In a boiler drum exist water and steam at saturation pressure and
saturation temperature. Furthermore, the water is mixed with steam bubbles in
different sizes. If one opens the valve to a steam consumer then the pressure
in the boiler drum falls and hence the steam bubbles expand and the water level
rises despite the fact that the water mass actually decreases. On the other
hand, pumping in cold feed water will cause the steam bubbles to collapse and
the water level falls when the water quantity actually increases in the steam
drum. These shrink and swell phenomena will complicate the boiler level
boiler with large amount of water and relatively low steam
production a single water level transmitter on the steam drum is sufficient
for the level controller to maintain a level with acceptable variation.
Far more sophisticated methods are required for a
boiler with high steam production and relatively
small water volume.
Boiler level control, the classic method
The classic three
point level control method works as follow: The outlet steam
mass-flow is measured and the inlet feedwater mass-flow is adjusted to the very
same amount. The
level transmitter is merely used to tune up the system so the
water level lies within the limits.
Thus, the input to the setpoint of the feedwater flow controller is the sum of
the outlet steam flow signal and the reversed water level controllers signal
minus 12 mA (or 9 psi if it is a pneumatic system).
Add to the monthly maintenance list:
Check the process impulse piping between the steam outlet line and the steam
flow transmitter. Fur deposit is very common in the orifices measuring
Check the process piping between the steam drum and the water level
transmitter. Condensate has to accumulate in the tubing before the transmitter
can work properly again. (Draining the level transmitter that often
wouldnt be necessary if you have stainless steel tubing between the
boiler drum and the level transmitter).
N.B. the feedwater flow controller has to be in manual
mode during these procedures.