Design bases are the physical attributes, loading and service conditions, environmental factors, and materials-related factors which must be considered in the detailed design of a piping system, to ensure its pressure integrity over its design life.
Physical Attributes
Physical attributes are those parameters that govern the size, layout, and dimensional limits or proportions of the piping system. Dimensional standards have been established for most piping components such as fittings, flanges, and valves, as well as for the diameter and wall thickness of standard manufactured pipe. Those standards are identified in the section ‘‘Use of Codes and Standards in Piping System Design.’’
Certain types of piping systems require special design practices for configuration control, to ensure constructibility, or in-service performance. For example, high temperature, high-pressure piping systems are usually designed with weld joints spaced a minimum of one pipe diameter apart to facilitate radiographic examination of the joints. Steam and wet-gas systems are designed to maintain the pipe runs at some minimum pitch to ensure adequate drainage of condensate or other liquids that may separate from the gas stream. Pipelines that are subject to frequent plugging should be designed with adequate clearance, and mechanically joined, to allow for ready dis-assembly and maintenance.
Loading and Service Conditions
Loading conditions, or loads, are forces, moments, pressure changes, temperature changes, thermal gradients, or any other parameters that affect the state of stress of the piping system. Typical examples of loading conditions include internal pressure, piping system deadweight, steady-state or transient temperatures, wind loads, or snow and ice loads. Loads may be external to the piping system, such as environmental temperature changes or wind loads; or they may be internal to the system, such as internal fluid pressure or temperature changes.
Service conditions are combinations of loads or load sets that occur simultaneously, therefore the piping system must be designed to withstand their combined effects. Occasionally, the service conditions will be specified by the piping design code. Examples are found in the American Society of Mechanical Engineers’ (ASME) Boiler and Pressure Vessel Code, Section III, Nuclear Power Plants Components, Paragraph NCA-2142, where service conditions are defined directly, and the ASME Code for Pressure Piping, ASME B31.1, Power Piping, Paragraph 104.8, where service conditions are specified under the topic of analysis of piping components due to the effects of sustained and occasional loads.
Where service conditions are not specified by a particular code, the designer should review the various loading conditions that the piping system is exposed to and formulate the combinations that must be considered in design. Reference to the most commonly used piping design codes listed in the section ‘‘Use of Codes and Standards in Piping System Design’’ will provide the designer with guidance in setting appropriate design stress limits.
Environmental Factors
The term environmental factors refers to operating conditions that result in progressive physical or chemically induced deterioration of the piping system which can ultimately lead to a breach of the pressure boundary or a gross structural failure. Failures that are the result of environmental factors are usually slow to progress and frequently involve localized areas of the piping system. The most common examples of environmental factors include corrosion, erosion, and physical damage. While corrosion and erosion mechanisms can act independently, a combined reaction known as erosion/corrosion, or flow-assisted corrosion, frequently occurs in wet-steam and water piping systems constructed of plain-carbon steels.
Materials-Related Considerations
Materials-related considerations are the specific chemical, metallurgical, and physical properties of a piping system’s material constituents that can ultimately determine its suitability for a particular service. Proper materials selection can be a crucial design consideration that will determine the adequacy of performance of a piping system where extremes of temperature, chemical attack, or erosion are significant factors in its operation.
Pressure Integrity
Pressure integrity is the maintenance of a leak-tight condition in piping systems’ pressure-containing boundaries coincident with the control of the level of stress or strain within predefined criteria limits. Pressure integrity is not synonymous with leakage integrity; the latter is only an assurance of a leak-tight condition without regard for the state of stress or structural stability of the pressure boundary. Maintenance of the pressure integrity of a piping system, within predefined criteria limits, is a major objective of the design process.
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