The drum level must be controlled to the limits specified by the boiler
manufacturer. If the drum level does not stay within these limits, there
may be water carryover. If the level exceeds the limits, boiler water
carryover into the superheater or the turbine may cause damage resulting
in extensive maintenance costs or outages of either the turbine or the
boiler. If the level is low, overheating of the water wall tubes may
cause tube ruptures and serious accidents, resulting in expensive
repairs, downtime, and injury or death to personnel. A rupture or crack
most commonly occurs where the tubes connect to the drum. Damage may be a
result of numerous or repeated low drum level conditions where the
water level is below the tube entry into the drum.
Boiler Steam Drum
flow 1Some companies have had cracked or damaged water tubes as a result
of time delayed trips or operators having a trip bypass button. When
the drum level gets too low, the boiler must have a boiler trip
interlock to prevent damage to the tubes and cracks in the tubes where
they connect to the boiler drum. The water tubes may crack or break
where they connect to the drum, or the tubes may rupture resulting in an
explosion. The water tube damage may also result in water leakage and
create problems with the drum level control. The water leakage will
affect the drum level because not all the water going into the drum is
producing steam.
Poor level control also has an effect on drum pressure control. The
feedwater going into the drum is not as hot as the water in the drum.
Adding feedwater too fast will result in a cooling effect in the boiler
drum reducing drum pressure and causing boiler level shrinkage. This can
be demonstrated by pouring tap water into a pan of boiling water.
Shrink and swell must be considered in determining the control strategy
of a boiler. During a rapid increase in load, a severe increase in level
may occur. Shrink and swell is a result of pressure changes in the drum
changing water density. The water in the drum contains steam bubbles
similar to when water is boiled in our homes. During a rapid increase in
load, a severe rise in level may occur because of an increase in volume
of the bubbles. This increased volume is the result of a drop in steam
pressure from the load increase and the increase in steam generation
from the greater firing rate to match the load increase (i.e., bubbles
expand). If the level in the drum is too high at this time, it may
result in water carryover into the superheater or the turbine. The
firing rate cycle can result in drum pressure cycles. The drum pressure
cycles will cause a change in drum level.
The firing rate change has an effect on drum level, but the most
significant cause of shrink and swell is rapid changes in drum pressure
expanding or shrinking the steam bubbles due to load changes. When there
is a decrease in demand, the drum pressure increases and the firing
rate changes, thus reducing the volume of the bubbles (i.e., bubbles get
smaller). A sudden loss in load could result in high drum pressure
causing shrinkage severe enough to trip the boiler on low level. A
boiler trip at high firing rates creates a furnace implosion. If the
implosion is severe enough, the boiler walls will be damaged due to high
vacuum in the furnace.
The Wall