Boiler Combustion PLC's/DCS

Turbine speed is usually not an input to the control scheme, particularly if there is a generator attached. The turbine has to spin at a constant speed in order to maintain the desired frequency. Turbine (and therefore boiler) loading is determined by steam flow.

My personal preference is the Foxboro I/A system, but most automated boiler controls have the same design philosophy. The primary feedback parameters are steam pressure, steam temperature and steam flow rate. Most boilers provide superheated steam (e.g., 1275 PSI, 950 °F), so the temperature & pressure are critical parameters. The steam flow signal usually has a higher gain attached to provide quicker response to load changes. Minor inputs (lower gain) are air flow & temperature, fuel flow, feedwater flow & temperature, and excess air.

The control scheme is usually separated into several secondary master control loops: air, fuel, water and steam may all have their own master control, with outputs to the various flow control devices and feedback from the flow monitoring sensors. All of the secondary masters provide inputs and feedback to the Boiler Master, which provides biasing and limits for the secondary loops.

Startups are often manually controlled for several reasons:

• The monitored parameters may be beyond the normal control range (if you have to control pressure 0-1500PSI, it's difficult to get the tight tolerances you want over the entire range).
• The sensors are designed for specific operating conditions, and are inaccurate outside of operating ranges For example, most steam flow detectors assume superheated steam, but until the boiler is up to temperature a lot of the output will be at saturated conditions.
• The rate of change for various parameters is well outside the controllable band. If critical speed of the turbine is below normal operating speed, the operator must quickly accelerate (say from a 2000 RPM point to a 2700 RPM point) to minimize time at or near critical speed, to prevent resonant vibrations from damaging the turbine. Such a change in steam flow would produce a huge feedback signal demanding massive increases in fuel and air to compensate. Since the ramp stops shortly thereafter, the fuel/air response would overshoot, possibly high enough to cause a system fault trip.

I hope this helps. Control system logic is a hefty subject to summarize.