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ow can steam drum level be controlled to maximize both stability and responsiveness? The best method is to feed forward (FF) the steam flow to the level PID loop and cascade the output of that loop to the setpoint of a boiler feedwater (BFW) flow loop.
This arrangement is called "Three Element Drum Level Control" (3E) because the algorithm uses level, BFW flow, and steam flow.
"Single Element" (1E) control should be used when either flow is too low or has bad quality . 1E is a simple PID loop with the level as PV, the output going to the valve, and no feed forward. Please review my previous blog, "Advanced PID Loop Tuning" for PID loop definitions, tuning advice, and generic programming methods.
http://innovativecontrols.com/blog/three-element-drum-level-control
The "three element control" in boilers stands for the three control element, each measuring an individual process variables;
For any boiler, controlling liquid level (at optimum range) is highest priority (high enough to assure that water is present in every steam generating tube, low enough to ensure sufficient steam disengaging space above liquid). For boiler operating at low pressure, it is comparatively inexpensive to design large steam drum for the system as for large drum, if there is any offset, liquid level moves very slowly. But for boiler operating at medium pressure or high pressure, it is expensive to design large steam drum, so size of steam drum is small in this case. For small steam drum, if there is any offset, liquid level fluctuate very quickly and to protect this fluctuation, advanced control scheme is needed. Three element control is employed in such system.
The question remains why 3 element and why not 2 elements.
Suppose we have 2 element control which directly control flowrate of boiler feed water (by opening/closing control valve) based on the level in steam drum in cascade mode. If level in steam drum decrease, control valve opening will increase to increase the flowrate of feed water. But lets say at the same time steam requirement fluctuate, it will fluctuate steam drum pressure. Now we will see consequences of this. Liquid level decrease in steam drum forced the opening of control valve increase (suppose 50% to 70%) to increase the flowrate so that it will balance system resistance. At this time, steam consumption decrease and pressure increase in steam drum. Control valve opening will still remain 70% because it will not consider decrease in steam flowrate. For this new condition, where system resistance increased, 70% opening of control valve will give the same flowrate which we were receiving before with 50% opening of control valve. In this case 2 element control will not work.
To avoid such issue, '3 element control' system is installed (third element is to measure steam flow rate) in cascade mode. So now, if the level in steam drum fluctuate and at the same time steam requirement fluctuate, steam flowrate signal will go in calculation box along with feed water flow signal. The magnitude of difference in these flowrate along with signal from liquid level will go in the flow controller of boiler feed water and accordingly it will adjust the opening of control valve to get the desired flow of water in steam drum to manipulate the level inside steam drum.
A detail answer is available at below Link.
http://blog.opticontrols.com/archives/165
