Boiler 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 shrinks and swell phenomena will complicate the control of the boilers steam drum level.
For a 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 boilers with high steam production and relatively small water volume.

This classic 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 orifice’s measuring holes.
• 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 wouldn’t 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.