Page 286 - Boiler_Operators_Handbook,_Second_Edition

Chapter 10

Plants and Equipment

operates at a pressure higher than 15 psig or a hot water
It would take volumes of books to adequately de- boiler that operates at a water temperature greater than
scribe all the variations of design and construction in 250°F or a pressure greater than 160 psig.
boilers since Hero first produced steam under (a little) Low pressure boilers are defined by ASME in Sec-
pressure. And that’s only boilers, nothing to do with tion IV of the BPVC “Rules for Construction of Heat-
all the other plant equipment and systems. You may ing Boilers.” It defines a low pressure boiler as a steam
encounter a design that isn’t described in this section. boiler that operates at a pressure no greater than 15 psig

I encountered two true Sterling design boilers, an 1890 or a hot water boiler that operates at temperatures not
design constructed in 1952, in 2004. I’ve limited the greater than 250°F and pressures not exceeding 160 psig.
descriptions in this section to what you would nor- Now you can understand why there’s so much
mally encounter. If you do encounter an older design confusion regarding medium pressure, there simply
it should be described in one of the references listed in isn’t any room for it! If the boiler makes steam it’s low
the bibliography. pressure until 15 psig and high pressure at any pressure
higher than 15 psig. Hot water boilers aren’t quite as
clearly defined but the temperature is normally the clue,
TYPES OF BOILER PLANTS almost any hot water boiler operating at temperatures
less than 250°F is a low pressure boiler.
When you want to get a definition right you go I zipped through that discussion of high and low
to the source and, in the case of boilers, the source is pressure without making note of some other defining
ASME, the American Society of Mechanical Engineers labels. The titles of the code sections is one key. A high
which produced and maintains its Boiler and Pressure pressure boiler is also called a “power” boiler, low pres-
Vessel Code (BPVC). The code is the accepted rule for sure boilers are called “heating” boilers and the defini-
construction of boilers and pressure vessels in the Unit- tions apply to those titles as well. A boiler plant that is
ed States, Canada, and much of the world. According to only used for heating but operates at steam pressures
the code a boiler is a vessel in which a liquid is heated above 15 psig or heats water to a temperature greater
or a vapor is generated under pressure by application of than 250°F is a high pressure plant with power boilers.
heat from the products or combustion or another source. A low pressure boiler could be used to power a steam
Vessel is the code word for an enclosed container under engine to generate electricity but it is still called a low
pressure. pressure boiler or heating boiler, the use has no bearing
Now let’s get the meaning of pressure straight. on the definition of the boiler.
You’ll encounter a large number of people in your career A superheated steam boiler is any boiler that raises
that have their own idea of what is low pressure and the temperature of the steam above saturation tempera-
high pressure then we all get to disagree on what we ture. It’s possible that low pressure steam could be su-
mean when we say medium pressure. The BPVC in its perheated but virtually all superheated steam boilers are
various documents defines high pressure and low pres- power boilers. On rare occasions you will encounter a
sure but never addresses the term medium pressure. separately fired superheater which is also a power boiler
High pressure boilers are defined by ASME in the by definition in Section I of the code.
first document prepared to address the construction of One other definition that isn’t clearly defined in
boilers and pressure vessels which is now known as the code but is commonly used is “High Temperature
Section I of the BPVC and it’s simply titled “Rules for Hot Water” abbreviated HTHW. When we talk about
Construction of Power Boilers.” That is a roman nu- these plants we typically say the initials rather than
meral one, not a capital letter i. All sections of the code the words. An HTHW boiler is simply a power or high
are numbered using roman numerals. Within section I pressure boiler that heats water rather than generating
a high pressure boiler is defined as a steam boiler that steam.
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