Consider State-Based Control

This under-utilized approach can enhance operations and the bottom line.

By David Huffman, ABB Inc.

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Plants often overlook their automation system as a resource for improving overall equipment effectiveness (OEE). They generally treat the system only as a tool for process operators to interact with valves, motors, control loops and other devices. However, a different approach can lead to significant progress in the quest for operational excellence.

Using a conventional control design, when changes to process conditions are needed an operator interacts with a series of faceplates to manipulate necessary control devices to a set of conditions dictated by immediate manufacturing requirements. This might be as simple as switching to a spare pump with only a few actions -- 24 are typical (Figure 1) -- or as complex as starting up or shutting down the entire process with hundreds or even thousands of actions.

If you operate a continuous process, chances are this describes how you use your automation system.

What's often misunderstood about continuous or even semi-continuous operations is that they never completely stay at one set of conditions. All processes operate in "states." Some processes have only a few but most have a wide variety of state conditions. Examples of states that generally apply include:

Maintenance. This often isn't considered a processing state but frequently you must monitor and alarm some measurements or some equipment remains running.

Process wait. Equipment isn't currently involved in production but is at or near operating temperatures and pressures and needs monitoring and alarming.

Starting. You are transitioning from process wait to steady-state operation.

Running. This is the normal state of the process. It frequently involves many states, defined by product grades, production rates or a variety of other factors.

Shutting down. The process moves from a running state to process wait conditions.

Many other states might apply to your process.

The State of Most Plants
Consider the number of actions that your operators must flawlessly execute to move between any of these states. How well do individual operators perform those actions? How much training do your operators need to learn and carry out all the individual actions? How often are these procedures executed? How frequently are they updated? How much effort is required to provide the appropriate level of detail for those procedures? If you're facing a pending shutdown, how many operating team members have been on site long enough to have taken part in the previous cycle, perhaps four or five years ago? Does process productivity decline with the loss of a senior shift member? Do some operators control the process better than others?