Shrinking and swelling effect in boiler
As the load on the boiler gradually reduces, the pressure inside the boiler drum will increase, which eventually would lead to reduction in bubble formation. With the reduction in the steam bubbles, the level of water inside the boiler drum would come back to normal. With the introduction of cold water inside the boiler, the temperature would further reduce and would result in a sudden drop in the boiler drum water level. This effect of drop in the water level inside the boiler drum is known as “shrinking”.
The level of the boiler water inside the drum is determined with the help of a water level gauge glass mounted on top of the boiler. It is a known and accepted fact that when the boiler is operating, the water level in the water gauge glass for a particular amount of water in the boiler drum, will be higher than that shown when the boiler is shut down. This happens because as the water reaches the boiling temperature steam bubbles that produces in the water increases the overall volume and pressure inside the boiler drum. This results in a higher level of the boiler gauge glass. As the pressure inside the boiler drum increases, some quantity of water will flash off due to the high temperature and would result in the formation of steam bubbles. These steam bubbles would further cause the drum level to rise.
However, if there arises a sudden demand for steam, the pressure inside the drum will drastically reduce. This would result in a condition of more steam than water inside the boiler drum It is to note that even if the volume of water reduces in the drum, the greater mass of steam will increase the level of water inside the drum. Moreover as the mass of the water further reduces, even more amount of steam is produced due to high temperature, which increases the level of water inside the boiler drum even further. This effect of water level rise is known as “swelling”.