Pipe stressing

Piping systems must be designed so as not to impose unacceptable stresses on the equipment to which they are connected. 

The weight of the pipes, their contents, insulation and any ancillary equipment. 

Loads imposed by the operation of ancillary equipment, such as relief valves. 

Thermal expansion is a major factor to be considered in the design of piping systems. The reaction load due to pressure drop will normally be negligible. The dead-weight loads can be carried by properly designed supports. 

Flexibility is incorporated into piping systems to absorb the thermal expansion. A piping system will have a certain amount of flexibility due to the bends and loops required by the layout. If necessary, expansion loops, bellows and other special expansion devices can be used to take up expansion. 

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The most recent developments in computational stmctural analysis are almost all based on the direct stiffness matrix method. As a result, piping stress computer programs such as SIMPLEX, ADLPIPE, NUPIPE, PIPESD, and CAESAR, to name a few, use the stiffness method. 

The bracket construc tion permits support of the exchanger without fixing the supports to the shell. This provides for thermal movement of the shells within the brackets and prevents the transfer of thermal stresses into the process piping. In special cases the brackets may be welded to the shell. However, this is usually avoided due to the resulting loss of flexibihty in field installation and equipment reuse at other sites and an increase in piping stresses. 

A typical cause of piping strain occurs when two flanges do not meet and pipefitters force them together. Pipe hangers that are poorly placed or tensioned can also cause significant piping stress problems. 

Figure 7-9C. Armored graphite disk. Note steel ring bonded to circumference of disk to increase safety in toxic or fiammable services and improve reliability by preventing unequal piping stresses from reaching the pressure membrane. Teflon coatings or linings are available on the entire disk. By pemnission, Zook Enterprises.

Although it is not the intent of this chapter to delve into pipe stress and vibration analysis, a few alerting comments appear to he in order. From this, the design engineer should he ahle to seek the proper qualified technical help to analyze the stresses associated with the piping layout. 

Efforts should be made to eliminate the use of expansion joints in process piping. However, if needed, the expansion joints are used to mitigate the pipe stresses caused by large thermal movements. Table 7-9 lists the recommended mechanical design criteria for expansion joints. 

Designers unfamiliar with plastic products can use the suggested preliminary safety factor guidelines in Table 2-11. They provide for extreme safety. Any product designed with these guidelines in mind should conduct tests on the products themselves to relate the guidelines to actual performance (Chapter 4, RP PIPES, Stress-Strain Curves). With more experience, more-appropriate values will be developed targeting to use 1.5 to 2.5. After field service of... 

A discussion of the methods used for the calculation of piping flexibility and stress analysis are beyond the scope of this book. Manual calculation techniques, and the application of computers in piping stress analysis, are discussed in the handbook edited by Nayyar et al. (2000). 

The casings should include a minimum of a Vs-in corrosion allowance. Casing strength and rigidity should limit change of alignment to 0.002 in if it is caused by the worst combination of pressure, torque, or allowable piping stress. 

Pipe stress and pressure vessel analysis Pressure, dead-weight, thermal expansion, vibration modal analysis of fatigue. Analysis to ensure that the piping and pressure vessels conform to the codes of ASME, API or WRC as the case may be... 

Thermal expansion and the resultant pipe stresses must be considered in any piping system design. For example, if the temperature changes from 50 to 600°F, the increase in length would be 4.9 in. per 100 ft for steel pipe and 7.3 in. per 100 ft for brass pipe. This amount of thermal expansion could easily cause a pipe or wall to buckle if the pipe were fastened firmly at each end with no allowances for expansion. The necessary flexibility for the piping system can be provided by the use of expansion loops, changes in direction, bellows joints, slip joints, and other devices. 

Computers have taken the hand calculation work out of piping stress analysis. However, because of time and cost, the computer is not available for many simple two-anchor pipe loops. 

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