Control Level with a Cascade

This requires different thinking about master and slave loops.

By Cecil L. Smith, Cecil L. Smith, Inc.

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Figure 3 illustrates the effect of switching the filters for a varying feed rate and noise on the level measurement. On each switch of the filters, the discharge flow sharply rises. This is apparent in the trend of the discharge flow but not in the plots of vessel level or level controller output. Variability of the feed to the vessel causes comparable changes in the magnitude of discharge flow during normal operations. However, the change on switching the filters is sudden. For most unit operations, abrupt changes to the feed are major upsets; the same change over a longer time period has fewer consequences.

Figure 4 presents the level-to-flow cascade control configuration. The cascade consists of two loops:

1. Inner loop. The flow controller positions the discharge control valve so discharge flow is at or near the set point provided by the level controller.

2. Outer loop. The vessel level controller provides the set point to the discharge flow controller.

Varying Feed Rate Figure 3. Higher discharge flow after switching filters isn't apparent from vessel level.  
Varying Feed Rate
Figure 3. Higher discharge flow after switching filters isn't apparent from vessel level.

Applications of cascade require more attention to tuning. When the inner loop is a flow controller, you can use typical flow-controller tuning, such as a controller gain of 0.2 %/% and a reset time of 3 sec. You must begin by verifying these coefficients function properly. When the outer loop is very slow (as is the case for the level controller), tightly tuning the inner loop isn't necessary.

For the level controller, the tuning for the simple feedback scheme was a controller gain of 0.4 %/% and a reset time of 120 min (as detailed in previous parts of the series). Adding the flow controller only nominally affects the dynamics of the process, so a reset time of 120 min remains suitable in the cascade configuration.

However, the controller gain for simple feedback isn't necessarily still appropriate. In the cascade configuration, discharge flow varies linearly with level controller output (which is the discharge flow set point). But in the simple feedback scheme, the relationship between discharge flow and level controller output (the discharge control valve opening) is more complex and rarely quantified. Depending on valve sizing and flow system characteristics, the control valve (and, consequently, the level controller output) frequently operates over about half the full range. For a properly sized equal-percentage valve, the range is 50–100%. For a grossly oversized equal-percentage valve, a range of 0–50% is more likely. Unfortunately, anything is possible. For cascade, a higher controller gain probably is required, but not necessarily one twice as high.

 Level-to-Flow Cascade  Figure 4. Level controller in outer loop provides set point for flow controller in inner loop.
Level-to-Flow Cascade
Figure 4. Level controller in outer loop provides set point for flow controller in inner loop.

Figure 5 presents the performance of the cascade configuration for a varying feed rate and noise on the level measurement. A controller gain of 0.4 %/% leads to occasional trips on low level, making a gain of 0.6 %/% more appropriate.

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