Schematics: The Bad and the Good

A brief introduction into the general guidelines of schematic building plus an example of a poor schematic and of a better version to illustrate some of the guidelines.

Introduction

Schematics are now the main display interface between the operator and the DCS thus it is imperative that they are well designed. A well designed schematic is a major benefit to operating a control system, a poor one is not only annoying but can lead to very inefficient plant operation and even mis-operation.

When schematics were first available on control systems the functionality was very limited, often to just basic line diagrams with numerical data on top. In recent years schematic functionality has been one of the major growth areas and on most modern control systems the possibilities are almost unlimited. This new functionality is of course welcome but as with any new features they are often used and abused.

Another change has been the basis for designing schematics. Traditionally P&ID diagrams have been and the schematics built by control system technicians. More recently specific diagrams have been generated for the design of schematics and they tend to be built by schematic builders. Whilst welcoming most of these changes one drawback that has been encountered many times is that the people specifying and building schematics are often very skilled in the building aspects of schematics but often they have little or no knowledge of human-computer interfacing (HCI) skills which are more physiological based than just being aesthetically pleasing.

To help you improve your schematic design it is best to start of with some guidelines with examples of both misuse of schematic design and better schematic design.

What does a poor schematic looks like ? Click here for an example.

General Guidelines

Lines

These should be kept straight if possible, without crossing other lines.

Text

Text should be kept to a minimum.

Superfluous text should always be avoided. The following are examples of superfluous text:

If an arrow is directing flow off the page there is no need to use a "to" prefix. Similar for flow onto a page and "from"

Try to use icons instead of text for alarms. For example use an up arrow to indicate high alarm, two up arrows to indicate high, high alarms. This is better than text.

Use lowercase not UPPERCASE, since lowercase is much easier to read.

Avoid technical control terminology, use words that the operators are comfortable with.

Colour

Colour makes all the difference to schematics, early black and white schematics were not very good. However, as with most things in life don't over do it. About 4 or 5 different colours on a schematic is considered optimal.

Dynamics

These should made very clear so that they stand out from the background, especially when they have noteworthy values or are in alarms.

Numerical Values

Don't fall into the trap of having too many significant figures. An valve output of 34.317% may look more impressive but is annoying to the operator because there will always be a figure changing on the screen. This will attract the operators eye even though the changes will be not significant.

Bar Charts

Vessels filled with bar charts look good but are annoying to the operator since you will have large areas of colour moving which is very distracting. They also take up lots of space. It is normally better to put a simple dip gauge inside the vessel.

Alarms

Try to avoid flashing whole text or numeric items. It is better to turn the text or numeric value red and to flash a small marker near the item.

It is also a good idea to signify what an alarm is with an icon rather than just annunciate that a numerical value is in alarm (it is not always obvious, for example is the above temperature high or low?).

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Equipment Representation Guidelines

Pipe Work

Pipe work is normally over emphasised on schematics. Its only purpose is to show how the flow propagates through the plant which is something an experienced operator should know well enough.

Try to use colours that allow the pipe work to be ignored, allowing the operator to concentrate on the measurements of the fluids in the pipe work.

Only two line thickness should be used for pipe work. A bigger thickness for the main process pipe work, and a smaller thickness for all other pipe work

The following table provides a starting point for pipe work colour and thickness selection:

Of course if your main process is the production of steam then you will need to amend the above selections :-) .

Vessels

Vessels are not actually that important to a process schematic. Try to use simple shapes, drawn in low intensity colours.

A good colour for vessels is low intensity white or grey.