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WATER TUBE BOILER/3 ELEMENT LEVEL CONTROL

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Saver2008

Saver2008

Mechanical
Oct 14, 2008
112
Hi

I have a question about where to take the water for desuperheater purpose. Attached you will find a schematic of 3 level control strategy for a water tube boiler and it is shown 2 cases:
Case 1
I take the water for desuperhetaer after the FT and FV for boiler feed water
Case 2
I take the water for desuperheater before the FT and FV for boiler feed whater

I have had some talkings with some engineers and told me that it is better to take it before FT and FV but as my perceptions, for water level control (using in this case 3 level control strategy) is better after because the FT for boiler feed water measures total feedwater to the steam drum and for desuperheating and then for superheated steam demand, I have another FT to measure the total demand of steam.....

Is there a rule or a recommendation to take where to use one than the other? What would be reasons to use case 1 and case 2?

Best Regards
Roland
 
 https://files.engineering.com/getfile.aspx?folder=5d1a1b24-edf8-4882-9dc8-71345568a68a&file=3_LEVEL_CONTROL_CASES.pdf
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Go for case 1, reasoning :-
This is a cascade control loop, where master LIC resets the FIC setpoint in order to keep level constant. Mean time, the FIC operates at the current setpoint to maintain constant flow. So you shouldnt be drawing water from this line (to some other consumer) once it has been measured. It defeats the whole purpose of the level control scheme.
BTW, I dont know if this feedforward control scheme- yes,this is feedforward, (with superheated steam flow FT feeding the LIC) actually works in your plant. In my experience, it is difficult to make such a feedforward control scheme work, modelling can be a nightmare, and I wouldnt rely on it.
 
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OP,
I am a little confused by your control diagram. If this a part of a larger P&ID? Is this an existing system you are modifying? Why are superheating and desuperheating shown at a simultaneous point? What are your process variables? Mass flow, volumetric flow?
I share georgeverghese's concerns about the feedforward model. I've seen many boiler controls much simpler than this design.
 
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Another reason for selecting Case 1:
During high steam flow, you may have plenty of pressure drop through the boiler tubes + superheater bank to keep the desuperheater working well. However, pressure downstream of FV in case 2 at low steam flow would be nearly the same as that upstream of desuperheater. So there would hardly be much pressure drop available for the atomisers within the desuperheater to work. This hydraulics reason would be valid even for the case when you do have a working feedforward model (and perhaps there is, on second thought, a fair chance you can devise a feedforward algorithm for this boiler operation).
 
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