Boiler Drum Level Dynamic Simulation

I am looking for help to build a dynamic simulation of a boiler drum level. Issues that are important to capture include:
- inverse response of the level to changes in drum pressure, steam flow, boiler feedwater flow and boiler firing. - difference in behaviour between small and large disturbance conditions. - level measurement errors as a function of pressure.
I am evaluating the drum level control tuning of different boilers in an oil refinery. One big problem is that the dynamic response of the steam drum levels on small and large disturbances could be very different. We want to know how optimum the tuning is under large steam upset conditions, and this is for obvious reasons difficult to test on a real plant. I want to build a useful simulation to tune the PID controllers and to test other rule and model based algorithms so that we can have confidence in the performance of the boiler drum level controllers.
As an example if what I want to do, I recently build a very simple yet very useful simulation of a compressor anti-surge control system and this allowed us to have enough confidence in that specific, previously suspect, control scheme. I am a bit unsure how to capture the shrink and swell effects of the boiler drum level adequately, especially the difference in behaviour under small and large upset conditions, to allow me to do the same for the boiler drum level applications.
I will appreciate help on this appliction.
Pieter Steenekamp
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Pieter, I may have something that will assist you. There's a very old article (60s or around then) titled "dynamic model of a boiler" (cool title what?), I have a copy somewhere. It addresses the very problem you describe. The sim models the thermosiphoning in the tubes, and it represents shrink & swell. The equations are presented so you should be able to convert it to code without too much difficulty. There's also some interesting frequency domain analysis that explains why the problem becomes increasingly acute as the boiler load drops away from MCR. IIRC, they say that for a lot of boilers running below 50% is effectively uncontrollable. To some degree that's my experience.
If I can locate it I'll post the details here. Cheers
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and remove the biggies
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You are really going to have to model the entire boiler to do this right, especially the water system, the drum, superheaters, and downstream load.
Working backwards, the steam flow rate out is going to be related to the header pressure, and the steam pressure. If the header pressure goes up, you have to put more heat into the generator to raise the steam pressure accordingly. The enthalpy increase required to do that is going to be acquired through increasing the time and/or submerged surface area in the generator section, that in turn will generally cause the feedwater rate to be changed, and also the amount of time spent in the superheater.
Consider spending a little time at a local powerhouse to see how these things run. The insight might be well worth your time spent. I would be surprised to find that there aren't any good models available.
Tappi probably has a pretty decent book on power boiler and recovery boiler operations. That probably has a model in it of sorts.
What you are embarking on isn't all that hard to get right, but it has a lot of details in it.
Michael
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