Boiler water level control
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 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).
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