A recent poll of advanced process control (APC) experts revealed unanimous agreement, users can expect APC applications to deliver sustainable, measurable benefits, but only when APC is built on a solid basic process control system (BPCS) foundation that:

• Develops control strategies from an understanding of the process and its properties;
• Ensures sensors, transmitters, and final control devices are working properly; and
• Ensures control loops are tuned and operating in automatic mode.

Wait a minute... if those three conditions are met, then the process would be as optimized as is possible, and APC wouldn't be needed! Well, yes and no.

Yes, the process would likely be operating with a minimum amount of variability, but no it's unlikely it would be optimized to the point where economic, productivity, and quality measurements were being pushed to their upper limits.

Operating with only the BPCS requires operators to continuously analyze the process and tweak loop setpoints to reach economic, productivity, and quality constraints. Perhaps if there were an operator for each processing unit only responsible for pushing the control system against constraints it would be possible to achieve an optimized process without applying APC; but how likely is that? Most control room operators have a plethora of duties and prefer to operate the process with a comfortable cushion.

The 'cushion' mentality raises two interesting questions.

• What's a two or three percent operational cushion cost?
• How much would it cost to add 2% capacity to your plant?

Those questions have been asked and answered for olefin processes. According to data collected by Solomon Associates, olefin plants using APC tend to have fewer unplanned slowdowns and shutdowns and generally operate about 2% better than olefin plants without APC. Two percent may not sound like much, but adding that much capacity to the average olefin plant costs about $10 million.

Only when APC is built on a solid control
system foundation can it push the limits of
quality (6s), productivity (#'s), and
economic ($) constraints.

Depending on the marketing literature you read, APC appears in a variety of forms including multivariable control, dynamic matrix control, and model-, general-, or horizon-predictive control. Adding to the confusion, APC can be deployed as centralized or decentralized applications.

Regardless of name or deployment, APC techniques establish a controller that cancels out or compensates for the natural dynamics of the process, using a desired speed of response. Notice APC does not eliminate variability-the best APC can do is reduce variation amplitude.

Whether APC should be centralized or decentralized requires considering two things:

• How fast can the process change?
• How many variables need to be controlled?

Centralized APC is generally deployed on its own application server over the top of the BPCS, manages 20 to several hundred controlled variables, and tweaks control-loop setpoints using the control network to interface with controllers in much the same way as operators tweak setpoints from an operator interface terminal.

Decentralized APC follows architectures of distributed control systems, programmable logic controllers, or hybrid control systems, and places APC application algorithms in the same controller as regulatory control, sequences, and interlocks. Processes-such as compressor surge control, boiler combustion and steam control, and some units of refining and pulp and paper processes-can change very quickly and thus require the APC be deployed as a decentralized solution.

Regardless of APC deployment, it's important to keep in mind APC adds another level of cascade control over the BPCS and brings with it the same control design considerations as any cascade application.

Everyone agrees, you can't build anything lasting on a poor foundation, and since the BPCS forms the foundation for building APC applications, it's important to ensure that:

• Control strategies are developed from an understanding of how the process reacts to disturbances;
• Sensors, transmitters, and final control devices are working properly; and
• Control loops are tuned and operating in automatic mode.

Control strategies can be developed to react to a process disturbance (feedback control), or to anticipate a process disturbance (feedforward-control).

All feedback control implementations have one thing in common; they react to a controlled variable disturbance after the disturbance is detected. Feedback control is simple to understand and most control systems provide a library of feedback control algorithms to choose from, complete with pre-built operator faceplate displays.

Feedforward is also simple to understand, but less commonly applied. Feedforward control measures a disturbance and introduces a dynamically compensated corrective action to the control algorithm before the disturbance affects the controlled variable. (See Feedforward is like insider trading sidebar.)

On occasion an APC application is discovered to be directly manipulating a final controlling device or valve. Experts caution this philosophy should be used sparingly, but nothing is a hard and fast rule. For example, operators sometimes use a manual loader to send a fixed output signal to a final control device and have learned exactly what to expect for each 5% change between 20% and 50%. It's also common to learn that under normal operating conditions, operators change the output signal a few percent once or twice a shift.

When these, or similar, conditions exist, let the APC mimic what the operator does using direct digital control. Learn what operators watch and how they decide when to change the output signal, how long to wait for a response, and then develop a corresponding algorithm in the APC application.

However, don't forget to carefully scrutinize the timing issues of applying direct digital control as part of a centralized APC application. Remember, BPCS-based control loops can read sensor inputs, execute the control algorithm, and change the output signal in milliseconds, and still not always cancel process disturbances. That makes it easier to understand how introducing a centralized APC's additional time lags in reading inputs, performing calculations, and getting the new output value to the final control device can amplify, rather than cancel variability.

Linearization is another area where experts agree it's far more important to ensure the loop-sensor, transmitter, BPCS and APC controllers, and final control device-are linear in the range of normal operation than to jump through hoops to achieve linearization across the entire measurement range.

The message is clear, BPCS and APC must be considered as a tightly integrated solution, and placement of control is not either/or. Understanding the process and its dynamics will guide controller algorithm placement. That doesn't mean BPCS loops are relegated to simple PID; quite the contrary. All available control tools, such as lead, lag, ratio, feedforward, cascade, adaptive/notch gain, limiters, high/low selectors, multipliers, adders, subtractors, etc., should be appropriately applied to create and deploy the most robust control solution possible based on knowing how the process performs.

To many practicing process control engineers and operators, APC's promise of optimization is a myth that works only in theory and can't be successful in a world where sensors plug, pumps and valves cavitate, and pipes leak. Maybe that was true in the past, but the analysis and modeling tools used to develop, deploy, operate, and maintain APC systems have significantly improved.

Today installing and expecting an APC system to actually maximize industrial productivity is no longer a myth, it's very doable.

In his book 'Optimization of Unit Operations' (Chilton, 1987), Béla Lipták defines optimization as, 'The integration of process control know-how to maximize industrial productivity.'

Mr. Lipták's definition of optimization could be met if process operations could:

• Avoid unplanned shutdowns caused by fouling, coking, plugging, and scaling;
• Avoid equipment constraint violations such as overspeed, cavitation, vibration, and surge;
• Implement optimum setpoint sequencing and ramping during feedstock or equipment switchovers;
• Continually monitor process stability;
• Push a process to economic, productivity, and quality constraints and then maintain that level of performance for days, weeks, or months:
• Ensure 'real' alarms are identified and managed by reducing or even eliminating nuisance alarms; and
• Provide operators with accurate and timely information about equipment performance so operators can become skilled caretakers of equipment.

An integrated BPCS/APC solution is capable of achieving all these things, but it requires approaching the solution with management's commitment, knowledgeable and committed people, healthy field instrumentation and control devices, regulatory control built on process knowledge, realistic and measurable improvement goals, and an appropriate APC application.

So, what would it cost to add 2% capacity to your plant? I'd wager it's more than installing an integrated BPCS/APC solution.