December 2002

Instrumentation/Control

Consider process control a specialty within chemical engineering – Part 3

By recognizing which process conditions are limiting, designs can avoid pitfalls in control strategies

Smith, C. L., Cecil L. Smith, Inc.

When the engineering process gain is zero, changes in the flow through the control valve have no effect on the controlled variable. Controllers simply cannot function under such conditions. In practice, the gain usually approaches zero but is not truly zero. But even this portends problems for the controller. Within the process, the relationship between the controlled variable and the flow through the control valve depends on certain mechanisms. Under normal process operating conditions, a certain mechanism is the dominant or controlling mechanism. As process conditions change, several possibilities may arise: The controlling mechanism becomes less effective.

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