Cascade, Feed Forward and
Boiler Level Control
By Allen D. Houtz
ABSTRACT: One common application of cascade control combined with feed forward control is in level control systems for boiler steam drums. The control strategies now used in modern industrial boiler systems had their beginnings on shipboard steam propulsion boilers. When boilers operated at low pressure, it was reasonably inexpensive to make the steam drum large. In a large drum, liquid level moves relatively slowly in response to disturbances (it has a long time constant). Therefore, manual or automatic adjustment of the feedwater valve in response to liquid level variations was an effective control strategy. But as boiler operating pressures have increased over the years, the cost of building and installing large steam drums forced the reduction of the drum size for a given steam production capacity. The consequence of smaller drum size is an attendant reduction in process time constants, or the speed with which important process variables can change. Smaller time constants mean upsets must be addressed more quickly, and this has led to the development of increasingly sophisticated control strategies.
Dynamic Shrink/Swell and Boiler Level Control
By Allen D. Houtz
ABSTRACT: As high pressure boilers ramp up to operating temperature and pressure, the volume of a given amount of saturated water in the drum can expand by as much as 30%. This natural expansion of the water volume during start-up is not dynamic shrink/swell as discussed later in this article, though it does provide its own unique control challenges. The expansion (or more precisely, decrease in density) of water during start-up of the boiler poses a problem if a differential pressure or displacer instrument is used for level measurement. Such a level transmitter calibrated for saturated water service at say, 600 psig, will indicate higher than the true level when the drum is filled with relatively cool boiler feedwater at a low start-up pressure. If left uncompensated at low pressure conditions, the "higher than true level" indication will cause the controller to maintain a lower than desired liquid level in the drum during the start-up period. If the low level trip device is actually sensitive to the interface (e.g. conductance probes or float switches), troublesome low level trip events become very likely during start-up.