Defining User and Functional Requirements for Process Manufacturing
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This web site is provided by ControlDraw Ltd to support the community of engineers who write Requirements Specifications for Process Control Systems.


At present this is focused on developing User Requirements Specifications (URS's) and Functional Requirements Specifications (FRS), in general for Process automation systems such and DCS and PLC's. 

It is primarily designed for Process and Control Engineers who have to develop a specification from Process Descriptions and Piping and Instrument Diagrams

If is also for those people who want to write better Process Descriptions.

The Life Cycle provides some explanation of the stages in developing a Process Automation System. 

But Life Cycles are a recent concept compared with Process Control existed before there were PLC's, DCS's or Microcomputers. A Brief History of Instrument Specifications describes how we managed before them.

For a more modern approach, see the Requirements Analysis Process 

The ISA S88.01 standard (S88) is an important international standard for describing batch processes but it is also highly applicable to other processes including continuous and discrete ones. It is good practise to write the URS using the terminology defined in S88.

In addition ongoing work is being carried out to develop and new standard at present called S106 to help with defining continuous control.

See also the Short S88 Tutorial


Establishing the Requirements

The detailed requirements are to be found in several places including

Engineering diagrams such as Process Flow Diagrams and Piping & Instrument Diagrams 

The Control and Operability Philosophy if one exists,  if it does not then write one.

Process descriptions

Instrument Specifications

Peoples Heads

Company Standards

Equipment suppliers control definitions

Elucidating the detailed requirements is where methodology such as a model based approach and a good Model Review Process is essential. 

Documenting the Requirements can be done using text and various types of diagrams.There are some software tools that can help too.

For many processes State Based Control is a good method way to describe the functional requirements.

Other Standards worth looking at include

ISA-5.06.01-2007 Functional Requirements Documentation for Control Software Applications

The Norsok I-005 System control diagram standard is designed for Norwegian Oil and gas projects, but is not only for that.


It may also be that your project is for the replacement of an existing system - see Migration Projects


See more on the URS Links page

Note that this web is still being developed, check What's New for recent changes.

And it you have comments then you can post them on the associated URS Writer Blog


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