Process Overview for Your User Requirement Specification

Emerson's Scott Turner


Author: Scott Turner

This is the second part of the five part series on how to write an effective User Requirement Specification. If you missed the first on, see Considerations for Your User Requirement Specification. The topic of this post is ‘Process Overview’. We will address Scope, Control Philosophy and then Hardware in future posts.

As before, I would very much appreciate your comments and input. The points which I would like authors of User Requirements Specifications to consider are as follows:

  1. A process flow diagram should be included with the user requirement specification (URS). This process flow diagram should illustrate how all of the tanks and vessels are connected with major process lines. The flow of process should also be indicated, it is generally considered good practice for the flow to be left to right. Each vessel contained in the process flow diagram should be numbered. The numbers should be the same numbers used elsewhere in the URS document. Do not forget to include the utilities. These are an important part of the story for a control system supplier or system integrator.

  2. For each vessel or tank identified on the process flow diagram, the URS should contain a high-level description. This description should explain the type of process which occurs within the vessel. For example, is it a reaction process or a stirring process? Is the tank used for inventory purposes only? Other points to describe include if it is batch, continuous or maybe an interface between batch and continuous process stages. Describe what products are made within the tank. This will help the control system supplier or system integrator become familiar with your terminology.

  3. Each major equipment item or unit should be fully described. This description should include information on its purpose, the process operation it performs and a general description of how it works.

  4. Equipment items can be arranged in trains (streams) or clusters. How are the equipment items on your plant arranged? If they are arranged in clusters, how are decisions made on which equipment items to use in a given situation? If they are arranged in trains or streams, is there any cross over between the streams?

  5. Separator P&ID - Click to enlarge

    Separator P&ID – Click to enlarge

    To help with understanding the functionality of the equipment items, it is important to consider any unusual features. For each equipment item or vessel consider if any unusual features exist or if there are any stringent or critical constraints on the process which occurs within them.

  6. P&ID diagrams (even conceptual ones) should be included with the URS. These will provide the system supplier or system integrator with a feel of the plant. They will be able to develop an understanding of how complex it is, the size of the control system required and what standard functionality may be included in the project.

  7. What Key Performance Indicators (KPI’s) require measuring? These may be plant wide, specific to individual equipment units or perhaps specific to the proposed project itself.

Remember to ensure that you check the information. The numbering and terminology contained on the process flow diagram and in the P&ID documents should be consistent with those in the written paragraphs. Any terms or abbreviations which the reader may not be familiar with should be fully explained.

I find that it helps to put a table at the front of the document explaining what the abbreviations are. If you can, please provide a legend with your P&ID diagrams. Different companies present instruments differently. A P&ID diagram legend sheet will reduce the number of questions you may be asked.

What tips would you add or like to see clarified? Add a comment below.

From Jim: You can connect and interact with other project professionals in the Plan & Design group in the Emerson Exchange 365 community.

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