Consider State-Based Control

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?

If your facility isn't using state-based control (SBC) then you probably don't have acceptable answers for most of these questions. Even worse, your business is missing an opportunity to achieve potentially large improvements in OEE and return on net assets (RONA).

SBC is an automation design philosophy that recognizes processes have identifiable states and the control system nearly always can be the best operator to keep the plant running within those states and during state transitions. It isn't a new concept — it's long been implemented for batch production. Indeed, the ability to use logic to sequentially control a complete series of plant operations has been available in many process controllers for several decades. At the core, that's what underlies SBC. Dow Chemical has applied SBC techniques for many years and has documented a two times better RONA for processes that use SBC versus those that rely on conventional automation designs (Figure 2).

However, in most automation systems designing, coding and maintaining this type of logic has been costly as each piece of logic usually was a unique code block with nearly no reuse of code between applications.