Stresses in a piping system

Stresses in a piping system

The allowable stresses or tensions are defined in terms of the mechanical resistance properties of the material obtained in tensile tests for different temperature levels and a global safety factor. In physical terms, the mechanical stress of a pipe is the force per unit area, usually expressed in PSI or KP. Another good definition of stress is the ability of a tube to withstand an applied load without failure or plastic deformation. Normally stresses are calculated at specific points in a piping system, for example, an elbow (change of direction), a support point (support), a specific fitting (tee, flange).

Design rules

The standards used in the analysis of piping systems are the joint standards of the American National Standards Institute, and the American Society of Mechanical Engineers ANSI/ASME.

Each of these codes gathers the experience of numerous specialized companies, researchers, project engineers and field engineers in specific application areas, among which they apply:

• B 31.1 Power piping and steam systems.
• B 31.3 Process piping.
• B 31.4 Transport of liquid hydrocarbons.
• B 31.8 Gas transmission and distribution pipes.
• ASME BPVC (Section II, V, VIII). General requirements.

ASME B 31.1 and B 31.3 standards stipulate two criteria for allowable stress. One is the so-called “basic allowable stress” in tension at the design temperature, with which those involved in the design of pressure equipment are familiar. The other is less known and is called “admissible stress range”, it is derived from the basic allowable stress and is used as a basis for the calculation of thermal expansion and for flexibility analysis.

The range of allowable stress or tension is calculated as follows:

• By support: SA = Sh.
• By dilation: SA = f (1.25 Sc + 0.25 Sh)
• By expansion and support: SA = f (1.25 Sc + 0.25 Sh)+ Sh

Where:

• SA = Maximum allowable stress.
• Sc = Allowable basic stress for the material at the minimum expected temperature of the material (normally room temperature) during a displacement cycle. (Appendix A ASME code B31.1)
• Sh = Allowable basic stress for the material at the maximum expected temperature of the material during a displacement cycle. (Appendix A ASME code B31.1)
• f = Reduction factor of the allowable stress range for cyclic conditions according to the total number of complete cycles of temperature changes over the expected useful life. f = 6 / N^0.2 < 1.0

Programs such as CAESAR II, ALGOR PIPEPAK, AUTO PIPE, TRIFLEX and BENTLEY are available to run detailed stress analysis in pipe systems with many branches. CAESAR II is the most advanced and used program in the world for the analysis of stresses and loads in pipes. This program is developed for the WINDOWS environment and its main function is the modeling, analysis and design of piping systems.