nucleate boiling) and direct evaporation at the steam-water interface (convective boiling), as shown in the sketch of different generated steam flow forms in Figure 1.1. 

Figure LI Steam generation from a heated surface, showing nucleate boiling, leading to bubbly, intermediate, annular and mist flow forms of convective boiling. Steam bubbles in water (a) leading to water droplets in steam (b).

Heat transfer rates in modern boilers are relatively high, and when the first stage of boiling (incipient boiling point) is quickly reached, small bubbles of steam begin to form on the heated, waterside metal surface (steam bubble nucleation) but initially collapse when cooled by contact with the bulk water. 

With continued heating, the local saturation temperature is reached and the steam bubbles move into the larger, bulk-water nucleate boiling region. Because the resulting steam bubble-water mixture close to the heated metal surface has a lower density than cooler water farther away from the heated surface, the steam bubble-water mixture rises. 

In high heat flux (heat transfer rate per unit area) boilers, such as power water tube (WT) boilers, the continued and more rapid convection of a steam bubble-water mixture away from the source of heat (bubbly flow), results in a gradual thinning of the water film at the heat-transfer surface. A point is eventually reached at which most of the flow is principally steam (but still contains entrained water droplets) and surface evaporation occurs. Flow patterns include intermediate flow (churn flow), annular flow, and mist flow (droplet flow). These various steam flow patterns are forms of convective boiling. 

Most steam generating plants operate below the critical pressure of water, and the boiling process therefore involves two-phase, nucleate boiling within the boiler water. At its critical pressure of 3,208.2 pounds per square inch absolute (psia), however, the boiling point of water is 374.15 C (705.47 °F), the latent heat of vaporization declines to zero, and steam bubble formation stops (despite the continued application of heat), to be replaced by a smooth transition of water directly to single-phase gaseous steam. 

Foaming occurs when steam bubbles arrive at a steam-water interface at a rate faster than that at which they can collapse into steam vapor. It is essentially a BW surface chemistry problem and develops from many different causes including ... 

In WT boilers operating at very high firing rates, the risk exists of steam bubbles developing in downcomers. Where this occurs, it causes a temporary halt in the natural steam-water circulation and instantly leads to surging or priming followed by carryover. 

Where air bubbles and other gases are entrained in turbulent FW and an abrupt reduction in pressure takes place, cavitation may occur. The result of the extremely rapid formation and collapse of steam bubbles on the suction side of feed pumps or the discharge side of valves produces erosive microjets that over time may promote severe cavitation-al metal wastage. 

Where particulate matter (in the form of corrosion products of iron oxide) is present in returning condensate, it often contains copper, nickel, and zinc oxides as well. This debris can initiate foaming (through steam bubble nucleation mechanisms) leading to carryover. It certainly contributes to boiler surface deposits, and the Cu usually also leads to copper-induced corrosion of steel. 

Caustic gouging usually occurs only in areas of high heat flux but may also result when heat transfer rates are low, as in horizontal or inclined WT boiler tubes under circumstances in which the steam-water velocity is particularly low. Here, the relatively small volume of BW surrounding the steam bubbles concentrates very quickly, the alkalinity soars, and caustic corrosion develops. 

Oily surfaces induce film boiling and the stabilization of steam bubble on the film surface, and overheating results. Oily surfaces also cause other problems, including ... 

While the PEM disk is in a beaker, there may be a tendency for the film to curl and lift on the steam bubbles, rising to the surface. It should be kept submerged so the top side doesn t get exposed to air. Use a clean inert polyethylene plastic or glass probe to keep it down in the dipping solution. 

The Wilson bubble rise model (a void fraction of steam bubbling through stagnant water)... 

For the pure eduminum tests (99.0% minimum purity), two tests were run in initial experiments using a glass-sided water tank. Each spill was only 0.15 kg. Motion pictures indicated that most of the aluminum entered as an irregular blob. Steam bubbles formed about the metal and collapsed. No explosions were obtained. In other tests, with the steel-pipe water vessel. No. 6 blasting caps were detonated after the aluminum entered the water. (The caps were located in the center of the pipe at the same level as the pressure transducer.) The cap detonation fragmented the metal, but no explosions were obtained. 

Further aluminum pour tests were made in a heavy-wall stainless steel tank fitted with Lucite side windows. The tank was supported on a force transducer and pressure transducers were located on either end. In a test, after the spill, there was a predetermined delay and then the wire was exploded. The aluminum usuaUy had puddled on the tank bottom before the wire explosion and steam bubbles could be seen. The shock from the wire explosion usually collapsed the film and, following this, the aluminum expanded. If the shock were sufficiently energetic, the aluminum soon fragmented and expelled the water from the tank in a thermal explosion. In such cases, the force transducers on the bottom ranged from 5 to 10 N sec. (The exploding wire alone led to impulses around 1 N sec.) Efficiencies of an explosion calculated as indicated above were low. 

As indicated in Sec. IIB, ordinary nucleate boiling is a two-step process. First, nuclei must appear. Second, the nuclei must grow into bubbles large enough to move away from the nucleation sites. The rate of heat absorption by the liquid may be controlled by either one or both of these two processes. The growth of a nucleus (tiny bubble) into ordinary bubbles has received attention recently. The theoretical attack of Forster and Zuber was discussed in Sec. IIB2. Inasmuch as the theory of Zwick and Plesset (P3, P4, Zl, Z2) represents another attempt to obtain exact expressions for bubble growth, and since the theory fits well with the few data for steam bubbles in superheated water, their theoretical method is summarized below. 

Fig. 38. Asymptotic growth of a steam bubble in water at 220.1 F., 1 atm. The line is a graph of Zwick s theoretical prediction (Zl).

Heat transfer to macroscopic bubbles has received but little attention. Jakob (J2) presents the growth curves for several steam bubbles up to a radius of 3.6 mm. The main experimental obstacle is that of determining the true volume of a nonspherical bubble. 

The density of steam (or vapor) above the liquid level will have an effect on the weight of the steam or vapor bubble and the hydrostatic head pressure. As the density of the steam or vapor increases, the weight increases and causes an increase in hydrostatic head even though the actual level of the tank has not changed. The larger the steam bubble, the greater the change in hydrostatic head pressure. 

Figure 1 Left Laminar flow field around a 400- xm diameter spherical steam bubble rising at 0.06 m/s. Right Mean flow around a 4-mm equivalent sphere diameter spherical-cap steam bubble rising at 20 cm/s in molten CaBr2 at 1 013 K.

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