Time for a little recap of what's been learned in the PTOA Introduction to Boilers and Steam Generation!

Let's review what has been learned about water tube boilers so far:

• PTOA Readers and Students are aware that boilers generate steam, a substance that has many industrial uses which include indirectly heating up process streams.
• PTOA Readers and Students can identify the hardware inside of a package boiler and the equipment that generates high pressure steam.
• PTOA Readers and Students understand the difference between Sensible Heat (detectable Heat) and the Heat of Vaporization (non-detectable Heat).

 

SYSTRAN HOUSTON'S GRAPHIC
OF A WATER TUBE BOILER

This PTOA segment explains how the Package Boiler works to produce Saturated Steam from Boiler Feed Water while referencing and giving full credit and thanks to Systran Houston for the very helpful visual aid shown below:

PTOA Readers and Students need to Imagine the flow of the Boiler Feed Water as it vaporizes into Saturated Steam as described below:

• The Boiler Feed Water (BFW), continuously flows through the green pipe into the gold steam drum (upper drum).
• The cooler BFW is more dense...and therefore more heavy... than steam. The heavier, cooler BFW flows to the bottom of the steam drum and thence into one of the red downcomers.
• In the downcomer, the water is indirectly heated by combustion gases in the firebox and starts changing state into steam.
• The steam is lighter than water and starts rising back into the steam drum via a riser (also shown in red).
• Guess what? Risers and downcomers are not dedicated piping; the nomenclature simply describes which way the fluid inside of them is flowing. A downcomer can become a riser and vice versa.
• Steam is also generated in the D tubes of the firebox; the D Tubes connect the blue mud drum (lower drum) to the gold steam drum (upper drum).
• Being a vapor, the generated saturated steam is much lighter than BFW and thus collects above the liquid level surface of the BFW in the steam drum.

 

SATURATED VERSUS SUPERHEATED STEAM

"Saturated" means "holding as much water as possible."

"Saturated Steam" describes steam that has just as many particles wanting to go into the vapor state (steam) versus wanting to stay in the water state.

In the covered pot nearby, five blue water particles are happy being vaporized into steam and five white steam particles are happy condensing back into water.

The water vapor collecting above the liquid level is thus defined to be saturated steam.

Any steam that is in contact with the liquid level in the steam drum is saturated.

Why?

Because because because because because!

(Your life will still be complete without understanding everything about vapor/liquid equilibria. Trust Your Mentor on that!).

The above bold statement means that ...

because any steam drum has a water level ...

all the steam above the water level in any steam drum is saturated steam.

 

 

Saturated steam does a great job of transferring heat...it's the steam type needed in a kettle-type reboiler.

 

But saturated steam is too wet to be used in a steam turbine.

The water that would easily condense from the saturated steam would destroy the turbine vanes.

 

Steam turbines require hotter, drier superheated steam.

 

All Boiler Process Operators Must Completely Understand
Saturated versus Superheated Steam

PTOA Readers and Students already know that it is impossible to make superheated steam in the steam drum of a boiler because a steam drum has a water level.

PTOA Readers and Students also understand that cranking up fuel to the burners won't increase the temperature sensed and observed on boiler TIs.

To reiterate ...(because it is an important point)..,

The PTOA Department of Redundancy Department issues the following Operational Warning:

If the Process Operator made the mistake of cranking up the burner fuel in an attempt to make superheated steam in the boiler, the result would be the vaporization of all the water in the steam drum, mud drum and risers...and all the while the TI would still continue reading 212 °F (100 °C).

The temperature observed on the TI would inform the Process Operator that the sensed temperature had adequately reached the boiling point temperature ...

but never reveal that the rest of the thermal energy provided by the firebox burner was evaporating all the water in the boiler!

Smart PTOA Readers and Students can inform that hapless Process Operator that the Heat of Vaporization...

the energy it takes to change state from a liquid to a gas...

cannot be sensed by a TI.

Only AFTER every single drop of water in the Boiler has been converted into steam ...

THEN the temperature of the steam shown on the TI would rapidly increase.

Unfortunately, the boiler may have blown a few tubes by then!

Boom Laka-Laka-Laka!

Boom Laka-Laka-Laka!

 

 

In summary, Process Operators don't need to know everything about vapor/liquid equilibria but they do need to understand why TIs alone cannot be used for determining the operating status of a Package Boiler.

In a future PTOA Segment ...

PTOA Readers and Students will learn that Process Operators must monitor the following process variables to get the complete picture of what's going on inside the boiler:

• Steam Drum Level.
• BFW Flowrate.
• Steam Production Flowrate.

 

SUPERHEATERS

The strategy for making hotter, drier superheated steam is to remove the saturated steam product away from the water level in the boiler and have it flow through specialized equipment and piping called a superheater.

The superheater can be seen in Systran Houston's graphic as a pipe connected to the top of the gold steam drum.

 

 

The superheater makes a pass through the D-tube firebox area before entering the steam header for distribution to steam users throughout the plant.

The superheated steam has now gained several degrees of superheat and can safely be used in a steam turbine.

 

 

This PTOA Segment #26 concludes the introduction to package boilers, popular process industry equipment that is used to generate steam on demand.

And by now all PTOA Readers and Students know that steam is a multiple-use utility that can also be used to indirectly heat up process streams.

 

Take Home Messages: The Systran Houston animated video shows how the internal elements of a package boiler make saturated steam from boiler feed water (BFW).

Any steam that is in contact with liquid is saturated steam which means just as many particles of water want to change state into steam as want to condense back into water.

Saturated steam is the best kind of steam to use for indirect heat transfer, like in a reboiler.Saturated steam is too wet to be used in a steam turbine.

Superheated steam is used in a steam turbine. Superheated steam is drier and hotter than saturated steam.Superheated steam is made in a superheater.

All Process Operators must thoroughly understand why the temperature sensed on the TI in a steam drum will not increase...even when the burners are cranked all the way up.

 

©2015 PTOA Segment 00026
Process Industry Temperature Changing Equipment

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