Primary Loads & Secondary Loads

Primary Loads & Secondary Loads in Piping Stress Analysis – Simple Explanation

In piping stress analysis, understanding different types of loads is extremely important. Not all loads behave in the same way, and not all loads cause failure in the same manner. To properly design a safe piping system, engineers must clearly understand the difference between primary loads and secondary loads.

This article explains Primary Loads & Secondary Loads in very simple language. It is written for students, fresh piping engineers, and working professionals who want a clear and practical understanding of this concept.

Why Load Classification Is Important

Piping systems are subjected to many forces during operation. These forces come from pressure, weight, temperature changes, equipment movements, and environmental effects. If all these loads are treated in the same way, the design may become unsafe or overly conservative.

To avoid this, piping design codes classify loads into different categories based on how they act on the system and how they cause failure. The most important classification is:

• Primary Loads
• Secondary Loads

This classification helps engineers apply the correct stress limits and failure criteria during design and analysis.

Background of Primary and Secondary Load Concept

The concept of primary and secondary loads comes from studies on fatigue and failure behavior of piping systems. Researchers observed that piping failures do not occur only due to one-time overloads. Many failures happen after repeated application of certain types of loads.

Based on this understanding, two different design criteria were developed:

• One criterion for loads that can cause immediate or catastrophic failure
• Another criterion for loads that cause fatigue damage over time

These two criteria correspond to primary loads and secondary loads.

What Are Primary Loads?

Primary loads are loads that are directly caused by external forces acting on the piping system. These loads are force-controlled and do not reduce automatically when the pipe deforms.

In simple words, if a pipe is subjected to a primary load, the load will continue to act even if the pipe starts yielding or deforming.

Characteristics of Primary Loads

1. Primary Loads Are Force-Driven

Primary loads are generated by direct forces acting on the piping system. Common examples include:

• Pipe weight due to gravity
• Internal pressure
• Spring support forces
• Relief valve thrust
• Fluid hammer forces

These loads are applied regardless of how much the pipe deforms.

2. Primary Loads Are Not Self-Limiting

One of the most important characteristics of primary loads is that they are not self-limiting.

This means that once plastic deformation starts, the load does not reduce automatically. The deformation will continue until:

• Force equilibrium is achieved due to boundary changes
• Material strain hardening occurs
• Or the pipe fails completely

Because of this behavior, primary loads are very dangerous if not properly controlled.

3. Primary Loads Are Usually Not Cyclic

Primary loads are generally steady or sustained in nature. They do not usually repeat in cycles.

Some loads like pressure fluctuation or pulsation may appear cyclic, but such loads often show characteristics of both primary and secondary loads.

4. Allowable Limits for Primary Loads

Allowable limits for primary stresses are based on classical failure theories such as:

• Von Mises theory
• Tresca (maximum shear stress) theory
• Rankine theory

These limits are directly related to:

• Yield stress of the material
• Ultimate tensile strength
• Creep rupture properties (for sustained high-temperature loads)

In other words, primary stress limits are designed to prevent yielding and rupture.

5. Failure Due to Primary Loads

Excessive primary load can cause:

• Gross plastic deformation
• Sudden rupture
• Catastrophic failure

Such failures can occur with a single application of the load.

In piping systems, failures caused by internal pressure (hoop stress) are typical examples of primary load failures. These failures are usually controlled by:

• Minimum wall thickness requirements
• Pressure design rules in piping codes

They are generally not checked by flexibility or stress analysis software.

What Are Secondary Loads?

Secondary loads are loads that arise due to imposed displacements rather than applied forces. These loads develop because the piping system is restrained and cannot freely deform.

Unlike primary loads, secondary loads reduce as the pipe deforms.

Characteristics of Secondary Loads

1. Secondary Loads Are Displacement-Driven

Secondary loads are caused by imposed movements or deformations. Common examples include:

• Thermal expansion of pipes
• Imposed anchor movements
• Equipment nozzle displacements
• Settlement of supports
• Vibration-induced movements

These loads occur because the piping system is restrained and cannot freely expand or move.

2. Secondary Loads Are Self-Limiting

One of the most important features of secondary loads is that they are self-limiting.

As the pipe starts yielding or deforming, the stress caused by secondary loads reduces automatically. The load relaxes through:

• Elastic deformation
• Plastic deformation
• Redistribution of stresses

Because of this behavior, secondary loads do not usually cause immediate failure.

3. Secondary Loads Are Usually Cyclic

Secondary loads are typically cyclic in nature. They repeat many times during:

• Startup and shutdown cycles
• Temperature fluctuations
• Operational transients

An exception is settlement, which may not be cyclic.

4. Allowable Limits for Secondary Loads

Allowable limits for secondary stresses are based on:

• Fatigue failure modes
• Elastic cycling requirements
• Material fatigue curves

The goal is to ensure that the pipe can withstand repeated stress cycles without developing fatigue cracks.

5. Failure Due to Secondary Loads

A single application of a secondary load will never cause failure.

Failure due to secondary loads occurs only after a large number of load cycles. Such failures are typically:

• Fatigue cracking
• Leakage near fittings
• Cracks at welds or stress concentration points

Even if a piping system has been operating successfully for many years, it does not automatically mean that it was properly designed for secondary loads. Fatigue damage accumulates slowly and may appear suddenly after long service.

Key Differences Between Primary and Secondary Loads

Aspect	Primary Loads	Secondary Loads
Nature	Force-controlled	Displacement-controlled
Self-limiting	No	Yes
Failure type	Yielding or rupture	Fatigue failure
Cyclic behavior	Usually non-cyclic	Usually cyclic
Design basis	Yield / strength limits	Fatigue limits

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Practical Importance in Piping Design

Understanding primary and secondary loads helps engineers:

• Apply correct stress limits
• Design proper flexibility in piping
• Avoid over-design or unsafe design
• Interpret stress analysis results correctly

Most piping stress analysis software evaluates primary and secondary stresses separately based on code requirements.

Conclusion

Primary and secondary loads behave very differently and must be treated differently in piping stress analysis. Primary loads are dangerous because they can cause immediate failure, while secondary loads are fatigue-related and cause damage over time.

A safe and reliable piping system design requires proper understanding and correct evaluation of both load types.

For every piping stress engineer, mastering this concept is essential for code compliance and long-term system safety.