I was dreaming of the past and my heart was beating fast...
I began to lose control... I began to lose control...

("Jealous Guy," by the great poet John Lennon, 1971)

 

EVOLUTION FROM "LOCAL CONTROL" TO THE FIRST CONTROL ROOMS

Local Control of a Process Plant

PTOA Readers and Students just learned in PTOA Segment #8 that "local control" means the "Outside" Process Operator …

meaning the Process Operator who periodically walks the process plant route and takes readings  …

keeps the plant operating within desired targets.

Likewise, "local control hardware" is situated out in the processing area with the pumps, and pipes, vessels, reactors, separators, etc.

The first local controllers used throttled air pressure signals (pneumatic signals). From the perspective of today's high-tech world these instruments looked like a complicated Rube Goldberg machine with lots of linkages causing hardware movements that provided limited automated control of the process.

 

These first pneumatic indicating, recording, and controlling instruments were physically much bigger than the Foxboro brand TIC featured in PTOA Segment #8.

"Local control hardware" was a great improvement for process control and had its day in the processing world. However, nowadays there is not much local controlling done by outside Process Operators.

 

The Automatic Control Game Changer: The Pneumatic Transmitter

Assume that the Temperature of a process must be controlled for process safety and optimum yield of the process product.

The pneumatic transmitter uses air pressure signals to translate a measurement of Temperature into a signal that a Controller can understand. The nearby photo is the workhorse of pneumatic transmitters … the Foxboro 13A

Most importantly, improvements in pneumatic signal transmission resulted in this important outcome:

The Controller did not need to be in the same local box but rather could be situated in what became a centralized Control Room.

This technological advance made it possible to do these things:

• Keep the functions of Detecting and Measuring a Temperature, and Transmitting a signal that represented the magnitude of the Temperature within the local processing area.
• Centralize the functions of Temperature Indication, Temperature Recording, and Temperature Controlling in a cozy area that could be located far away from the processing area.

 

Plant Owners and Corporate HQ administrators quickly upgraded to centralizing process control. Why?

• Because centralizing control reduced the number of "Outside Process Operators" that needed to be on the payroll. One Outside Process Operator could take readings and do "the rounds" for several process units in the same area of the plant.
• Advanced Automatic Control was better than human-dependent process control. Automatic  process control maintained the plant processes at desired operating ranges better than a human being could do...  most of the time.

 

Voila!...

The "Control  Room" and "Control Board/Control Panel" came into existence.

 

ENTER THE CONTROL BOARD OPERATOR

The job description for a "Control Board Operator" likewise evolved into existence.

Advances in automated control technology changed the job description of most of the former "Outside Process Operators" into Control Board Operators.

From the vantage point of a seat at the Control Board Panel, it was now possible for the Control Board Operator to understand how multiple processing plants in the complex interacted with each other.

The Control Board Operator had to intimately understand when an override of automatic control was warranted and s/he could operate the process "manually" while still in the Control Room.

Otherwise stated …

Control Board Operators must possess a core competency with respect to understanding the limitations of process automation installed to control the process plants that they are responsible for. From time to time, the collective process data received from all instruments will not be in sync.

During those times, the Control Board Operator needs to override the automation and re-establish "manual control" to return the process to safe and efficient operations.

The Control Board Operator of a very complex, integrated processing facility understands that each processing unit in the complex had a different criteria for making maximum yield. In this case the Control Board Operator has to operate the Control Board to optimize production.

And, of course, the modern Control Board Operator had to be fluent in decoding process plant schematics like PFDs and P&IDs.

 

THE FOUR MAIN PROCESS VARIABLES:
TEMPERATURE, PRESSURE,
FLOW(RATE) and LEVEL

Up to this point, the PTOA has been focusing generally on the concept of Temperature and how this parameter can be changed or change on its own in a processing facility. .

Temperature is one of four "Process Variables" that must be controlled to churn out desired products.

The other process variables are Pressure, process stream Flow(rate), and container Levels.

Of course there are other industrial Process Variables but they tend to be specific to the processing plant and will not be featured in the PTOA content.

Every process plant will have some type of instrumentation to control Temperature, Pressure, Flow(rate), and Level for the purpose of operating the processing plant.

 

For this reason, The PTOA will limit its focus to understanding the four main Process Variables.

 

"Temperature" will hence be referred to as "PV Temperature."

 

"Pressure" will be referred to as "PV Pressure."

 

"Flow(rate)" will be referred to as "PV Flow(rate)."

 

"Level" will be referred to as "PV Level."

 

 

Otherwise stated:

All Control Board Operators spend their entire shift keeping dozens of PV Temperatures, PV Pressures, process stream PV Flow(rates), and PV Levels on target to make the desired. final products safely and efficiently.

All Control Board Operators are control freaks.

The PTOA content will first feature the PV Temperature followed by the PV Pressure, then the PV Flow(rate) and finally the PV Level. PTOA Readers and Students who continue the challenge will learn how these Process Variables are controlled in a processing facility.

 

Take Home Messages: The invention and transmitting improvements of the pneumatic transmitter separated the Sensing, Measuring, and Transmitting functions of a control loop hardware from the Indicating, Recording, and Controlling functions which were now in a centralized area far away from the local-control processing area.

Otherwise stated, the invention of the pneumatic transmitter and pneumatic controller also created Control Rooms, Control Boards/Control  Panels, and Control Board Operators.

All Control Board Operators spend their shifts controlling the main Process Variables: the PV Temperature, the PV Pressure, the PV Flow(rate), and the PV Level.

Control Board Operators must understand the process and the instrumentation competently so they know when automatic control must be overridden and return the process to manual control for safe and efficient operations.

The PTOA content will first focus on the PV Temperature. PV stands for "Process Variable."

©2015 PTOA Segment 00009
Process Industry Automation

You need to login or register to bookmark/favorite this content.