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Posted by Bob D. on August 06, 2007 at 13:05:59:
In Reply to: steam collapse posted by John on August 03, 2007 at 14:44:23:
Hi, sorry for the delay in responding, but it was a beautiful weekend.
Anyway - usually when you hear of a steam bubble collapse they are talkng about a steady-state condition of saturated liquid and saturated vapor (water-steam) control volume that is chaged in some way that causes a phase change of the steam to a liquid, such as the introduction of colder water that "sucks" the BTUs out of the steam, causing the condensation. This happens pretty quickly, and can create a "vacuum" condition, drawing even more water into you control volume at a fairly high velocity. Since the volume of water has momentum (mass x velocity) significantly higher that the same volume of steam that just "collapsed" (condensed into the liquid phase, at "normal" conditions anyway, not supercritcal or anything fancy like that where the specific volume doesn't change), the stage is set for some pretty nasty water hammer events.
A steam "bubble" is also used as an "expansion volume" used to maintain fairly constant pressures in liquid systems (since water systems tend to be incompressible). With judicious use of local heaters within a "pressurizer" expansion tank, system pressure can be raised or lowered pretty much at the Operator's control regardless of main system temperature. This characteristic is used a lot in many processes, and even in "pressurized water" light water reactor plants. If you let the pressurizer get a little to cool (off the P-T saturation curve), the steam bubble can collapse as described above, and your system goes completely "solid", with almost total loss of pressure control, rapid depressurization, possible lifting of safety valves, and maybe other un-good things.
Hope this helps.