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