This is a collection of Links to useful resources for Requirements Specifications, not only for process control system,
JETT is a consortium of pharmaceutical users (manufacturers), equipment suppliers, and consultants seeking to improve communications between Users and Suppliers to more effectively meet the "validation" requirements of the pharmaceutical industry.
SearchEng Web for Electrical, Control and Instrumentation engineers.
Goal-oriented Process Control System Design A King's College London project
Volere This is the home of the Volere Requirements Specification Template. Downloads are available in .pdf and Word formats note these are IT oriented but worth looking at
What is a Requirement? A useful document from Coley Consulting, a UK IT consultant.
Requirements management From Wikipedia
Why Johnny Can't Write Requirements an interesting perspective from Compliance Automation, Inc.
RoyMech provides a considerable amount of information on the engineering of process plants
You can Download a Word URS Template for basic purpose
ANSI/ISA-5.06.01-2007 Functional Requirements Documentation for Control Software Applications