Types of Stresses in Piping Systems

Primary, Secondary and Occasional Loads

From a piping stress analysis point of view the following are the main loads to be considered for the design

Primary load occurs from Sustained loads like dead weight, live weight, internal pressure etc. and are called non-self-limiting loads. Pressure thrust from an expansion joint is used in this article.

Secondary loads occur from thermal expansion loads like temperature change, anchors and restraints etc. and are called self-limiting loads. 

Thermal expansion in a horizontal pipe loop is used in this article.

Occasional loads occur from static wind and seismic loads and are considered to act occasionally. Seismic load on a vertical pipe loop is used in this article.

Primary Stress

Primary Stress is generated by internal and external force and moments. Primary stress is not self limiting – even if a part moves,the load causing it does not reduce. In this example, an expansion joint without restraining hardware creates a primary stress on a pipe.

Pipe loops with tied and untied expansion joints.The foreground joint is 

tied – it has tie rods to prevent axial growth of the expansion joint. The 

.background joint has no tie rods

Deflection of the two expansion joints under pressure. The tie rods limit the expansion of the tied joint. The untied joint increases in length the same as if it is a hydraulic cylinder applying bending stresses to the pipe

Pipe stress as reported by Caesar. The untied joint is applying a bending force, which, depending on the stress level, Caesar can report as a fail. 

This design does not meet the expansion joint manufacturers 

requirements for guiding and anchoring. The pipe with the tied joint is okay.

***This primary stress is caused by pressure of the fluid multiplied by the area of the pipe. It occurs all the time the system is pressurized. No matter how much the pipe displaces, the untied bellows keeps pushing on it

Because primary stresses are not relieved by the piping moving or yielding, primary stress limits are set lower than other allowable stresses. For example, if primary stresses managed to get above the yield point, the piping would balloon out and explode. The piping codes keep the primary stresses below the yield point by a factor of safety.

Secondary Stress *

Thermal expansion and contraction happens when a pipe heats up and 

cools down. The piping system must have enough flexibility to handle 

.the expansion