As the pressure of steam reduces, turbulence at the water surface increases, thereby increasing the moisture carryover in the steam. To ensure high dryness fraction of steam, the boiler should be operated close to design pressure.
Often boiler operators operate the boiler at pressures much lower than the design pressure primarily to reduce the fuel consumption. However, operating the boiler at low pressure leads to significant deterioration of the steam quality. To understand this, it is necessary to first understand how carryover occurs in a boiler.
The entrainment of moisture (or carryover) occurs mainly at the point when the steam bubble leaves the surface of the water. As a steam bubble rises through the water and reaches the surface, it breaks through the final layer of water and enters the steam space. The steam bubble ruptures through the thin layer of surrounding water and produces an initial rush of high-velocity steam. Because of the high-velocity, this steam carries a small amount of that thin water layer into the steam space.
Simultaneously, the loss of the steam bubble from the water surface briefly creates a crater on the water surface. Water rushes in from all sides to fill this crater, colliding at the center of the crater. This collision produces a tiny splash near the center of the crater. The water droplets from these splashes then get easily entrained in the rising steam. These two events increase lead to moisture being carried over with the rising steam.
The size of the steam bubbles is directly related to steam pressure, lower the pressure bigger the steam bubble. Thus low-pressure boiler operation produces more and larger steam bubbles creating bigger craters and bigger splashes at the water surface. This increased turbulence leads to increase in the water carryover in the steam. Additionally, operating the boiler at low pressure results in higher steam velocity which combined with the high moisture carryover tends to carry water droplets into the steam systems rather than allowing them to fall out by gravity. Poor steam quality deteriorates the health of the steam system though corrosion and erosion.
Thus boilers should be operated at a pressure closer to the design pressure to deliver higher quality of steam to the process as well as to enhance the life of the steam system.