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Boiler Continuous Blowdown Control

Hey all,

Has anyone ever done control of the continuous blowdown on a boiler before? Our plant has 3 100,000 lb/hr power boilers and we have just automated our blowdowns. Previously they were done manually by the operators based on phosphate residual to maintain 50 cycles of concentration in the boiler drum. Now that they are automated we are using conductivity and our chemical residuals will be controlled with chemical pump flowrates.

Anyway, the point of continuous blowdown as I understand it is to keep solids from building up in the steam drum. To target 50 cycles of concentration in the steam drum, means that 2% of the boiler water flowing into the boiler should be blown down. Now I could have put a flow transmitter in, and ratioed blowdown flow to steam flow, but I think I would have been "over-instrumenting" this, and I don't think it's necessary for boilers of this size. I also wanted to keep the control as simple as possible for the operators. I wanted the controller to look like a typical controller on the DCS without too much stuff that they'd forget about and then 6 months down the road everything is on manual...

After I installed the conductivity analyzer and the valve (Fisher 3/4" ET), I did some testing of the system. I found that when I was at steady state with a valve loading, and then moved the valve 10%, it took 12 - 15 hours for the conductivity to reach steady state again (boiler was base loaded at a constant rate). (I don't remember what the process gain was and my books are all at work) However with a load change, conductivity would change almost instantaneously, and was directly proportional to the load change.

So what I did is base loaded the boiler at 30%, 45%, 60%, and 75% and found the valve position that maintained my target of 1400 micromhos. I took this info, build a table with a function generator block, and used it as a feed-forward input into a PID controller. The PID controller is tuned very slow, but only 'trims' the valve anyway. I have kind of done a poor man's ratio control. It seems to work quite well, but I find it works best when the boiler load is changing consistently. Otherwise I have to wait for the PID to work. I seem to be on target about 85% of the time, but when there is large load swings I thing it would be impossible to stay on track all of the time.

So the feed-forward seems to work well, and the operator still just sees a Manual/Auto station on the Console like any typical controller.

If anyone else has done this type of control before I'd be interested in hearing what was done. Maybe I can tweak a few things, and compare different ways of doing things.

Thanks.

Curtis

Reply to
Curtis
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It would seem that a simple ratio control would have probably been the easiest thing to put in and probably would do the job you wanted. I would suggest that at this time you have spent more funds in devising a method to get around the flowmeter than it would have cost in the first place.

Before we go further, we might want to look at the rest of the system. Are you adding the phosphate and chemicals in proportion to the boilerwater feed rate or in response to something else? Is this metered properly?

Why not add a fast acting PID loop with a limited range? It will drop the conductivity to the proper level right off and close as necessary. Add a little derivative control to speed it up and perhaps a touch of integral control if your PID algorithm isn't incremental. If you are really nervous, put in a ramp to prevent too fast of movement. Consider a faster blowdown in the even that conductivity gets above some particular level. Note that fast acting should not be confused with nervous.

Unless this blowdown valve is large compared to the drum, there isn't going to be a great need of worrying about upsetting the boiler by blowing down properly.

If your controls routinely allow large deviations in the conductivity level, then your operators will turn them off in order to protect the boiler and themselves. Make sure you present the operators with the pertinent data in concise form. They need to know the conductivity and that it is within range. Keep statistics on conductivity deviations before and after your control system is in. Management and operators will be interested in seeing if you are indeed doing better than they were.

Michael

Reply to
Herman Family
2% blowdaown on a boiler of this size & I assume 600# or less based on phosphate treatment, seems low, but your conductivity seems to be in control.

With a slow moving process like this a ratio control with a little PID is perfect, because your 1400 target is not critical. A little high or a little low, no big deal. The conductivity spikes are not that critical, as long as they don't stay too high for too long, it leads to boiler scaling, but you probably already knew that.

I have d>Hey all,

Reply to
me

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