suggest a control scheme for regulating three control valves in Auto


Thread Starter


Problem: We are not able to run the Furnace in Temperature Auto mode(1200 deg C) & we do not know the control philosophy

There are three control valves:
For Air with Flow meters ( 4-20 Ma O/p)
For BF with Flow meters ( 4-20 Ma O/p)
For Propane with Flow meters ( 4-20 Ma O/p)
Two S type T/c are there ( Range 0 to 1400 deg C) (4-20 ma)

The Mixed Gas ( BF & Propane ratio is:
Bf Gas : 94% ,calforic value: 700K cal/m3( 2930 kj/m3)
Propane gas: 6 % ,,calforic value: 22690 k cal/m3

As Blast Furnace gas ( BF Gas) is free & Propane is very costly , intially For statring of furnace Propane is used but when temp is about 700 deg ,less propane & more BF gas is used
Can you help in suggesting a control scheme for regulating three control valves in Auto.
Well, it sounds like you have two operating modes:

1. Startup when < 700 degrees is a regular PID control of temperature using the Propane Valve Only. The Air flow PID setpoint is calculated to give the correct Propane/Air ratio, based on measured propane flow.

2. Operation when > 700 degrees. At this point you want to Maximise Bf Gas. To do this set up the temperature PID control as a split range controller. For a temperature PID output of 0-50%, the BF gas valve is controlled 0-100%. When the output of the temperature control is 50 - 100%, Bf Gas flow is maxed out and Propane gas valve is controlled 0 - 100%. Again, calculate the Air flow setpoint based on the measured flows of Bf & Propane.

There are some complications you will need to deal with ....

1. The tuning of the temperature control will probably vary depending on whether you are using Bf or Propane. You should be able to solving this by using 2 sets of Adaptive tuning values

2. You probably can't just swap from startup mode to operating mode without causing a bump in the temperature. You might need to have some sort of transition logic to ramp down the propane valve & ramp up the Bf gas valve.

You could also use a Cascade arrangement with temperature as the Master PID loop and Bf & Propane Flows as slave PID loops, but I don't think there is any point to this as it complicates the control for little advantage.

My 2c

Another part of the control scheme that is often overlooked when you are transferring from one control source to another is de-coupling.

Your two source of fuel, BF gas and propane have different heating values, pressures, valve types/characteristics, physical line differences, transport times, and time constants.

While it is common to simply have a PID controller for each valve these two controllers need to be decoupled. As the propane valve moves the BF valve needs to be aware of the movement for the process responds. Like wise when the propane valve moves the BF valve needs to be aware of it before the process responds.

By carefully setting up a decoupling network you can greatly improve the robustness and controllability of your system.