Flow loops are so fast that even conservative tuning provides acceptable performance, with little or no incentive to improve performance of the loop. However, this isn’t necessarily the case for the temperature-to-flow cascade. Because the liquid outlet temperature loop is relatively fast for temperature loops, a conservatively tuned steam flow loop might not be five times faster than the temperature loop. So, if you experience difficulties tuning the temperature loop, consider more-aggressive tuning in the steam flow controller.
For the primary controlled variable, the process operating line for cascade control often markedly differs from that for simple feedback control. Figure 2 shows the operating line for liquid outlet temperature controlled via a temperature-to-flow cascade. Because the output of this temperature controller is the set point for the steam flow controller, the operating line presents the liquid outlet temperature as a function of steam flow.
The operating line is essentially linear. Furthermore, it’s unaffected by the inherent characteristics (linear, equal-percentage or other) of the steam valve. The flow controller has to contend with all issues associated with the valve. In effect, the flow controller isolates the temperature controller from such issues.
The PID controller is a linear controller. With a linear process operating line, the process is linear (at least for a given throughput). Consequently, a given value for the controller gain will deliver consistent performance for all steam flows. In some exchanger applications (but not most), this could be a major advantage, making the outer loop far easier to tune in the cascade configuration that the corresponding loop in the simple feedback configuration.
Minimum Heat Transfer Rate
The operating line in Figure 2 suggests that the minimum steam flow is approximately 31 lb/min. This corresponds to a steam valve position of 29% (equal-percentage valve), a liquid outlet temperature of 200°F and atmospheric pressure in the shell of the exchanger.
The cascade configuration can suffer the same cycling described in the first article in this series for the simple feedback configuration. This phenomenon reflects the characteristics of the process. The process just isn’t capable of smooth operation at lower heat-transfer rates. Control system sophistication can’t address this problem nor can it be “tuned out.” However, it’s reasonable to expect that the controls recognize the limits of process operation and not attempt to operate beyond those limits.
Figure 3 -- Shell pressure override:
Based on the operating line in Figure 2, the temperature loop shouldn’t specify a steam flow set point less than 31 lb/min. In digital systems you easily can place limits on the set point for the inner loop of a cascade; such systems also will address the windup issues in the outer loop of the cascade. The shortcoming of this approach is that the value of 31 lb/min for the minimum steam flow isn’t precise and also depends upon process variables such as the throughput.
The first article in this series proposed a shell pressure override loop for the simple feedback configuration. A temperature-to-flow cascade also can incorporate a shell pressure override (Figure 3) — the output of the high select serves as the set point of the steam flow controller. Otherwise, the issues are the same. Implementing this configuration requires an additional measurement device for shell pressure. For exchangers that discharge condensate to a drain, the set point for the shell pressure controller would be slightly above atmospheric. For exchangers that discharge into a condensate return system, the set point must slightly exceed the pressure required to force the condensate back to the boiler house.