Section VIII – Division 2 Criteria and Commentary
|Publication Date:||10 June 2009|
Part 1, paragraph 1.2 defines the scope of coverage for VIII-2. The term scope refers to both the type of pressure equipment being considered in the development of these rules, as well as the geometric scope of the vessel that is stamped as meeting VIII-2.
In accordance with Part 1, paragraph 188.8.131.52, pressure vessels are defined as containers for the containment of pressure, internal or external. This pressure may be obtained from any external source, or by the application of heat from a direct or indirect source, as a result of a process, or any combination thereof.
The manner in which the scope of the standard is described follows very closely to the introduction section of VIII-1. In the following paragraphs, a discussion of requirements is provided only where a significant difference exists between VIII-2 and the scope definition from VIII-1, or where a major change was made from Old VIII-2.
With regard to pressure vessels installed in non-stationary applications, Part 1, paragraph 184.108.40.206.b now permits stamping of VIII-2 vessels installed on motor vehicles and railway cars. This particular application was prohibited in the Old VIII-2. Construction and stamping of VIII-2 vessels in nonstationary applications requires a prior written agreement with the local jurisdictional authority covering operation and maintenance control for a specific service. This operation and maintenance control must be retained during the useful life of the pressure vessel by the user in conformance with the Users Design Specification.
Part 1, paragraph 220.127.116.11.e.3 defines pressure vessels in which steam is generated but which are not classified as Unfired Steam Boilers that require construction in accordance with the rules of Section I or VIII-1. A third category for a vessel that generates steam that may be constructed to VIII-2 was added: vessels in which steam is generated but not withdrawn for external use.
One significant difference between VIII-2 and the Old VIII-2 is special service vessels such as those in lethal service. In Old VIII-2, paragraph AG-301.1(c) , the user and/or his designated agent had to define in the UDS if a vessel was intended for lethal service. If lethal service was specified, then additional technical requirements (e.g. enhanced NDE, restrictions on material, etc.) were imposed on this vessel. In VIII-2, additional requirements are not specified for lethal service or any other special service condition. The rationale behind this change is that the user and/or his designated agent are responsible to describe in the UDS (see Part 2, paragraph 2.2.2), the intended operation of the vessel, and if a vessel is intended for a service that is dangerous to life and property, then the user should specify any additional requirements to mitigate the risks. Just as it has been the rule in ASME that its standards would not define when a vessel is in lethal service, what additional requirements would be appropriate for any given vessel are best defined by the user, and not by the Committee.
Additional Requirements for Very High Pressure Vessels
The rules of VIII-2 do not specify a limitation on pressure but are not all-inclusive for all types of construction. For very high pressures, additions to these rules may be required to meet the design principles and construction practices essential to vessels for such pressures. However, only in the event that, after application of additional design principles and construction practices, the vessel still complies with all of the requirements of the Code, may it be stamped with the Code symbol. As an alternative to VIII-2, it is recommended that VIII-3 be considered for the construction of vessels intended for operating pressures exceeding 68.95 MPa (10,000 psi).
Geometric Scope of This Division
Similar to the Introduction chapter of VIII-1, the description of pressure equipment covered by the scope of VIII-2 is handled by listing the type of equipment that is not covered under the scope of the standard. However, there is one significant difference between VIII-1 and VIII-2 in this regard. Both standards will allow a pressure vessel that is otherwise outside the scope of the standard to be stamped with the appropriate code symbol stamp so long as all of the applicable requirements of the standard are satisfied. In VIII-1 this includes vessels that are otherwise covered under the scope of another standard. However, in Part 1, paragraph 18.104.22.168, if a pressure vessel is included in the scope of another ASME code section, then it may not be constructed and stamped (U2) per VIII-2. The rationale for this has to do with the fact that the rules in any ASME standard are developed by experts in a particular field or type of equipment. In the case of VIII-2, experts in the fields of design, fabrication, inspection and testing of pressure vessels developed the rules in this standard. For example, the developers of VIII-2 were not experts in the construction of power boilers, thus it was deemed inappropriate to allow a power boiler to be certified to VIII-2 even if it did comply with all of its rules.
Classifications Outside the Scope of this Division
Vessels that are not included in the scope of VIII-2 are shown below. Again with the exception of subparagraph a), below, all of the remainder of the use exempted vessels may be constructed and stamped to VIII-2 if all of the applicable requirements are met. But similar to VIII-1, the Local Jurisdictional Authority at the location of an installation of a vessel establishes the mandatory applicability of the Code rules.
a) Vessels within the scope of other ASME B&PV Code Sections but not other design Codes (e.g., EN 13445, BSI PD-5500, etc.).
b) Fired process tubular heaters as defined in API RP 560.
c) Pressure containers that are integral parts or components of rotating or reciprocating mechanical devices.
d) Structures consisting of piping components whose primary function are the transport of fluids from one location to another within a system.
e) Pressure containing parts of components, such as strainers and devices that serve such purposes as mixing, separating, snubbing, distributing or controlling flow, provided that pressure containing parts of such components are generally recognizes piping components or accessories.
f) A vessel for containing water under pressure where the design pressure does not exceed 2.07 MPa (300 psi) and the design temperature does not exceed 99°C (210 °F).
g) A hot water supply storage tank heated by steam or any other indirect means, and the heat input does not exceed 58.6 kW, the water temperature does not exceed 99°C (210 °F), and the nominal water containing capacity does not exceed 454 L.
h) Vessels with an internal or external design pressure not exceeding 103 kPA (15 psi).
i) Vessels with an inside diameter, height or cross-section diagonal not exceeding the 150 mm (6 in).
j) Pressure vessels used for human occupancy.
When a pressure vessel unit consists of more than one independent pressure chamber, only the parts of chambers that come within the scope of this Division need be constructed in compliance with its provisions (see Part 4, paragraph 4.1.8).
Field Assembly of Vessels
The rules for field assembly of vessels are given in Part 1, paragraph 1.2.6, and they are essentially unchanged from the rules that were published in the Old VIII-2.
Pressure Relief Devices
The scope of this Division includes provisions for pressure relief devices necessary to satisfy the requirements of Part 9.