APPS: Boiler Drum Level

Fri Mar 27 10:09:03 EDT 2009

==> Automation List post by Ritika...
To view or reply to this thread: http://www.control.com/thread/1238136342

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APPS:  Boiler Drum Level
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I have a question to the experts of this forum:

1) If the feedwater supply pressure is fairly constant, then is it possible to eliminate the third element - feedwater flow sensor?

2) I want to get some numbers about three element control. Say, 
FT Steam = FTS;
FT Water = FTW;
LT Drum = LT

Request to kindly help me understand the interconnections wrt to a 3-element Drum Loop. 

I understand that 4-20mA of LT gets connected to the LIC_Drum. The difference of the Drum SP of say, 50% (12mA) gets subtracted from instantaneous (say 8mA/25%) 4-20mA from LT. This error (4mA/25%) gets acted upon by the PI/PID algorithm of Drum LIC. The LIC_Drum output is a 4-20mA PID output, say, 75% (16mA) (assuming a direct action controller output). 

THIS IS WHERE MY QUERY COMES IN. 

After this step the FTS's (FT_Steam) 4-20mA should get added to this LIC Output as a Feedforward path, right? If, say, FTS is 12mA (50% steam flow), then will this 12mA get DIRECTLY ADDED to the LIC_Drum's mA PID Output? i.e., Will LIC_Drum output of 16mA get added to 12mA of FTS? But this doesnt make sense since the summation is 28mA now? Do we need to scale the FTS's mA? If so then by how much? Will the summation of LIC_Drum Output & FTS never cross 20mA? Should the Feedforward Path Gain be chosen to scale FTS to keep this summation below 20mA always under all conditions? I am really lost on this. Please help me understand, what is it that I am missing.

Thanks for your time
Regards
Ritika
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