Detailed Design

Detailed design of process control systems starts with a plant wide approach early in the overall plant design stages. 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?

Only when the overall control scheme is known can individual loop design be considered, which will define the ' User Requirements' from the control system and operator interfaces. This should be well documented and a recommended reference publication is IEE guidelines for the documentation of computer software for real time and interactive systems (Link).

This should take place in close liaison with the process engineers. For example a process engineer will want to design a pump for minimum pressure differential to save on pipe pressure rating and running costs - the process control design may demand more differential pressure to give sufficient operable range to a flow control valve in the line.

The selection of output variables, manipulated variables and measured variables is important. Input and Output variables may both be measured, preferably on line - the measurement should be reliable, accurate, sufficiently sensitive and if possible minimise time delays and time constants. It might not be possible to measure on line all of the output variables. So for example composition may be controlled by regulating top or tray temperature on a distillation column.

Our approach to process design integrated with process control will ultimately give a high quality product in a safe and efficient plant.

That overall approach can be applied to any type of control system whether it be a PLC, DCS or SCADA or a group of single loop stand alone controllers. We have experience of many manufacturers systems including:

• Eurotherm Process Automation (previously Turnbull Control Systems)
• Eurotherm Temperature controllers
• Emerson Process Systems- DeltaV and Provox with Fieldbus which includes iFIX graphics using Visual Basic for operator displays (previously Intellution)
• Historian Systems such as AspenTech data historian

These involve programming in proprietary languages from some of the above manufacturers - but also includes programming in a variety of other languages, including ACCOL (Bristol Babcock's proprietary language), ABACUS, S5 (Siemens) and a number of less well known languages. We have programming experience in RSX11, VAX-VMS, DOS, various Windows varieties, UNIX, and Linux Operating Systems, and Fortran, Algol, Pascal, Basic, Perl, Tcl/Tk, and C.

Which ever system we work with we adhere to recognised standards such as IEC-61131-3 Programming Industrial Control Systems; ISA S88 Batch Control (Parts 1 to 3) and from the EEMUA (Engineering Equipment and Materials Users Association) Process Plant Control Desks Utilising Human-Computer Interfaces. A Guide to Design, Operational and Human Interface Issues: Publication No. 201:2002; Alarm Systems. A Guide to Design Management and Procurement. Publication No. 191:1999.

CHAZOP.

Control HAZOP (or Computer HAZOP both shortened to CHAZOP) is the same process as process HAZOP with the specific task of assessing the risks involved in control systems deviating from their intended function. It will cover hardware - from field to control room including networking; software - from operating systems on the DCS/PLC/Operator Workstations to application programs; power supplies and environmental effects on the whole system. It can also include an alarm review - though this can also carried out as a separate exercise. The input information into a CHAZOP and guide-word / parameters used differ from a HAZOP but the principals of the exercise are the same. Our HAZ1508 software is suited to carrying out a CHAZOP and structures the exercise to match HAZOPs undertaken with the same software.HAZ1508