Three-element control is similar to the two-element system, except that the water flow loop is closed rather than being left open. In this way, pressure disturbances that would affect feedwater flow are handled immediately by the fast response of the feedwater flow loop. There are several ways of connecting a three-element feedwater system. Figure 2 below illustrates the most common way of connecting this system.
In addition to the three primary control variables (three elements) -- drum level, steam flow, and feedwater flow -- drum vapor-space pressure can be utilized to compensate for density changes. The pressure is passed through a calculator, DY-118 in Figure 2 (above), that calculates a multplier to be applied to the raw level signal by LY-108. The value of the multiplier is obtained from the density change versus pressure for saturated steam, taken from the steam tables.
Gain adjustments of the three-element feedwater system are made by first determining the relative gains between level and flow loops. By observing a change in boiler load one can note the particular boiler “swell” characteristics. Maximum system stability results when the negative effect of swell equals the positive effect of flow. For example: If a 20 % of maximum flow change produces a 2.4 PSI (0.16 bar) change in a pneumatic flow transmitter output and this flow change also produces a 3 in. (75 mm) swell on a 30 in. (750 mm) range transmitter or a 1.2 PSI (0.08 bar) transmitter output change, then the gain of the level loop should be double the gain on the flow loop.
A feedforward variation is recommended by Shinskey to maintain a steam-water balance, reducing the influence of shrink-swell and inverse-response phenomena. The system shown in Figure 3 (below) causes feed-water flow to match steam flow in the absence of action by the level controller. The two flowmeters have identical ranges, and their signals are subtracted. If the two flow rates are identical, the subtractor sends a 50 % signal to the flow-difference controller. An increase in steam flow will call for an equal increase in feedwater flow to return the difference signal to 50 %.
Errors in the flowmeters and the withdrawal of perhaps 2.5 % water as “blowdown” (which is not converted to steam) will prevent the two flow signals from being identical. Any error in the steam-water balance will cause the level to fall or rise. Therefore, the level controller’s output readjusts the set point of the flow-difference controller to maintain the steady-state balance.
The system assumes the use of orifice-type flow sensors and does not use square-root extractors, because the period of oscillation and dynamic gain of a two-capacity level process varies directly with flow. This way the gain of the feedwater control loop is automatically compensate for the variations in process gain.
Figure 3 (above) also shows an external feedback from the flow-difference measurement, which is applied to the level controller. This preconditions the level controller during startup and when the feedwater controller (DFIC-109 ) is in manual it protects the level controller from reset windup. Otherwise, an increase in steam or blowdown flow will immediately increase the feedwater flow, without depending on the level controller. This means that the feedback portion of the loop (LIC-108) will need only to trim the DFIC-109 set point to correct for flow metering errors.
In this configuration, the level upsets are minimized, because the role of the feedback portion of the loop is reduced from manipulating feedwater flow across its entire range to adjusting it for flow meter errors only. In this configuration, controller mode settings are not critical and inverse response caused by shrink, swell are also reduced.
Another variation of steam drum level control proposed by Shinskey to overcome shrink-swell effects involves the use of proportional feedback from a level measurement that detects a wide range (LT-108b). On large boilers, primary drum level control is often accomplished with a narrow-range transmitter (LT-108a) for more accurate control, with a wide-range transmitter present to respond to large excursions and handle alarm trips outside the narrow control range.
The wide-range transmitter can be utilized to provide proportional feedback to sum with the output of the narrow-range level control (LC-108). When an increase in load, causes the water to swell, the wide-range measurement, which is sensing much more of the total water inventory, will tend to give a lower measurement, while the narrow-range instrument may indicate a rising level. Used with the proper filtering and tuning, the wide range signal can be used to offset the narrow-range control loop's inverse response.
Bela Liptak, CONTROL Columnist