Steam boilers are used to heat up water in order to produce steam. Steam is widely used in the process industry for heating purposes. In this particular application, the steam was used to dry sludge in an installation in the Netherlands.

The figure above shows a P&ID of the relevant installation. The dryer uses steam to dry products. The amount of steam (further referred to as Disturbance Variable, DV) is controlled by another control system that is not considered here, but can be measured. The steam that passes through the dryer, is produced from two heat sources: gas from an electricity generating turbine, and, if those gasses are insufficient, 2 gas fired burners. These heat sources are controlled by valves, further referred to as MV1 and MV2, respectively. These valves need to maintain the boiler pressure at setpoint. Note that MV1 is the bypass valve opening, so that most heat is used for steam when MV1 = 0.

 

Description of the problem

After load changes, for instance when switching on/off a dryer (i.e. DV changes from 0 to 100% or back). The steam pressure would show large deviations from target and it would oscillate for a long time before it damped out. Figure 1 shows an example of these oscillations. The period of oscillations was almost one hour. Frequently, the deviations would exceed 20 bar, causing the complete installation to trip.

 
 

Figure 1: excessive pressure oscillations after a load change.

 

Figure 2 shows a block-diagram of the old old boiler pressure controller (PC1). It consisted of a PID controller, followed by a split range block. The controller was tuned (wrongly) with fixed settings, since the pressure responds different to MV1 than MV2.

 
 

Figure 2: block diagram of the old control system

 

New control system

Figure 3 shows a blockdiagram of the new control system. Both the PID controller and the feedforward controller (FF) had to be tuned optimally. Since MV1 and MV2 can have very different effects on the pressure, we applied the PID Tuner in such a way that we could perform step tests with both MV1 and MV2. Indeed, the step responses differed considerably.

As a result, the settings of the PID and FF had to be gain scheduled (one set for MV1, and one for MV2). We used the PID Tuner to compute the PID settings (proportional gain, integral time, differential time), and the FF gain.
 
 

Figure 3: new control system

 

The control system was implemented by CONET in a Siemens PCS7 computer control system. They prepared the implementation using the SIMATIC PCS 7. CONET used a simulation model of the relevant process (digital Twin) that acted as an OPC client and communicated via the Siemens OPC server with the SIMATIC PCS7, (just like the PID Tuner).

Figure 4 shows simulation results with the new control system. Initially, the burners are active, since the bypass valve is already completely closed. After the steam load drops at t= 400 seconds, the burners are switched off and the bypass valve is used to control pressure. Clearly, the pressure variations, after a load change, are small, and dampen out quickly.

 
 

Figure 4: simulation with new control system (horizontal axis shows time in seconds)