Consider Cascade Control

Proper plant operation often depends upon effective control of steam-heated shell-and-tube exchangers. Two previous articles (see sidebar) examined control of the liquid outlet temperature using a control valve on either the steam supply or the condensate return. This article will focus on cascade control configurations for these two cases. A future article will look at liquid bypass arrangements.

Figure 1 depicts simple feedback and temperature-to-flow cascade configurations for controlling liquid outlet temperature using a control valve on the steam supply. With the advent of digital systems, the use of cascade configurations significantly increased. The only additional hardware for implementing the cascade configuration is a measurement device for steam flow. All other components are implemented in software.
To distinguish between the two loops, we’ll use the terms “inner” and “outer.” For the temperature-to-flow cascade, the flow loop is the inner (or “slave”) loop and the temperature loop is the outer (or “master”) loop.

Figure 1 --Valve on steam supply: Two approaches can provide liquid outlet temperature control via a valve on the steam supply. Click on illustration for larger image.

Cascade configurations usually are justified on improved dynamic response to disturbances to the inner loop. Suppose significant variations occur in the steam supply pressure. For the simple feedback configuration (Figure 1a), the temperature controller wouldn’t respond until the change in steam supply pressure affects liquid outlet temperature. But for the cascade configuration (Figure 1b), the flow controller responds when the change in steam supply pressure affects steam flow. The cascade configuration would respond to this disturbance far more rapidly than the simple feedback configuration.

For a cascade configuration to properly function, the inner loop must be faster than the outer loop — the general guideline is at least five times faster. For most temperature-to-flow cascades, the dynamic separation between the loops would exceed 5:1.

However, as temperature loops go, the liquid outlet temperature loop responds quite quickly. Furthermore, a common practice is to very conservatively tune flow loops. Typical suggestions for the tuning coefficients include the following:

Figure 2 -- Process operating line: Temperature-to-flow cascade control provides an essentially linear operating line. Click on illustration for larger image.

1. controller gain 0.1%/%; reset time 2 sec.;
2. controller gain 0.2%/%; reset time 3 sec.; and
3. controller gain 0.3%/%; reset time 5 sec.

In practice, these settings give about the same performance. They’re also very conservative. If the flow controller doesn’t function with one of these settings, the first step should be to understand why.

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.