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Process Control in Plant Design

By: , Posted on: August 2, 2015

“Much of what is taught in Chemical Engineering courses under the heading of process control is out of date, irrelevant and impractical, with the result that most new process plant designers have little idea of how to design process control aspects of their plant.

Once they start as practitioners, feedback to beginning designers from commissioning and control engineers or their attendance at HAZOPs will hopefully eliminate features which lead to poor controllability, but I would like to give newbies more of a head start than is presently usual.

Integration of process control and design by professionals is far more intuitive and qualitative than mathematical. Beginners definitely seem in my experience to need to be given a place to start. Interactions with more experienced engineers will refine their understanding, but what they need in the first instance is a way as an absolute minimum to put the basics on their P&IDs.

Matching design rigor with stage of design

At conceptual design stage, very little or no consideration needs to be given to process control issues, unless the plant has some novel or very hazardous components which are likely to present entirely new or very high-risk process control problems.

At the detailed stage of design, a fully thought out and instrumented P&ID needs to be produced, and ideally, precise models of instruments specified. As a minimum, realistic instrument choices and specifications should be produced.

Perhaps more importantly, the number of inputs and outputs to the control system cannot be determined unless the instrumentation has been thought through to this degree.

The software for a manually operated plant is defined in the Operation and Maintenance manual, and for automatic plants in the Functional Design Specification. As most plants have some automation nowadays, a combination of the two documents will be required to understand how the designer thinks the plant will be controlled.

Operation and Maintenance Manuals

Operation and Maintenance (O+M) Manuals are written for (almost) every process plant, describing how it is to be operated and maintained, and how to troubleshoot any problems which occur.

They are to me largely a type of process control software, as on a fully manual plant they describe in detail the control actions which people will undertake to achieve the things which a PLC would do on a fully automatic plant.

Most plants are, however, not fully automatic. There are automatic control actions, and there are manual interventions.

Some plant conditions are thought trivial enough to allow the system to automatically restart itself via remote command. Some are thought dangerous enough that the system forces someone physically to press a button before restart is possible and the O+ M manual tells them that they must to go and look at the kit before they press the button.

So decisions have to be made about safe operation of the plant; and software and operating procedures work together to ensure safety.

Control philosophies always to my mind make implicit assumptions about how the plant will be operated, which it is better to include in the document, as another reader may make different assumptions if they are not made explicit.

Specification of Operators

The level of education and training of operators has to be specified to determine the degree of automation which a plant requires.

In choosing whether to have a highly automated plant, one needs to consider the advantage of operators over instrumentation – operators can detect not just specified conditions, but unspecified and unexpected conditions.

The more we expect our operators to do about the things they monitor, the higher their required level of skill and understanding needs to be.

A fully manual plant will need a high availability of highly experienced staff. A fully automatic plant may need no permanent staff on site at all, especially now that we can access plant telemetry and SCADA systems remotely via IP technology.

Automatic Control

We don’t build fully manual plants in the developed world nowadays. Computers are too cheap and reliable, and operators too expensive and human for routine operation activities to be best done by people.

Control is mostly done using a combination of PLCs, PCs, and high level control system (DCS or SCADA), though there may be a few field mounted controllers specified for a number of reasons.

Process plant designers need to know how to specify instrumentation and control hardware, populate their P&ID s with these items, and write FDSs so that software engineers can design and price their software.

Process plant designers need to have an idea of what neighboring disciplines do, and what they need to do their jobs. We don’t, however, need to be able to do their jobs, we have to be broad-brush people. We can’t sweat all the details.”

Read more from Sean on SciTech Connect


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About the Author

sean moranProfessor Moran is a Chartered Chemical Engineer with over twenty years’ experience in process design, commissioning and troubleshooting. He started his career with international process engineering contractors and worked worldwide on water treatment projects before setting up his own consultancy in 1996, specializing in process and hydraulic design, commissioning and troubleshooting of industrial effluent and water treatment plants.

In his role as Associate Professor at the University of Nottingham, he co-ordinates the design teaching program for chemical engineering students. Professor Moran’s university work focuses on increasing industrial relevance in teaching, with a particular emphasis on process design, safety and employability.

Connect with Sean on LinkedIn here, check out his Facebook page here and stay up-to-date on his thoughts, research and practice at his personal blog here.

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