Superheat Temperature Control
Superheated steam temperature control is very straightforward. Normally, steam leaves the drum and travels through a primary superheater(s) before entering the desuperheater, where attemperating water is mixed with the steam to modulate its temperature before it enters the next superheater section. After the steam passes through that superheater, the outlet temperature is measured.
If the inlet temperature to the superheater is a measured variable, the preferred method of control is a cascaded loop. In this case the outer controller uses the superheater outlet temperature as the process variable. The output from the outer controller is the inlet temperature set point. The output from the inner controller is spray water demand. If the superheater outlet temperature is the only available measurement, then we are forced to use a single-element control loop. In either case, it is important that the controls are equipped with a feedforward signal.
A variety of signals can be used for the superheater temperature control feedforward. Usually, the boiler demand is a good starting point for the feedforward because this signal anticipates the measured temperature signals. My experience is that the boiler demand usually has a well-defined relationship with the superheater temperature.
Other measured variables are available to supply the feedforward signal. Throttle pressure is usually used in tandem with the throttle pressure set point as an indication of over- or underfiring of the boiler, but throttle pressure is transient in nature. Airflow versus fuel flow or steam flow may be used in the same way. The ratio of fuel flow to the top mill versus the other mills is a good indicator of the changing dynamics in the boiler, especially if the boiler is large and has many burner levels. In this case it is a good rule of thumb to think of the top elevations as affecting temperature more than pressure, and the lower elevations as affecting steam pressure more than temperature. Finally, the reheater temperature control affects the superheater temperature to a greater or lesser degree, depending on the type of boiler manufacturer and its method of control.
The feedforward signal development may include both static and dynamic functionality. The static cases are basically a function of the variable that you are using. Dynamic feedforward refers to a derivative kick based on the movement of the chosen variable. For example, the ratio of airflow to steam flow might be used as an indicator of the boiler’s movement up or down, and the feedforward then can be manipulated accordingly.
Patience is a virtue when tuning these feedforwards, because steam temperature processes may have long time constants.
Reheat Temperature Control
It is an interesting fact that superheater spray adds to the efficiency of a unit but reheater spray flow decreases the unit’s efficiency. Maximum boiler efficiency is always the goal, so boiler manufacturers have developed alternative approaches to control reheat steam temperature.
Babcock & Wilcox uses a gas recirculation fan to move flue gas from the outlet of the boiler back into the furnace, either directly or through the secondary air wind box. More recirculation yields higher furnace temperature and, therefore, higher steam temperatures. Combustion Engineering, now Alstom Power, is famous for its tangential, tilting burner design that can move the furnace fireball vertically to control steam temperatures. Foster Wheeler boilers use a superheat/reheat gas bypass damper to shunt flue gas to the appropriate gas pass ducts to control reheat temperature. Spray valves are also used in each design, although the reheat temperature set point to the spray valve controller is usually several degrees higher to keep the reheater spray to a minimum.
The setup for the reheat temperature spray valve control is the same as that for the superheat temperature control: two valves (modulating valve and block valve), an attemperator or desuperheater, and a reheater section. However, reheat steam temperature control is not normally a cascaded loop. Assuming that the primary method of control (gas recirculating fan, tilting burners, or bypass damper) is operating, the sprays are held in reserve. The operator-adjustable set point is used directly by the primary control mechanism. A sliding bias is added to the set point before it is sent to the spray controller. Usually, the spray set point is set higher than the primary reheat temperature control set point before the sprays are enabled, to reduce the reheater spray flow.
Part II will look at fuel flow control, pulverizer air control, and overall plant control options such as boiler- and turbine-following modes and plant coordinated control.
--Tim Leopold (tim.leopold@hotmail.com) is a field service engineer with ABB and has more than 20 years' experience tuning controls on power plants around the world. His book You Can Tune a Boiler But You Can't Tuna Fish is slated for publication in March.
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