Module 2 — Loads on Piping Systems and Resulting Stresses
If you've ever wondered why an engineer spends so much time worrying about a pipe that's "just sitting there" carrying fluid from one tank to another, this post is for you. Piping stress analysis is the branch of engineering that makes sure pipes carrying steam, oil, gas, or chemicals through a plant don't crack, leak, or collapse over their working life — and it all starts with understanding loads: the forces and movements acting on a pipe every single day.In this guide, we'll break down the three categories of loads every piping system experiences, explain why some loads are far more dangerous than others despite seeming "smaller," and show how all of this translates into the actual stress happening inside the pipe wall. No prior engineering background required — we'll use everyday analogies throughout.
You can click here to watch the full video.”
What Is a "Load" in Piping Systems?
In plain terms, a load is anything that pushes, pulls, twists, or bends a pipe. That could be the pipe's own weight, the pressure of the fluid inside it, the wind blowing against it, or even the pipe expanding as it heats up. Every one of these loads falls into one of three categories: primary, secondary, or occasional.
1. Primary Loads: The Ones That Never Go Away
Primary loads are always present, every day the pipe is in service. They include:
- The weight of the pipe itself
- The weight of the fluid flowing inside it
- Internal pressure pushing outward from within
- For buried pipelines, the weight of the soil sitting on top
Internal pressure alone generates what's called hoop stress, calculated with a simple formula: pressure multiplied by the pipe's outside diameter, divided by two times the wall thickness. You don't need to memorize the formula to understand the point — it's a direct, constant relationship between pressure and stress, with no complicated timing involved.
Here's what makes primary loads distinct, and worth remembering:
- Force-driven: they come from an actual push or pull — weight pressing down, pressure pushing outward.
- Not self-limiting: imagine a heavy object sitting on a shelf. If the shelf starts to crack, the weight doesn't get any lighter — it keeps pushing at full strength right up until the shelf breaks. Primary loads behave exactly the same way.
- Not cyclic: they don't turn on and off. The pipe's weight is there constantly, at roughly the same level, every day.
Because primary loads don't ease off, failure can happen from a single event — a pipe bursting under pressure, or a support collapsing under weight. There's no need for repeated cycles; one bad moment is enough.
2. Secondary Loads: The Ones That Come and Go
Secondary loads behave almost the opposite way. They show up because of how the system operates, not simply because it exists. The most common example is thermal expansion: when hot fluid runs through a pipe, the metal heats up and physically grows longer — the same way a metal ruler expands slightly when left in the sun. Other secondary loads include a support gradually settling into the ground, or vibration from a nearby pump or compressor.
Key characteristics of secondary loads:
- Displacement-driven: caused by movement or deformation, not a direct force.
- Self-limiting: as the pipe expands and pushes against whatever restrains it, the pipe itself flexes slightly, relieving some of the built-up stress on its own.
- Typically cyclic: thermal expansion is present when the pipe is hot and running, and disappears when it cools down during a shutdown — so the pipe flexes back and forth repeatedly over its lifetime.
This cyclic, back-and-forth behavior leads to the real danger of secondary loads: fatigue. Think of bending a metal paperclip repeatedly — the first bend doesn't break it, but enough repetitions eventually cause it to snap, even though no single bend was especially forceful. That's why a piping system can run successfully for years, even with a design flaw, before a crack finally develops from accumulated cycling.
3. Occasional Loads: The Rare but Serious Events
Occasional loads only affect the system some of the time — not continuously, and not in every operating cycle. The two main examples:
- Wind load: lateral force from wind blowing against above-ground piping and pipe racks.
- Earthquake (seismic) load: dynamic loading transmitted into the piping from ground motion during an earthquake.
Even though these events are rare — and ideally never occur during a plant's operating life — engineers still design for them as a safety precaution.
Primary vs. Secondary Loads: A Side-by-Side Comparison
If you remember only one distinction from this article, make it this one, since it drives how engineers actually evaluate piping systems:
| Characteristic | Primary Loads | Secondary Loads |
|---|---|---|
| Driving mechanism | Force-driven (weight, pressure) | Displacement-driven (thermal growth, settlement) |
| Self-limiting? | No — keeps acting even as the system begins to fail | Yes — stress relaxes as the system deforms |
| Cyclic behavior | Essentially constant | Typically cyclic (e.g., present when hot, absent when cold) |
| Mode of failure | Can fail from a single load application | Fails from fatigue after multiple cycles |
This is exactly why engineers set different allowable stress limits for primary and secondary loads — because they fail in fundamentally different ways.
How These Loads Create Stress in the Pipe Wall
Once we know what's pushing on a pipe, the next step is understanding what that does inside the metal. In engineering terms, "stress" isn't a vague feeling — it's a precise, measurable quantity: force spread over an area. There are four main stress types that show up in a pipe wall:
Longitudinal Stress
Runs along the length of the pipe — the same direction the pipe travels, similar to stretching a garden hose from end to end.
Radial Stress
Occurs across the thickness of the pipe wall, caused by internal pressure pushing outward — like squeezing the wall from the inside toward the outside.
Hoop (Circumferential) Stress
Wraps around the pipe's circumference, much like the metal hoops that hold together a wooden barrel. Internal pressure tries to split the pipe into two halves lengthwise, which makes hoop stress usually the single biggest factor engineers consider when deciding how thick a pipe wall needs to be.
Shear Stress
Comes from twisting or from forces sliding layers of material past one another — similar to wringing out a wet towel. In piping, this typically comes from torsional loads or transverse (sideways) forces.
Connecting Loads to Stresses
In practice, one stress type can come from more than one kind of load. Longitudinal stress, for instance, can result from a straightforward axial force (like weight pulling along the pipe's length), from internal pressure itself, or from bending — which happens when weight, pressure, thermal expansion, or wind causes the pipe to flex like a diving board. Hoop and radial stress mainly come from pressure, while shear stress traces back to transverse forces or torsion.
Notice that many of these stresses can be triggered by either a primary or a secondary load. That's why understanding which category of load created a stress matters just as much as calculating the stress value itself — it tells you whether you're dealing with a single-event risk or a long-term fatigue risk.
Key Takeaways
- Loads are classified as primary (always present), secondary (tied to operating conditions), or occasional (rare events like wind and earthquakes).
- Primary loads are force-driven and not self-limiting; secondary loads are displacement-driven and self-limiting.
- Secondary loads are typically cyclic, and their failure mode — fatigue — requires many load cycles to develop, not a single event.
- Together, these loads generate longitudinal, radial, hoop, and shear stress in the pipe wall, with hoop stress usually governing the required wall thickness.
Frequently Asked Questions
What's the main difference between primary and secondary loads?
Primary loads come from direct forces like weight and pressure and never ease off, even as a system fails. Secondary loads come from displacement, like thermal expansion, and are self-limiting — the stress relaxes as the pipe flexes.
Why is hoop stress so important in pipe design?
Hoop stress, caused by internal pressure acting around the pipe's circumference, is usually the largest stress a pipe wall experiences, which makes it the governing factor when engineers calculate the minimum required wall thickness.
Can a pipe fail even if it was designed correctly?
Yes — if the failure is due to a secondary load like thermal cycling, it can take years of repeated cycles before fatigue causes a crack, even in a system that initially appeared to run just fine.
What comes after understanding loads and stresses?
The next step is learning how these individual loads are combined together following ASME code rules, and how that process is carried out in practice using industry-standard piping stress software such as CAESAR II.You can click here to watch the full video.”