Spurious process variables

 Not all the variables that you may name about a process are true process variables.

How to determine if a variable is a true process variable?

As was mentioned in the post Process variables, these variables have the following features

• change in time and
• can be measured.

If the so-called variable does not have both of these features it is not a process variable.

One further feature of a process variable is that: this variable must be controlled so that the process runs as close as possible to its optimum estimated conditions.

Examples of spurious process variables

While design stages of process instrumentation and control variables need to be correctly identified. In this stage is were errors may occur by chosing a spurious process variable.

Here is a short list of spurious process variables examples:

• Distillation column number of plates. A column plate is a mechanical component of a major equipment which is attached to the main body can hardly change in time. A change in the number of plates would imply to shut down the process and then start a labourious work of modifying a distillation column wich not only could be expensive, according to its size, but lengthly. This can not be done in minutes and even less in seconds!

• Pipe roughness. When pipes are manufactured the roughness is already set according to the process of fabrication and quality control of that supplier. It changes in time but very slowly (perhaps in the order of months or years!) so that it is impractical for a process that last a day at most. Form the point of view of a process that last a few hours, the pipe roughness changing every three months is virtually a stationary parameter. Pipe roughness can be measured but you would need to dismantle all pipes for a given portion of the process plant. Measurement of pipe roughness is then useless.

• Mixing tank diameter. Thermal expansion and/or contraction occur in metallic equipment but it is hardly noticeable in a tank of 1m  diameter. For a change in diameter to play an important role in the control of a process it would need to be in the order of meters and no metallic tank on earth does that (at least that I know). Now, if you had such a diameter-changing tank you probably need enormous amounts of energy so that the termal expansion/contraction could occur.

• Pump motor power. Power cosumption of an electric motor changes with load or work to be done. It certainly changes in time and can be measured. However, there is other variables that may be used instead: angular velocity (rpm), pressure and flow rate. These last 3 variables are more common to a chemical process than electric power.

Any question? Write in the comments and I shall try to help.

Other stuff of interest

• LE01 - AC and DC voltage measurement and continuity test
• LE 02 - Start and stop push button installation 24V DC
• LE 03 - Turn on/off an 24V DC pilot light with a push button
• LE 04 - Latch contact with encapsulated relay for turning on/off an AC bulb light
• What is a process variable?
• What are the most important process variables?
• Time dependence of process variables
• A list of process variables

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Ildebrando.