Control Philosophy

Designing a controllable process is a fundamental part of designing a process that is safe and profitable. So the design of a satisfactory process control system should be an integral part of process design. This requires a 'strategic' understanding of the overall control system, as well as the detailed design of the control loops and controllers.

This approach to control system design requires consideration of control at the early design stages of the process, rather than a "bolt-on" once the process design is "finished". This leads to simpler and safer process control. It also allows consideration of the plant wide control scheme. In a continuous process is the scheme structured for on demand product flow or fixed fresh feed flow? In a batch process have sufficient units of each type in the batch train been provided to allow optimum batch cycle time?

Alternative solutions to control of a process could then be subject to the normal HazOp and risk assessment procedures one would expect as part of each stage of a design. If the control solution was not acceptable for safety reasons, it could be considered further at the earliest stages rather than not being considered because the control "bolted on" at a later stage.

Similarly, the basic process design should not attempt to use control design to "work-round" a bad equipment sizing or design - especially if safety rather than loss of efficiency is involved. The control system design should enhance safe design concepts and act as further layer of stability and hence safety, not the ultimate layer of safety.

When writing down the requirements for a control system the IEE guidelines for software documentation give clear guidance on the content and purpose of the necessary specification for a successfull project. More information on the user specification can be found here.

EN 61508 / IEC 61511 / S84 / UKOOA Guidelines