Jul-2012

APC2: aligning people with control

A lead practitioner explains how refinery-wide involvement of people underpins a major advanced process control programme

Raymond Coker
BP Texas City refinery

Article Summary

Advanced process control (APC) can take on many flavours and levels of sophistication. With regard to digital control systems (DCS), advanced regulatory controls can create value using property predictions cascaded to DCS controllers, or feed forward or decoupling signals can be added to improve stability and robustness of controls. In order to take on entire units of control, medium to large multivariable controllers have been developed to handle process interactions and to drive the unit to the most economical combination of constraints. Multi-unit control and optimisation in the form of a larger linear program provides a tool to coordinate flows and optimal targets across the refinery. More detailed effects can be modelled and included in the linear program solution as new knowledge is incorporated into the solution. Fuel gas, steam, hydrogen, amine, cooling water, tanks, shipments, electricity and process line-ups create opportunities to synergise across multiple units.

Aligning people with control (APC2) provides the mechanism to design controllers, commission controllers and sustain value long term. Critical resources include advanced control engineers, unit control engineers, process engineers, operations personnel, and economics and planning personnel. Economics and planning provide the big picture view of the refinery and help identify the gold nuggets for control personnel. Advanced control personnel help planning by adding greater fidelity to the models and trade-offs between quality and production, as well as by commissioning model-based controllers to coordinate moves across each unit and to reach the targets set by planning. Unit control personnel help with control system configurations, regulatory controls and advanced controls. Process and operations provide critical process and constraint information.

The extended team includes management sponsors, instrument and analyser personnel, environmental personnel, laboratory personnel, corporate technical leaders and sponsors, commercial managers, upstream suppliers and downstream customers.

Create the vision
In order to get an APC programme off the ground, there must be a credible story to initiate interest and support from management at multiple layers. Using rules of thumb (3-5% improvement, 90% utilisation, average stream margin and average stream flow rate) for APC benefits for each unit, a table of estimated benefits can be calculated (see Figure 1). The priority or order of execution of the projects depends on the benefit-to-cost ratio (planning and economics and project management), condition of unit, condition of instruments, condition of systems, TAR schedule, other project schedules and availability of key resources (operations, process engineering, control engineering, instrument and analyser maintenance, and system support).

It is very important to build good alignment with your management team early in the APC programme. Without their initial support, it will be difficult to drive through the obstacles ahead. The following people are critical to the success of the programme:
• Management Supports resources, projects and funds
• APC engineers Lead APC studies and projects
• Unit control engineers and digital control personnel APC design, regulatory controls, database points, displays
• Planning and economics Stream values and utility costs, quality and production targets
• Process engineers Equipment and process limits, process models
• Instrument and analyser staff Support valve, instrument and analyser repairs
• Operations staff APC design, operating objectives, dynamic and steady-state concerns
• Environmental engineers Environ-mental limits and permits, compliance requirements
• Safety personnel Ensure management of change provides proper communication and approvals, compliance
• Information and network technology personnel Develop and maintain system and network architecture.
The following are important for the proper execution of the APC studies and projects:
• APC study report All disciplines and stakeholders required to define scope, schedule, cost, benefits and infrastructure
• APC pretest report Control, process, operations and maintenance resources required to update design, transform, calculate, repair instruments, modify regulatory controls and prepare for test
• Inferential calculations Control, process and analyser maintenance resources to develop and implement inferential calculations with updates from lab, analyser or rigorous calculations
• Dynamic models Control, process and operations resources to test, develop and verify dynamics and steady-state gains
• Controller performance Control, process, planning and operations resources to commission controller economics, speed and priorities
• As-built documentation Control, process, operations and training to document final solution in sufficient detail to train operators and support applications.

Benefits drive APC
Once the APC projects are ordered by year, the estimates can be further refined to improve the scope, costs, schedule and benefits (see Figure 2).

These costs reflect only the project costs. The maintenance costs for APC range from 15-20% of the original project effort per year. The largest factor in the maintenance costs is how often and how much the unit changes over time.

Start with the end in mind: the benefit mechanisms drive the APC solutions. APC studies are an excellent way to leverage the knowledge of operations, process engineers, process consultants and modellers, control engineers, instrument and analyser maintenance, environmental engineers and safety personnel. There are many gold nuggets (benefit opportunities) in the minds of these people working on the units or supporting the units. By asking leading questions and understanding the process and constraints, the APC leader can discern the most important benefit mechanisms to go after and develop the control objectives.

The APC design will grow as new mechanisms are identified. The product of the study is an APC design that includes the manipulated, disturbance (or feed forward) and controlled variables.