WHY PROCESS OPERATORS ARE NEEDED
TO MONITOR LOCAL PROCESS TEMPERATURES

In these modern times, why would a processing plant hire a set of shift workers to record local temperatures and pressures and other readings?  Certainly the technology exists nowadays to automate the function of recording vital processing data.

Some processes, like a nuclear reactor, cannot have human beings safely working in the environment. These processes are totally automated and are designed with redundant (back up) control systems.

But nothing beats a pair of human eyeballs to verify and report on a process condition wherever it is possible to do so.

So outside Process Operators make their rounds and verify the status of process plant operations.

Some excellent reasons to verify the temperature of the fluid flowing through the pipes are mentioned below.

For right now, take another look at the picture of an outside Process Operator taking readings.

Maybe the outside Process Operator in the picture is expecting the TI to read 260°C or maybe he expects the TI to read 500°F (different scales, same temperature).

Maybe he expects the temperature to be 45°F (which is equivalent to 7.2°C).

There's not enough information in the picture to know what the desired operating temperature at that location is.

See all that silver stuff around the pipes in the above photo? That's called insulation.

Insulation reduces the impact of the outside ambient temperature on the process.

The insulation is a hint that the Process Operator is expecting the temperature to be lower or higher than the ambient temperature.

The purpose of the insulation is to help keep the temperature of the fluid flowing through the pipes at the desired operating temperature just like a coat helps to keep you warm on a cold day. 

 

GOOD REASONS TO KEEP MONITORING TEMPERATURES
DAY IN AND DAY OUT

All TIs give a hint whether or not the process is at the desired operating temperature.The placement of TIs and other process status indicators is not arbitrary; there's a good reason why that temperature needed to be monitored exactly where that TI was placed.

Here’s a list of good reasons to install and monitor a  TI:

1) Maximizing Production of Desired Product (aka Yield).

One good reason to install a TI is to confirm that the fluid flowing through the pipes is at the temperature needed to maximize the output of desired product(s).

The amount of desired product made from an amount of feedstock(s) is called yield.

A deviance from the desired operating temperature can often result in less yield of the most valuable final products. Maximizing yield is in everybody's best interest because the cash to make the payroll comes from the sale of the products.

The graphic on the left illustrates the typical yield of fuel products produced at an average U.S.A. refinery from one barrel of crude oil.

The graphic shows that half of one barrel of crude oil feedstock is converted into gasoline.

How much is converted into "Residual Fuel Oil?"

The graphic on the left is a pie chart showing the different varieties of Sam Adams beer produced.

Apparently the brew called "Seasonal" is the most produced beer. What brew is produced almost as much as "Seasonal?"

 

 

 

2) Confirm That A Safe Operating Temperature Exists.

The TI might also be installed for safety reasons.

For example, an intermediate or final product that is too hot when it is sent to a storage tank can damage the tank. A strategically placed TI will help protect personnel and equipment.

 

3) Confirm the physical state (aka phase)
of the fluid in the pipes.

PTOA Readers and Students are already familiar with changing the physical state of water at home.

Making ice cubes involves freezing water that has been poured into ice-cube trays.

And how many PTOA Readers and Students have waited impatiently for water to boil while making a cup of tea or oatmeal or a few cups of rice?

Likewise, industrial process technology can be used to alter the physical state of feedstocks as needed to make more desirable final products.

Any solid can be changed into a liquid by increasing the temperature.

PTOA Readers and Students already know that kind of change of state (aka phase change) is called melting.

Every solid has a melting point temperature.

If the temperature keeps being increased, the liquid will eventually change into a vapor (more commonly called 'gas').

Fun Fact:

When a vapor bubble forms below the liquid surface that's called boiling. When a vapor bubble forms above the liquid surface that's called evaporation.

 

Any gas/vapor can be changed into a liquid by a lowering the temperature until the gas/vapor condenses into a liquid.

Keep lowering the temperature and the liquid will turn into a solid which PTOA Readers already know is called "freezing."

PTOA Readers that live in cold climates have observed that the several meters/feet of snowfall does not all melt into water.

Some snow skips the step of melting and goe straight into a gas/vapor phase. This change of state is known as "sublimation."

In summary,

The  scientific sounding phrases "changing the physical state of matter"  and "phase change" are actually familiar to PTOA Readers and Students.

There is no need to be anxious about the terms "sublimation," "evaporation," or "condensing."

They are helpful describing a particular kind of change of state just like the more familiar phase changes known as "melting," "freezing," and "boiling."

And all "changes of physical state" aka "phase changes" are caused by raising or lowering temperature.

Local TIs help confirm that the temperature of the process is low enough to keep liquids from vaporizing and, vice versa, make sure that the temperature of a process is sufficiently high enough to keep a gas from condensing into a liquid.

 

 

4) Backup Control Room Temperature Readings.

The best designed process plants may have a local TI installed just to back up the automated temperature reading that is transmitted into the Control Room.

Automation requires integration of hardware and software and any of the components in a control loop can be faulty.

 

 

A Control Board Operator will direct the outside Process Operator to verify the status of a temperature as needed instead of rely totally on the temperature visible on the Control Board.

The worst nuclear accident in US history at Three Mile Island, PA was caused by instrument failure compounded by human misinterpretation of the situation. A link to the wiki page on the incident is below.

In that incident, the Control Board Operator would have appreciated an eyeball report on the situation before making decisions...but that was not possible.

http://en.wikipedia.org/wiki/Three_Mile_Island_accident

Take Home Messages: The reasons to raise or lower the temperature of the fluid flowing through pipes include making the most product possible (maximizing yield) and safety. Since everything has a specific temperature at which it will boil, evaporate, condense, melt, freeze, or sublime, a local TI can be used to verify that the stuff in the pipes is in the physical state desired. Local TIs are also used to confirm if the temperature reading in the Control Room is accurate. 

© 2015 PTOA Segment 00002
Process Variable Temperature

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