Drum water level contol


Thread Starter

Mario Roberto

I would like receive information and experiences on drum water level control (single or multi-component PID control controller in cascade).
Application is: steel making plant steam production by fumes cooling.
Responding to Mario Roberto's query:

Be careful about naming heat recovery boiler a "flue gas cooler". Plant I worked for was heavily fined when the country in which it was located ruled that it was indeed a Heat Recovery Unit and not a gas cooler.

As to PID info, search List archive for Steam Drum Level topic. I can provide study showing the importance of drum size as related to transient response (load swings) and the impact on PID performance.

Phil Corso, PE
(Boca Raton, FL)
If you need a precise control of the water level I would recommend using two cascaded loops: the main loop produces a signal equal to the water flow demand (on the basis of the level measurement), which serves as a setpoint for the second loop that controls the water flow into the drum. The process you control has an integrating property. For that reason you cannot apply much I-action in the main loop. To eliminate the static error you need to apply a feedforward signal equal to the mass steam flow out of the drum so that the input signal for the slave loop is the sum of the output of the first controller and the above feedforward signal.

Dr. Igor Boiko
Consulting in Control is available
(including modeling, simulations and control design)
[email protected]
Tel: 1-403-294-2745

Michael Blaschke

A good description of the single-, two- and three-element controls is in the ISA standard ANSI/ISA-77.42.01-1999 (available at "http://www.isa.org":http://www.isa.org ). It also includes a good description of when to use each and their limitations, as well as a SAMA diagram for each.

Single element (drum level control only) is generally used only when the steam flow rate of change is minimal and feedwater supply pressure is essentially constant. Proportional-plus-integral-action is generally used. This minimal control can generally only be applied where steam flow is constant or at low loads when steam flow measurements are not available or inaccurate. It is inadequate because the shrink and swell characteristics of the boiler produce level changes during boiler load changes in a direction opposite to that to which level would change under steady state conditions, and results in incorrect control action anytime the boiler load changes. However, this may be tolerated if the boiler is small, has relatively large water storage and the load changes slowly. Derivative-action control has been used to try to compensate for shrink and swell, but it must be negative (increasing level decreases flow).

Two-element control (drum level control with steam flow feed forward) is the minimum needed for a variable steam-flow application. The output from the proportional-action level controller is summed with the steam flow signal to determine the demand to the final control element. This type of control is not used much for new installations, as it is only partially effective in correction for drum shrink and swell and because it depends fairly much on final control element linearity and repeatablility throughout the load range. This type of control requires careful gain and bias adjustment so that the steam flow and level signals exactly offset each other. Proportional-only control action is generally used, because the "wrong way" action due to shrink and swell makes any integral action add instability.

Three-element control (drum level control with steam flow and feedwater flow control) is now generally used for variable steam-flow applications. It corrects the problems with two-element control by replacing the open-loop characteristic of the feedwater valve with closed-loop feedback control of the feedwater. There are several ways to arrange a three-element control system, but the most common uses the output from the proportional-action level controller summed with the steam flow signal to become the feed forward setpoint to the secondary feedwater flow controller. Essentially, the feedwater flow follows steam flow, and uses the deviation in level as a resetting action to bring the required water inventory back to balance. Proportional-only control action is generally used because the "wrong way" action due to shrink and swell, any integral actions adds instability.

Michael Blaschke