Classifying Piping System Loads

A World of Forces

Before we dive into the code's specific terminology, it's helpful to build an intuitive understanding of the physical phenomena we need to account for. A piping system in an industrial plant is not a static object; it is a dynamic component subjected to a variety of internal and external forces.

To get a sense of these real-world effects, the following video provides excellent qualitative examples of the issues that pipe stress analysis is designed to solve.

The video illustrates that a piping system must be designed to handle everything from its own weight and the pressure of its contents to environmental forces like wind and seismic events. The code provides a systematic framework for analyzing these effects by grouping them into categories.

The Three Major Load Categories

ASME B31.3 groups loads based on their duration and nature. This is because the failure mechanism associated with each load type is different. A constant, heavy load might cause a pipe to slowly sag and eventually collapse (a primary failure), while a repeated heating and cooling cycle might lead to a fatigue crack over many years (a secondary failure).

The following article provides clear definitions for these categories. We will use it as our guide.

Let's synthesize the information from the reading and connect it to the code's requirements.

1. Sustained Loads

• Definition: These are loads that are present continuously throughout the normal operation of the piping system. They are considered primary loads because they are necessary to satisfy static equilibrium. If not properly managed, they can lead to gross failure (rupture or collapse).
• Source Examples:
  • Weight: The dead weight of the pipe, fluid, insulation, valves, and any other permanent attachments. This is often abbreviated as 'W' or 'GR' (Gravity) in stress analysis reports.
  • Pressure: The internal design pressure. While pressure creates hoop stress (which we use for wall thickness calculations), it also creates a longitudinal stress that is considered a sustained load. This is often abbreviated as 'P'.
• Code Treatment: The stress from sustained loads, $S_L$, must be less than the material's basic allowable stress in the hot condition, $S_h$. This check prevents failure from long-term, steady loading.

2. Displacement Loads (Expansion Loads)

• Definition: These are loads generated by the restraint of displacement. The most common source is thermal expansion or contraction. As the pipe heats up, it tries to expand. If it is anchored at both ends, this expansion is constrained, inducing stress. These are secondary loads and are considered self-limiting. If the stress becomes too high, the material may yield slightly, which relieves the stress without causing immediate failure.
• Source Examples:
  • Thermal expansion and contraction between operating and ambient temperatures.
  • Movement of anchors or connected equipment (e.g., thermal growth of a vessel nozzle).
  • Forced displacement during installation to align pipe ends.
• Code Treatment: Because the primary danger is fatigue from repeated cycles (start-ups and shut-downs), the code evaluates the stress range, not the absolute stress. The calculated displacement stress range, $S_E$, must be less than the allowable displacement stress range, $S_A$.

3. Occasional Loads

• Definition: These are loads that occur infrequently and for a short duration.
• Source Examples:
  • Wind: Forces acting on exposed piping.
  • Seismic: Inertial forces during an earthquake.
  • Water Hammer/Surge: Pressure waves created by rapid valve closure or pump trips.
  • Relief Valve Discharge: Reaction forces generated when a pressure relief valve opens.
  • Snow or Ice: Weight accumulation in cold climates.
• Code Treatment: Because these events are short-lived, the material can safely tolerate a higher stress level. The code allows the combination of sustained and occasional stresses to exceed $S_h$, typically up to 1.33 times $S_h$.

This image provides a concise summary of the stress limits for the two primary categories we'll be analyzing throughout this course.

Load Cases in Practice

In a real-world project, you won't analyze these loads in isolation. Instead, a pipe stress engineer combines them into specific load cases to evaluate the system's behavior under different scenarios. A sample engineering report provides a perfect example of how these are defined.

As you can see from the report, the load cases directly correspond to the categories we've discussed:

• SUS (Sustained): Combines Weight (W) and Pressure (P1). This is checked against the $S_h$ limit.
• EXP (Expansion): Considers the thermal load (T1) alone. This is used to calculate the displacement stress range, $S_E$, which is checked against the $S_A$ limit.
• OCC (Occasional): Combines the sustained loads (Weight + Pressure) with an occasional load (e.g., Seismic, U). This is checked against the increased occasional allowable stress (e.g., 1.33 $S_h$).

Finally, let's look at the output from a pipe stress analysis software. This table shows the final code compliance check for a specific point in a piping system.

In this table, you can see the practical application of everything we've covered:

• The Load combination column shows various cases, such as GR + Max P (Gravity + Pressure), which corresponds to the Sustained (SUST) load type.
• The TR:Amb to T1 combination represents the temperature range from ambient to operating, corresponding to the Displacement (DISP) load type.
• The highlighted line, Sus. + E1, combines sustained loads with an occasional load (Earthquake), corresponding to the Occasional (OCC) load type.
• For each case, the software calculates a Code Stress and compares it against the Code Allow. (Code Allowable). The design is acceptable only if the calculated stress is less than the allowable limit for every required load case.

Conclusion

In this lesson, you've learned to classify the various forces acting on a piping system into the three fundamental categories defined by ASME B31.3. This classification is not just an academic exercise; it is the basis for how the code ensures safety against different types of failure.

Here are the key takeaways:

• Sustained Loads are constant forces like weight and pressure. They are checked against the hot allowable stress ($S_h$) to prevent plastic collapse.
• Displacement Loads are caused by restrained thermal expansion. The code checks the stress range ($S_E$) against an allowable range ($S_A$) to prevent fatigue failure.
• Occasional Loads are short-duration events like wind or seismic activity. The code permits a higher stress level for these loads due to their infrequent nature.
• Stress analysis is performed using load cases, which are specific combinations of these loads designed to simulate various operating and environmental conditions.

You now have a complete picture of the "Design Conditions and Allowable Stresses" module. We've defined the pressure and temperature, and now we've classified the loads. The final piece of this puzzle is determining the allowable stress values themselves. In our next lesson, we will learn how to look up the basic allowable stress value, $S$, from the material tables in ASME B31.3. This value is the cornerstone from which the specific allowables for sustained ($S_h$) and displacement ($S_A$) stresses are derived.