Boiler Pressure 2

[TendaiM]

Good Day

. The problem is I have a pressurized boiler and the feed water is pumped into the boiler using a pump which gets its input from a frequency inverter. The frequency inverter gets its input from a controller which monitors the water level in the boiler. The concern however is that the pump is only able to overcome the boiler pressure, hence pump water, into the boiler after a certain frequency, which thus is giving the problem because pump in excessive amounts than is required.

The controller being used is a PI controller. We cannot replace the pump and we need to develop a revised water regulation algorithm. I am working on a model to calculate the expected amount of water to get into the boiler by using a look up table of speed at which pump can pump water versus the boiler pressure. Since I know pump capacity I can then calculate the amount of water which will be pumped in at a frequency at which the pump can overcome the boiler pressure.

Also I can find from a level transmitter the amount of water which has to go into the boiler, hence when water is eventually pumped, I just return the error amount back to the reservoir.

However I feel that they might be a better and cheaper way of control. Maybe introduce a derivative term.

Can you please help me out on this one.

 

[GroovyGuy]

Howdy,
A couple of questions come to mind;
1) Did you set the minimum speed on the VFD high enough such that the pump's discharge pressure > steam drum pressure? (This solution may not work too well if your boiler pressure is all over the map, but it could provide at least a minimum speed for the lowest pressure.)
2) Is there a flow-transmitter on the pump discharge that can prove flow from the pump to the steam drum? (I suspect that a FT does not exist as you would have mentioned it in your description above.)

GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[djs]

Sounds to me like your existing controller is not tuned correctly. Again like GG said, you have to take into account min speeds including the output from the controller. In any case it looks like your existing controller is overshooting which is caused in part by incorrect tuning.

 

[controlsdude]

PID tuning methods, I would google that and then try and find a document that would let you use the method of entering a set of variables into the tuning parameters.

There are also a ton of tuning applications out there that you can even download onto your smart phone or maybe even from the controller manufacturer.

There are multiple ways to tune just pick the one that makes sense to you.

Wiki link for a start

https://en.wikipedia.org/wiki/PID_controller

I know on some/most controllers you can put limits +/- on as in your case the VFD but inside the controller PID that sends the command to the vfd. That way your process is not going haywire but only going between certain rail (+/-) limits that you set in the controller. So that your not going into left field with your pump trying to settle on PI values.

 

[IRstuff]

There is a control systems forum here: forum830 I suggest you re-post there.

TTFN (ta ta for now)
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