Pipe Stresses: Its Types and Difference

Piping stress analysis is an analytical method to determine all the different stresses generated in a piping system. What causes stress in the piping system? Sources such as weight, internal/external pressure, temperature, loads, or forces that produce stress in the piping system.

In this article, you will discover the main forces responsible for the generation of stress in the piping system.

The two distinct groups of forces include:

1. Primary loads
2. Secondary loads

What is Primary Load?

Primary loads are the force-driven loads that occur from sustained types of stress. These incorporate gravitational forces, relief valve discharge, internal or external fluid pressure, spring forces, seismic loads, and more. Hence, some kind of forces or loads acting on the piping system develops first loads.

What is Secondary Load?

Secondary loads are movement driven loads. The different types of imposed displacement in the piping system includes thermal expansion, anchor or restraint movements, vibration, and more. The secondary loads are often cyclic in nature, but not always. (For example, tank settlement).

For instance, the pipe connected to a storage tank may be under stress if the tank nozzle to which it is attached moves down due to tank settlement. Also, the pipe may vibrate due to the vibrations caused by the rotating equipment attached to it.

Let’s discover the significant differences between Primary Loads and Secondary Loads based on various characteristics:

1. Occurrence:

a. Primary Load: Force driven (e.g., gravity, pressure, spring forces, and more)

b. Secondary Load: Displacement driven (e.g., thermal expansion, settlement, vibration, and more)

2. Self-Limiting Nature:

a. Primary Load: Primary stresses are not self-limiting, and failure can occur on a single application of load. Once plastic deformation starts, it continues until force equilibrium is achieved or cross-section fails. Failure can be avoided only by the removal of loading or strain hardening in the material.

b. Secondary Load: Secondary loads are self-limiting. These loads tend to dissipate as the system deforms through yielding or deflection.

3. Cyclic Nature:

a. Primary Load: Non-cyclic

b. Secondary Load: Cyclic (exception: settlement)

4. Failure Modes:

a. Primary Load: Catastrophic, quick, and sudden failure based on one or more advanced theories by Von Mises, Tresca, or Rankine.

b. Secondary Load: Failure is time- consuming, fatigue, and non-catastrophic in nature.

5. Application of load:

a. Primary Load: A single application of excessive primary load may cause design failure through gross plastic deformation and rupture. Failure may occur in primary loads with a single application of the stress.

b. Secondary Load: A single application of load never produces failure in secondary stress. Usually, the high number of applications of load causes failure.

6. Load Duration:

a. Primary Load: Few primary loads are sustained types of load that remains present throughout the plant life cycle.

b. Secondary Load: Secondary loads are generally present only when the plant is operating.

Conclusion

Piping systems are essential for various processing plants and facilities. So it is necessary to design and fabricate pipes with precision and care. Stress analysis of piping systems is critical for piping design. It helps predict stresses in piping and loads on equipment from weights, pressure, thermal gradients, and more.