Consider Cascade Control

Such configurations sometimes offer benefits for tubular exchangers.

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

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The cascade configuration in Figure 5b presents the problem that it can’t guarantee the required 5:1 separation of dynamics between the two loops of the cascade. When the condensate valve is used to control steam flow, the dynamics of this flow loop are far slower than that of most flow loops. In addition, the dynamics of the liquid outlet temperature loop for the exchanger are much faster than the dynamics of most temperature loops. So, it’s unlikely that the flow loop will be five times faster than the temperature loop. You’ll be able to tune the flow loop but will experience difficulties in tuning the temperature loop. The only way to get the temperature loop to operate on automatic is to use very conservative settings (low controller gain, long reset time) that make the loop respond very slowly. The simple feedback configuration in Figure 5a will perform better than the temperature-to-flow cascade in Figure 5b.

cascade control figure 6
Figure 6 -- Process operating line:
For temperature-to-flow cascade,
as throughput increases gain or
sensitivity decreases.
Click on illustration for larger image.

 

Variable Throughput
Figure 6 depicts the process operating lines for various throughputs for the control valve on the steam supply. The operating lines are identical except that the minimum flow is zero for the control valve on the condensate.
At a given throughput, the process operating line is essentially straight. The slope of the operating line is the process gain or sensitivity. The exchanger typifies most processes in that as the throughput increases the gain or sensitivity decreases.

Processes with variable throughput raise some tuning issues. You must tune the liquid outlet temperature controller at a low throughput, where the process sensitivity is the highest. However, as the throughput increases, the process sensitivity decreases, which will lead to a slower-responding loop. If a measurement of liquid flow rate is available, you can use scheduled tuning to increase controller gain with liquid flow.

However, if you have a liquid flow-rate measurement, the temperature-to-flow-ratio configuration (Figure 7) is a better approach. It uses a flow-to-flow controller (FFC) or a flow ratio controller. One input is the liquid flow rate, which is referred to as the “wild flow,” because either upstream or downstream processes determine this value. The other input is the steam flow, which is referred to as the “controlled flow,” because the position of the steam control valve governs its value. The set point for the flow-to-flow or ratio controller is the desired ratio of steam flow to liquid flow. The output of the liquid outlet temperature controller is this set point.

For the temperature-to-flow-ratio cascade, the process operating line is a plot of the liquid outlet temperature as a function of the steam-to-liquid flow ratio. This plot will be a straight line whose slope is independent of the throughput. The temperature-to-flow-ratio cascade will respond quickly to changes in liquid flow rate (and also quickly to changes in steam supply pressure). The temperature controller must respond to other disturbances such as changes in the liquid inlet temperature. But, because the operating line is linear and independent of throughput, it will give a consistent response.

cascade control figure 7
Figure 7 -- Better approach:

If a liquid flow-rate measurement
is available, use temperature-to-flow-ratio
cascade.
Click on illustration for larger image.


With the control valve on the steam supply, issues regarding the minimum and maximum possible heat transfer rates also apply to the temperature-to-flow-ratio cascade:

Minimum steam flow. If the shell pressure drops below that required for the condensate to flow out of the exchanger, the cycling situation described in the first article will occur. This is best prevented by measuring shell pressure and incorporating a shell pressure override on the output of the liquid outlet temperature controller.

Maximum steam flow. If the flow-ratio set point is too high, the resulting steam flow will exceed the maximum possible steam flow. This raises the potential for windup. So, incorporate one of the three suggested windup-prevention methods into the configuration of the temperature-to-flow-ratio cascade.

A temperature-to-flow-ratio configuration also is possible where the control valve is on the condensate.

However, the issues are the same as for the temperature-to-flow configuration discussed previously. The steam flow responds slowly to changes in the condensate flow. Consequently, the flow ratio controller will respond slowly. With the valve on the condensate, the ratio configuration can’t respond effectively to rapid changes in the liquid flow. And, as before, the configuration can’t provide the necessary 5:1 separation in dynamics between the temperature loop and the flow ratio loop.


Cecil L. Smith is president of Cecil L. Smith, Inc., Baton Rouge, La. E-mail him at cecilsmith@cox.net. He is the author of “Practical Process Control,” which John Wiley is publishing early this year.

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