Boiler Efficiency and Steam Quality: The Challenge of Creating Quality Steam Using Existing Boiler Efficiencies http://www.nationalboard.org/index.aspx?pageID=164&ID=235
... Lower Pressure Increases Entrainment As a steam bubble rises through the water and reaches the surface, it finally breaks through the final layer of water and enters the steam space. This final act of leaving the water causes water entrainment in several ways. Initially, the bursting of the steam bubble or the rupture of the thin layer of water surrounding it produces an initial rush of high-velocity steam that carries a small amount of that thin water layer into the steam space. Then, the loss of the steam bubble from the water surface briefly creates a crater on the water surface. Water rushes in to fill this crater, colliding with water rushing from the other sides of the crater, and produces a tiny splash near the center of the crater. The water droplets from these splashes are then easily entrained in the rising steam. The size of the bubbles is directly related to steam pressure. Low-pressure operation requires a larger volume of steam to carry the required heat energy. This low-pressure operation produces more and larger steam bubbles and creates greater turbulence on the water surface. These bubbles produce more craters and larger craters, as well as more and larger splashes as they leave the water surface. In addition, low-pressure operation results in a higher vapor velocity which, when combined with the high turbulence of low-pressure operation, tends to carry water droplets into the steam systems rather than allowing them to fall out by gravity. The solution is to operate the boiler at its maximum design pressure and use pressure-reducing valves at the point of use where required. ... (Lots of related articles ... eg "How to Destroy a Boiler" and " Anatomy of a catastrophic boiler accident". ) No specific numbers on the range of steam quality from a kettle boiler.
