Drum Three Element Level Control


Thread Starter


All boilers contain a drum and have a vital role in boiler proper operation.

i have three element level (actual level, input feedwater, output steam from drum) of HRSG boiler. I want to know advantages and disadvantages of this method of level control and if there any improvements of this system?
The advantages of 3 element cascade level control:

-feedwater flow loop compensates for variation in BFW flow and supply pressure

-feedwater flow loop means that level control loop can be tuned for drum level control and does not have to be as fast acting with the flow loop taking care of feedwater variations

-feedforward from steam flow provides quicker action on load changes

I have found that adaptive gain on the level PID allows for less gain in a narrow band around setpoint but if level starts to deviate then the loop gain increases.


I don't see any demerits of three-element level control. The loop is inherently stable. Which means that even an untuned loop will be able to control the level. If fine-tuned, the control loop give very good response and enables stable control of level and feedwater flow.

At different times, I have transferred three such loops to DCS from conventional single loop controllers and implemented the level loop of a new waste-heat boiler on DCS. These loops were the easiest to start functioning, besides steam temperature control and fuel control loops.

As a further improvement, we implemented a density correction function using a characterizer block which estimated the density of water in steam drum based on its pressure and used that density to correct the level reported by transmitter.

This correction allowed the level controller to read correct level, even during startup and shutdown of the boiler, when the pressure of steam drum is largely deviated from its normal operating pressure and due to which, the level reported by a DP level transmitter is pretty much wrong.