Improving drum level response in industrial boilers

Hi, I'm working on control of an existing industrial plant powerhouse, boilers and TGs, the disturbance level can be very high, and boiler trips
due to drum excursions are a constant threat. We'd really like to be able to improve the performance of the drum level controls, which are the ubiquitous 3-element configuration (feedforward from steam rate and water flow) and PID on level. Is anyone aware of / can anyone point me to references to something smarter than this traditional approach? TIA
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Noodnik wrote:

I don't really do process control at all, so I can't recommend literature.
But I can ask questions!
Have you analyzed the current system to make sure that it is harmonized? I.e. is everything in the system limiting it's response speed about the same amount? If there's some salient part of the system -- a sensor, or a valve, or something -- that just can't keep up, then fixing that may allow you to tune the whole thing for a higher bandwidth and therefore go faster.
Ditto if there are nonlinearities or pure delays in the system that are being handled by de-tuning a linear control rule rather than by introducing 'smart' nonlinearities into the controller.
Literature about this would (in my curmudgeonly view) fall into two categories. The first will be those that address the questions above, in a manner that is specific to your problem (i.e. discussing specific actuator and sensor technologies, expected nonlinearities, etc.). The second would be documents written by some egghead in a university who had no clue how the real world was put together.
I'm interested in seeing if there's someone out there who can answer your _real_ question.
--
Tim Wescott
Control system and signal processing consulting
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I have had some experience at this from a long time ago when I was in the navy. My first question is are there bigger variations in demand than when the boilers were first installed? The PID control will only respond so quickly. Are the feed forwards right set right? At a constant load the PID should cause the level to reach the set point. If the feed forwards are setup to match the flow demand the PID should be contributing little at this point. You need to test this at two power level and maintain them to get the feed forwards gains right.
I would look at the webpages here. 3 methods of control are listed here. http://www.controlguru.com/wp/p48.html
I was involved in power generation over 30 years ago. I know of the problem but are probably better ways to solve it. Back then we only had mag-amps.
Peter Nachtwey
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On Sat, 01 May 2010 13:34:27 +0800, Noodnik wrote:

Your control scheme does not take "swell" into account, and that needs to be done for optimal boiler drum level control. I don't know of any literature offhand, try searching for "boiler swell compensation" or similar.
The swell effect is the increase in steam fraction on increasing firing rate, which causes the water level to rise and produce an error signal to decrease feedwater flow, partially negating the steam flow signal to increase feedwater flow and tending to cause overshoot on the correction. If you produce your error signal not directly from drum level but from drum level plus a leaky (decaying with time) swell predictor, you can eliminate the undershoot in the response. Of course decreasing demand has the opposite effect; lower steam fraction causes the water level to drop while feedwater demand is also dropping.
It is not possible to eliminate boiler swell, it is only possible to prevent it from winding up your PID error signal and causing overshoot, which is usually the biggest cause of poor water level control on boiler load change.
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Thanks, and to the other respondents. The main disturbance here is dips in HP header pressure, causing sudden increases in steam rate for all the boilers on the header. Swell occurs also in this situation. We'll try your suggestion of a leaky predictor (haven't come across that terminology before, nice) on a simulator and if it looks OK give it a try at the sharp end.
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You mentioned a 3-element boiler steam-drum level control with feed-forward from steam rate. This is not the way that such a control is supposed to work (i.e. feed-forward); instead the measurement of mass-flow of steam is meant to set directly the set-point for the mass flow of make-up water, the PID control on level is then supposed to make a slow additional correction for errors in the mass-balance measurements. Provided the steam:water mass-balance is maintained then there can be no better control, and precise control of level is not necessary or practical. The swell-capacity of the drum must be capable of handling the swell due to transient pressure changes, and that's a drum design issue which, if wrong, cannot be improved by fancy control.
I have seen many properly-designed traditional 3-element feedwater control systems replaced by systems designed by process-control engineers without specific boiler-control know-how, in which the steam:water mass-balance performance has been destroyed by downgrading the steam measurement to a mere feed-forward onto the make-up flow, with consequent frequent trips. It may be worth checking this hasn't happened to you.
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