Shrink and Swell effect in boiler

Shrink  effect 

Now as the steam pressure starts increasing steadily the boiler pressure increases and boiling point of water also starts increasing which lead to decrease in bubble formation and level starts dropping and water level seems decreasing. Now automatic water level control valve opens and cool water enters the system and bubbles burst leading to decrease in level of water more. This effect is called shrinkage.

Swell effect 

Let us suppose discharging of cargo in a tanker is going on. Now due to sudden increase in steam demand due to starting of another copt or due to sudden increase in rpm of copt’s there is a sudden drop in boiler pressure. This sudden drop in pressure leads to vapourization of water as boiling point of water decreases with decrease in pressure. Hence the formation of bubbles inside the boiler makes the level of water looks like increased which in real in not there. This phenomenon is known as swelling which leads to closure of automatic water control valve and water stop entering the boiler. This can lead to unsafe condition of boiler.