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Companion Guide to the ASME Boiler and Pressure Vessel Codes - Volume 1

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Organization: ASME
Publication Date: 1 January 2018
Status: active
Page Count: 854

Section I applies to several types of boilers and components of boilers, such as economizers, superheaters, reheaters, and in some circumstances feedwater heaters. Although its title is Power Boilers, the scope of Section I is somewhat broader. The Preamble to Section I explains that it covers power boilers, electric boilers, miniature boilers, high-temperature water boilers, heat recovery steam generators, solar receiver steam generators, some fired pressure vessels, and organic fluid vaporizers. Since the precise definitions of these various types of boilers are not generally known, the following definitions, found in footnotes to the preamble, are helpful:

(1) Power boiler-a boiler in which steam or other vapor is generated at a pressure of more than 15 psi (100 kPa) for use external to itself.

(2) Electric boiler-a power boiler or a high-temperature water boiler in which the source of heat is electricity.

(3) Miniature boiler-a power boiler or high-temperature water boiler in which the limits in PMB-2 are not exceeded.

(4) High-temperature water boiler-a water boiler intended for operation at pressures in excess of 160 psi (1.1 MPa) and/or temperatures in excess of 250°F (120°C).

(5) Heat Recovery Steam Generator (HRSG)-a boiler that has as its principal source of thermal energy a hot gas stream having high ramp rates and temperatures such as the exhaust of a gas turbine.

(6) Solar receiver steam generator-a boiler system in which water is converted to steam using solar energy as the principal source of thermal energy. The solar energy is typically concentrated onto the solar receiver through the use of an array of mirrors that focuses solar radiation on the heat transfer surface.

(7) Fired pressure vessel-reheaters, isolable superheaters, economizers located outside the limits of boiler external piping, and non-integral separately fired superheaters.

Section I doesn't provide an explicit definition of an organic fluid vaporizer, which is a boiler-like device that uses an organic fluid instead of steam as the working fluid. However, the last paragraph of the Preamble states that a pressure vessel in which an organic fluid is vaporized by the application of heat resulting from the combustion of fuel shall be constructed under the provisions of Section I. (Those provisions are found in Part PVG.) Thus, so far as Section I is concerned, an organic fluid vaporizer is a boiler-like device in which an organic fluid is vaporized as just described. (Note that a key factor is the vaporization of the organic fluid. If the organic fluid is merely heated without vaporizing, the device does not fall within the scope of Section I; it might fall instead under the scope of Section VIII as a pressure vessel.) The Preamble then provides a notable exception to the Section I definition of an organic fluid vaporizer: "Vessels in which vapor is generated incidental to the operation of a processing system, containing a number of pressure vessels such as are used in chemical and petroleum manufacture, are not covered by the rules of Section I." Again, if Section I rules do not cover these vessels, what rules do? The answer is the rules of Section VIII, Pressure Vessels. Those rules cover all kinds of pressure vessels, including in some cases fired pressure vessels.

Although the origin of the above exception to Section I dominion is uncertain, a possible explanation can be surmised. Note that the vessel in question would probably be used in a chemical plant or petroleum refinery. Such plants are normally owned and operated by large companies with a capable engineering staff and well-trained operators. Those companies can usually demonstrate a good record of maintenance and safety. Furthermore, vaporizing organic liquids inside pressure vessels is a routine matter for them. They might also argue that it really does not make much difference whether a properly designed vessel is built to the rules of Section I or Section VIII, nor does it matter whether the source of heat is from direct firing, from hot gases that may have given up some of their heat by having passed over a heat-transfer surface upstream, or from a hot liquid that is being processed. There are also economic reasons why an Owner might prefer a Section VIII vessel over a Section I vessel, as explained later in the discussion of fired versus unfired boilers.

The Preamble does not explain the precise meaning of "vapor generation incidental to the operation of a processing system." It apparently means the generation of vapor in a vessel or heat exchanger that is part of a processing system in a chemical plant or petroleum refinery where this vapor generation is only a minor or secondary aspect of the principal business of the plant, such as refining oil. Thus, certain equipment normally constructed to Section I rules could, under this exception, be constructed instead to the rules of Section VIII, provided the appropriate authorities in the jurisdiction where the equipment is to be installed have no objection. These so-called jurisdictional authorities have the last word in deciding which Code section applies (see How and Where Section I is Enforced and Effective Dates in Section 1.6).

There is also some imprecision in the use and meaning of the term power boiler. This term is sometimes understood to mean a boiler with steam that is used for the generation of power, as opposed, for example, to a boiler with steam that is used for chemical processing or high-pressure steam heating. However, according to the definition in the Preamble, a boiler that generates steam or other vapor at a pressure greater than 15 psi for external use is considered by Section I to be a power boiler, irrespective of how the steam might be used. Although exceptions exist, most jurisdictional authorities follow the ASME Code in defining and categorizing boilers and pressure vessels.

Note from the definition of a power boiler that the steam or other vapor generated is for use external to the boiler. This is supposed to distinguish a power boiler from certain other pressure vessels, such as autoclaves, that may similarly generate steam or vapor at a pressure greater than 15 psi but not generally for external use. These pressure vessels, often used as process equipment in the chemical and petroleum industries, and for cooking or sterilization in other industries, are designed to meet the rules of Section VIII.

From the Preamble definitions, it is apparent that a hightemperature water boiler, which generally produces pressurized hot water for heating or process use, is not considered a power boiler. However, as a practical matter, the particular characterization of a device by Section I as a power boiler or something else is less important than the fact that it is indeed covered by Section I rules.

The Preamble explains that the scope of Section I covers the complete boiler unit, which is defined as comprising the boiler proper and the boiler external piping. This very important distinction needs further explanation. The term boiler proper is an unusual one, chosen to distinguish the boiler itself from its external piping. The boiler proper consists of all the pressure parts comprising the boiler, such as the drum, the economizer, the superheater, the reheater, waterwalls, steamgenerating tubes known as the boiler bank, various headers, downcomers, risers, and transfer piping connecting these components. Any such piping connecting parts of the boiler proper is called boiler proper piping. The boiler external piping is defined by its extent: it is the piping that begins at the first joint where the boiler proper terminates and extends to and includes the valve or valves required by Section I.

The importance of all these definitions and distinctions is that the construction rules that apply to the boiler proper and boiler proper piping are somewhat different from those that apply to the boiler external piping. This is explained in Section 1.5, Distinction Between Boiler Proper Piping and Boiler External Piping.

The Preamble also explains that the piping beyond the valves required by Section I is not within the scope of Section I. Thus these valves define the boundary of the boiler, and Section I jurisdiction stops there. Note that the upstream boundary of the scope of Section I varies slightly, depending on feedwater valve arrangements. Different valve arrangements are required for a single boiler fed from a single source, as opposed to two or more boilers fed from a common source, with and without bypass valves around the required regulating valve (see the solid and dotted lines shown in Figure PG-58.3.1(a), reproduced in this chapter). The required boiler feed check valve is typically placed upstream of the feed stop valve, except that on a single boiler-turbine unit installation, the stop valve may be located upstream of the check valve. Changes in the Section I boundary can have some significance in the choice of design pressure for the feed water piping and valves, which is governed by the rules of B31.1, Power Piping (see paragraph 122.1.3).

An exception to this coverage of boiler piping is found in the treatment of the hot and cold reheat piping between the boiler and a turbine, (see Figure PG-58.3.1(c), reproduced in this chapter) which is excluded from the scope of Section I. Occasionally, someone asks how and why reheat piping was left outside the scope of Section I. The explanation offered some years ago by a senior member of the Committee is as follows: Although reheaters date back to the earliest days of Section I, the rising steam pressures employed in large utility boilers in the 1940s led to their increased use. The reheat piping became larger, heavier, and more complex. In those days, the General Electric Company and Westinghouse made virtually all of the turbines used in the United States, and it was customary for those turbine manufacturers to take responsibility for the design of the reheat piping. Whenever some Committee members suggested that it might be time to consider bringing reheat piping into the scope of Section I, those two companies objected. On the basis of their long, successful experience, they convinced the Committee that such a change was unnecessary and that they were perfectly capable of designing that piping. Thus the reheat piping remained outside the scope of Section I.

In the late 1980s, failures of hot reheat piping occurred at two major utilities, with injuries and loss of life. This reopened the question of whether Section I should cover reheat piping and whether the failed piping, designed in the 1960s, might not have failed had it been within the scope of Section I. (Current Section I rules call for all boiler components to be manufactured, inspected, certified, and stamped under a quality control system, a requirement imposed in 1973. Moreover, any piping within the scope of Section I is generally inspected yearly, with the rest of the boiler, although finding potential leaks or failures under the insulation is not so readily accomplished.) However, investigators were not able to agree on the causes of the failures, and the idea of bringing the reheat piping within Section I jurisdiction died for lack of support.

Code coverage of this reheat piping and also of piping beyond the boiler external piping varies. Normally in power plant design, the owner or the architect engineer will select the B31.1, Power Piping Code, for the reheat piping, and either that Code or the B31.3, Process Piping Code [14] (formerly called the Chemical Plant and Petroleum Refinery Piping Code), for the piping beyond the boiler external piping. The reader should also note that the jurisdiction (state or province) may have laws mandating use of one or the other of these two Codes to cover piping that isn't within the scope of Section I.

Some unfired steam boilers are constructed to the requirements of Section VIII, as permitted by the preamble. Section VIII rules deal only with the vessels themselves; they do not deal with any piping attached to the vessels. In such cases, the choice of an appropriate design Code for the piping may be left to the plant designers; otherwise, the jurisdictional authorities may mandate a particular piping Code.

Document History

January 1, 2018
Companion Guide to the ASME Boiler and Pressure Vessel Codes - Volume 1
Section I applies to several types of boilers and components of boilers, such as economizers, superheaters, reheaters, and in some circumstances feedwater heaters. Although its title is Power...
January 1, 2012
Companion Guide to the ASME Boiler and Pressure Vessel and Piping Codes - Volume 1
INTRODUCTION This fourth edition is in two volumes composed of 11 Parts, with Parts 1–5 in Volume 1, Parts 6–11 in Volume 2. Common to both volumes is the front matter, including the Organization of...