We have 2 units of steam boiler, with 18 ton/hr each, but operate in modulating mode, i.e. steam is produced on demand. The steam demand normally 10 t/hr to 24 t/hr, most of the time steam demand was so surge that it creates turbulent and crazy water fluctuations in the boiler.

We have installed 3-element control for our steam boiler to replace the single element, and it seemed to work fine at first. However, because the water level fluctuates rapidly, so does the control valve positioning, therefore the boiler is subjected to high water and low water condition. Are there any controls that I can add in to dampen the effect of water fluctuation so that the control valve becomes more stable?

 

There are two things I would do:

1. Study the steam demand control. Somewhere there should be a steam pressure control loop. You may have a poorly designed or tuned control loop. It is difficult to design a closed loop control where you have a setpoint for steam demand and measurement of steam flow used as the feedback variable. Most attempts that I have seen have been failures.

2. If you are convinced the steam demand controller is stable(look at the fuel flow trend), then I would see if the unit is exhibiting shrink-swell of the drum level.If it is, then I would place a first order filter in series with the steam flow signal being used for the three-element control. I would set the time constant around 30 seconds(0.5 minute). This usually is benefical for boilers with shrink-swell problems.