By David W. Spitzer
AN ANALYSIS of boiler level controls shows the importance of the interrelationships between the process, measurement instruments, and control strategy used to achieve the control objective. In short, the boiler burns fuel to convert water into steam, and makeup water is added to replace the steam (water) that leaves it.
Operating at a high water level in the boiler can carryover water into the steam header. Operating the water level too low can damage the boiler. Neither of these conditions is desirable, so the boiler should be operated so its water level is such that neither the high nor low-level switches shut it down. The overall control objective is to replace the water that leaves the boiler in such a way that its water level remains constant.
A single-element boiler control strategy (See Figure 1 below) achieves this objective by measuring boiler level, and using a controller to manipulate the makeup water feed valve. Consequently, increasing steam demand causes more steam to leave the boiler, which lowers the water level in the boiler. The controller senses the low level (feedback), and reacts by increasing the makeup water flow to increase the level to its setpoint.
FIGURE 1: SINGLE-ELEMENT BOILER LEVEL CONTROL
A single-element strategy keeps a boiler’s water level constant by measuring level, and using a controller to manipulate the makeup water feed valve.
The nature of this control strategy is such that changes in the makeup water flow are initiated when the boiler level isn’t at its setpoint. Because makeup water flow will vary with demand and firing rate, the boiler level will deviate from its setpoint during normal operation. This strategy may be adequate for many boilers, especially in applications where steam demand is constant or varies slowly.
Complicating the issue is the transient inverse response of the water level in the boiler. When more fuel is burned, the water’s temperature increases and it expands, thereby raising its level. On the other hand, more water is boiled when more fuel is burned, thereby lowering the level. In actual operation, the boiler level initially increases in response to expansion followed by a drop in level due to the loss of water. Therefore, the level controller can react incorrectly to a fuel increase by initially reducing makeup water flow in response to the increasing level measurement. More sophisticated controls may be necessary to compensate for this effect.
For instance, implementing a two-element boiler control strategy by adding a cascade-makeup, water-flow control loop to the boiler level control (See Figure 2 below) can improve control by reducing level variations from setpoint and makeup water flow fluctuation. This strategy cuts the effect of disturbances in the flow loop, and allows the level loop to be tuned for better performance.
FIGURE 2: TWO-ELEMENT BOILER LEVEL CONTROL
A two-element strategy adds a cascade-makeup, water-flow control loop to improve control by reducing level variations from setpoint and makeup water flow fluctuation. This cuts the effect of disturbances in the flow loop, and allows the level loop to be tuned for better performance.
To reduce the effect of changing steam demand on boiler level, a three-element boiler control strategy can be used (See Figure 3 below). In this strategy, steam flow from the boiler is measured, and used to adjust the output of the level controller, so that a change in steam flow results in a like change in makeup water flow. This control strategy operates as feed-forward because it manipulates the makeup flow before the boiler level has time to change.
FIGURE 3: THREE-ELEMENT BOILER LEVEL CONTROL
In three-element boiler control, steam flow from the boiler is measured, and used to adjust the output of the level controller, so a change in steam flow results in a like change in makeup water flow.
Finally, four-element boiler level control (See Figure 4 below) can be implemented by taking boiler blowdown into account.
FIGURE 4: FOUR-ELEMENT BOILER LEVEL CONTROL
Four-element boiler level control is implemented by taking boiler blowdown into account.
Greg Shinskey, a process control consultant from North Sandwich, N.H., says, “The magnitude of shrink-swell (inverse response) varies with the type of steam generator. In nuclear power plants, pressurized water reactors exhibit much more shrink-swell compared to boiling water reactors where large volumes of water surround the nuclear fuel.” He suggests using a non-linear filter in the controller to let the feed-forward do most of the work in pressurized water reactors (see “Taming the Shrink-Swell Dragon,” March ’04).
Fossil fuel boiler designs are more similar, so shrink-swell varies with the density difference between the water and steam. In higher pressure boilers, the density difference is less than in lower pressure boilers. Therefore, boilers operating at higher pressures generally exhibit less shrink-swell than lower pressure ones.