[From the ControlGlobal LinkedIn group, Somnath Kelkar of Walchandnagar Industries Ltd. asks, "What is the best method to tune PID values for three-element drum level control in the boiler?"]
It depends on your logic. If you have cascade control, i.e., level control and flow control, then tune it separately. Also, while tuning, make your feed-forward signal zero.
Rahul Singh, Sepcoiii Electric Power
Three-element level control incorporates feed-forward control on steam demand, either through a bias or a ratio. This is necessary because of the complex dynamic behavior of the level when the boiler feed water flow is changed. It's common to see shrink/swell as the header pressure changes, and inverse response if the water is below the bubble point at header pressure. The feed-forward part gives the necessary tight control of the level without the need to put tight tuning in the level PID. Indeed, tight tuning is likely to make the control unstable. I would suggest you first calculate the tuning for averaging level control.
If you're using bias feed-forward, you can apply this directly. If you're using ratio feed-forward, you may have to recalculate Kc to allow for the change of range caused by the ratio. You can then slowly speed up the controller by increasing Kc and reducing Ti, but watch out for oscillatory behavior. Derivative action should not be used in this case.
Myke King, Whitehouse Consulting