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Test Your Knowledge: The Three Element System

The three element system for controlling the boiler drum level considers which three parameters?

1.Feed Water Flow, Fuel Flow, Liquid Level
2.Feed Water Flow, Combustion Air Flow, Liquid Level
3.Feed Water Flow, Fuel Flow, Steam Flow
4.Feed Water Flow, Steam Flow, Liquid Level

And the answer is…

4. The “three element control” in boilers stands for the three control element, each measuring an individual process variables;

  • Flowrate of boiler feed water
  • Flowrate of steam leaving the boiler
  • Liquid level inside steam drum

For any boiler, controlling the liquid level at the optimum range is the highest priority. It needs to be high enough to ensure that water is present in every steam generating tube, yet low enough to provide sufficient space for steam above the liquid.

For boilers operating at low pressure, it is relatively inexpensive to have a large steam drum in which the liquid level moves very slowly. But for boilers operating at medium or high pressure, larger steam drums are more expensive, so in most cases these steam drums are small. This means that liquid levels can fluctuate very quickly. To protect against this fluctuation, the three element control is typically used.

Why 3 elements instead of 2 elements?

Suppose we have a 2 element control which directly controls the flowrate of boiler feed water (by opening/closing control valve) based on the liquid level in the steam drum. If the level decreases, the control valve opening will increase to increase the flowrate of feed water. But lets say at the same time the steam requirement changes whichl fluctuates the steam drum pressure.

For instance, if the liquid level decrease in the steam drum that forces the control valve to increase the flowrate (suppose 50% to 70%) so that it will balance the system. At a result, steam consumption decreases and pressure increases in the steam drum. In this scenario the control valve opening will still remain 70% because it will not consider the decrease in steam flowrate. Therefore the 70% opening of the control valve will give the same flowrate it was getting before with the 50% opening. In this case, the 2 element control will not work.

To avoid theissue, the 3 element control system is installed (the third element measures the steam flow rate). Now if the level in the steam drum fluctuates at the same time as the steam requirement, the steam flowrate signal will go into the flow controller of the boiler feed water and adjust the opening of control valve accordingly to get the desired flow of water inside the steam drum.

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