Control Notes

Steam temperature is one of the most challenging control loops in a power plant boiler because it is highly nonlinear and has a long dead time and time lag. Adding to the challenge, steam temperature is affected by boiler load, rate of change of boiler load, air flow rate, the combination of burners in service, and the amount of soot on the boiler tubes.

After separation from the boiler water in the drum, the steam is superheated to improve the thermal efficiency of the boiler-turbine unit. Modern boilers raise the steam temperature to around 1000F (538C), which approaches the creep (slow deformation) point of the steel making up the superheater tubing. Steam temperatures above this level, even for brief periods of time, can shorten the usable life of the boiler. Keeping steam temperature constant is also important for minimizing thermal stresses on the boiler and turbine.

Steam temperature is normally controlled by spraying water into the steam between the first and second-stage superheater to cool it down. Water injection is done in a device called an attemperator or desuperheater. The spray water comes from either an intermediate stage of the boiler feedwater pump (for reheater spray) or from the pump discharge (for superheater spray). Other methods of steam temperature control include flue gas recirculation, flue gas bypass, and tilting the angle at which the burners fire into the furnace. This discussion will focus on steam temperature control through attemperation. The designs discussed here will apply to the reheater and superheater, but only the superheater will be mentioned for simplicity.

BASIC FEEDBACK CONTROL

The simplest method for controlling steam temperature is by measuring the steam temperature at the point it exits the boiler, and changing the spray water valve position to correct deviations from the steam temperature set point (Figure 1). This control loop should be tuned for the fastest possible response without overshoot, but even then the loop will respond relatively slowly due to the long dead time and time lag of the superheater.

Figure 1. Simple Steam Temperature Control

CASCADED STEAM TEMPERATURE CONTROL

Because of the slow response of the main steam temperature control loop, improved disturbance rejection can be achieved by implementing a secondary (inner) control loop at the desuperheater. This loop measures the desuperheater outlet temperature and manipulates the control valve position to match the desuperheater outlet temperature to its set point coming from the main steam temperature controller (Figure 2). This arrangement is called cascade control.

Figure 2. Cascaded Steam Temperature Controls

The spray water comes from upstream of the feedwater control valves, and changes in feedwater control valve position will cause changes in spray water pressure, and therefore disturb the spray water flow rate. The desuperheater outlet temperature control loop will provide a gradual recovery when this happens. If the spray water flow rate to the attemperator is measured, a flow control loop can be implemented as a tertiary inner loop to provide very fast disturbance rejection. However, in many cases spray water flow rate is not measured at the individual attemperators and this flow loop cannot be implemented.

GAIN SCHEDULING

The process dead time of the superheater increases with a decrease in boiler load because of the slower rate of steam flow at lower loads. This will have a negative impact on the stability of the main steam temperature control loop unless gain scheduling is implemented. Step tests need to be done at low, medium, and high boiler loads, and optimal controller settings calculated at each load level. A gain scheduler should be implemented to adjust the controller settings according to unit load. Because of the changing dead time and lag of the superheater, the integral and derivative times must be scheduled in addition to the controller gain.

The gain of the desuperheater outlet temperature loop will be affected greatly by steam flow rate. Changes in steam flow rate will affect the amount of cooling obtained from a given spray water flow rate. Less cooling will occur at high steam flow rates. In addition, at high loads the pressure differential between the feedwater pump discharge and steam pressure will be lower, reducing the spray flow rate for a given spray valve position (assuming the absence of a flow control loop on the desuperheater spray flow). To compensate for these nonlinear behavior, controller gain scheduling should be implemented on the desuperheater outlet temperature loop too. Fugure 3 shows the basic design of the steam temperature controller gain scheduler (cascaded controller is not shown for clarity). Similar to tuning the main steam temperature control loop, step tests must be done at low, medium, and high boiler loads to design the gain scheduler.

Figure 3. Steam Temperature Controller Gain Scheduling

FEEDFORWARD CONTROL

During boiler load ramps in turbine-following mode, the firing rate is changed first, followed by a change in steam flow rate a while later. With the increase in steam flow rate lagging behind fuel flow rate, the additional heat in the furnace can lead to large deviations in steam temperature. To compensate for this, a feedforward control signal from the boiler master to the steam temperature controller can be implemented.

The feedforward can use the rate of change in fuel flow or one of several other derived measurements to bias the steam temperature controller’s output. In essence, when boiler load is increasing, the spray water flow rate will be increased to counter the excess heat being transferred to the steam, and vice versa. The feedforward can be calibrated by measuring the extent of steam temperature deviation during load ramps.

Stay tuned!
Jacques Smuts – Author of the book Process Control for Practitioners