R

a b f s passes only the transient part of the feedforward signal. This eliminates or avoids scaling problems with the summer since the steady-state or dc component of the feedforward signal is blocked. It can be shown that best results are usually achieved if the impulse relay time constant is set equal to the PI controller's reset time.1 The theory is discussed briefly in Chapter 12.

9.7 OVERRIDES FOR COLUMN OVERHEAD SYSTEM

Let us assume we have a conventional column with the following normal controls:

—Condensate receiver level controls top-product drawofF.

—Base level controls bottom-product drawoff.

—Reflux is ratioed to feed.

—Steam is ratioed to feed.

As indicated by Figure 9.9, this column has a horizontal condenser and vertical, cylindrical condensate receiver or reflux drum. We will assume that the level controller is of the PI type with set point at midscale of the level transmitter span and that gain 2 auto overrides are employed. The level overrides then function as described in the following.

FEED FORWARD SIGNAL

FIGURE 9.8

Impulse feed forward with PI controller and overrides

FEED FORWARD SIGNAL

FIGURE 9.8

Impulse feed forward with PI controller and overrides

FIGURE 9.9

Overrides for column overhead system

FIGURE 9.9

Overrides for column overhead system

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