Consider Cascade Control

Such configurations sometimes offer benefits for tubular exchangers.

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

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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.

cascade control figure 1
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:

cascade control figure 2
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.

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