Double, double toil and trouble;
Fire burn and cauldron bubble.
Cool it with a baboon's blood;
Then the charm is firm and good.

(From "Macbeth," by the bard William Shakespeare, 1606)

Okay, nothing as interesting as baboon's blood has ever bubbled out of a package boiler.

In the real process industry world, the desired product from a package boiler is dry, high pressure steam that can be used as described in the previous PTOA segment.

 

INTRODUCTION TO PACKAGE BOILERS

Using steam to perform useful work has been going on for 240 years, long enough to know that experts need to fabricate boilers so that they don't fail.

Boilers are  built by certified experts and then transported to the facility as a 'package' to be installed in the utility area of the plant.  Hence the name "package boiler."

Package boilers must be inspected by state officials on a regular basis. The state Boiler Inspector is trained to spot potential tube and refractory failures. In the picture to the left, areas of concern on boiler tubes are encircled.

The two main types of package boilers are water-tube and fire-tube.  The PTOA will focus on water-tube package boilers.

The term "package boiler" also helps delineate this style of boiler from a type called a "waste heat boiler."

The focus on waste heat boilers is coming up soon.

 

SIMILARITIES BETWEEN
FIRED HEATERS AND PACKAGE BOILERS

Package boilers and fired heaters have many similarities. The following simplified schematics of a fired heater and package boiler illustrate the many common features.

PTOA Readers and Students should locate the common hardware found in each of the schematics.

Schematic of Fired Heater Internals

Both the fired heater and boiler have fire boxes that are lined with refractory.

Both have burners and both combust fuel to generate hot water vapor (H20), carbon dioxide (CO2), and carbon monoxide (CO).

These combustion products  exit the fired heater and boiler through stacks.

The combustion products made of hot gases indirectly raise the temperature of the fluid flowing through the tubes by heat transfer.

Here's a big  difference:

The fluid flowing through the tubes of a fired heater is the process fluid that needs to be heated to a target temperature.

The fluid flowing through the tubes of the package boiler is water that is in the process of changing phase into water vapor (aka "steam"). The physical state of the water is changed inside the boiler.

Saturated steam (defined and explained in an upcoming PTOA segment) is the product drawn directly off the steam drum of the package boiler.

 

UNIQUE FEATURES OF PACKAGE BOILERS

There is not a commonly accepted ISA symbol for a boiler.

The P&ID page(s) for a package boiler typically illustrate how the specific boiler purchased works with automatic instrumentation to control the Boiler Feed Water (BFW) Flowrate, Steam Drum Level, and Steam Product Flowrate. This "3-Element Boiler Control" is the subject of future PTOA segments.

The graphic below would be a suitable ISA symbol candidate for a water tube package boiler.

All package boilers have a bigger top circle (the steam drum or upper drum) connected to a smaller little circle on the bottom (the mud drum or lower drum) by tubes (risers and downcomers).

The area in the shape of  a Capital D or backwards Capital D is the firebox; the firebox water tubes are fabricated into the "D" shape. These D tubes connect the mud drum (lower drum) to the steam drum (upper drum).

The hole in the firebox of the above icon is where the burner assembly and flames would poke through.

Many water tube package boilers are constructed with the burner assembly (and hence, flames) protruding into the firebox on a short-end wall.   In very large D-structure boilers, the burners may be on the bottom, just like the burners of a fired heater.

The picture below is a picture of a real water tube package boiler before it was enclosed in protective housing. The capital "D" tubes are particularly easy to see.

Compare the above photo with the photo to the right.

In the package boiler photo shown at the right, PTOA Readers and Students should be able to distinguish the steam drum from the mud drum as well as be able to visualize where the risers, downcomers, D tubes and  firebox would be located.

The burner assembly is shown covered by the protective plastic.

PTOA Readers and Students can now identify the internals of a package boiler. The next two PTOA segments will explain how a package boiler makes steam.

 

Take Home Messages: Saturated Steam is generated in the water tubes of Package Boilers. The steam is generated by indirectly heating water that is flowing through the boiler's tubes until it changes phase into steam.

The common features of Package Boilers and Fired Heaters are Fireboxes, Refractory, Burners, Combustion Gases/Combustion Products, Stacks, and Tubes.

Unique features of Package Boilers include:

• Steam Drum or Upper Drum
• Mud Drum or Lower Drum
• Risers and Downcomers that connect the Mud Drum and Steam Drum.
• "D" shaped Water Tubes that are more exposed to the combustion products because they are shaped around the walls of the firebox.

A 3-Element Control Scheme is used for automated Boiler Control. The 3 Elements are Boiler Feedwater (BFW) Flowrate, Steam Drum Level, and Steam Production Flowrate.

Package Boilers are built by specialists off site, then moved and installed at the processing facility. Package Boilers are inspected routinely by state officials.

 

©2015 PTOA Segment 00024
Process Industry Temperature Changing Equipment

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