PA FLOW: A CAUTIONARY TALE

This power plant called me to come out because they were having trouble maintaining control as the load was lowered. At higher loads the control of throttle pressure and megawatts seemed fine. But on a load decrease the boiler master and the turbine master would lose the ability to control in a way that looked suspiciously like the phenomena of hysterisis in a valve. Time was spent tuning and de-tuning the boiler and turbine controls. But no matter what we did the results were disappointing. See the figure below.

As can be seen, as the load demand and the load decreased there came a point when the throttle pressure began to rise precipitously. Various tuning was tried but all to no avail.

But then, in conversation, it came to light that this problem had shown itself in late January after testing had been performed at the plant to develop new primary airflow curves for the mills. The old and new PA flow curves are shown below:

The interesting portion of this graph lies between 45% and 60% feeder demand. The old curve, prior to testing, had begun to rise at 45% demand. This curve leveled out at about 92%. The new curve did not begin to rise until the feeder demand reached 60%. The feeder minimum speeds are about 45% at this plant. This means that the motive power provided by changing the PA flow was lacking from 45 to 60% feeder demand. Between these demands the feeder speed was changing, but the PA flow remained constant. This means that the ability to modulate the amount of coal going into the furnace was severely restricted.

We decided to modify the PA flow as shown below:


There is a fine line here between the desire to allow the PA flow to modulate between 45 and 60%, and the need to keep the air close to the amount required to maintain proper velocities in the coal pipes.

The results of this small change were immediate and very dramatic. See the figure below:



As a result of this change the unit megawatt control, and the throttle pressure control became well behaved.