Pipe Stress Analysis: How To Get Your Plant Piping Right

As an engineer, you know there’s a lot more to pipe design than routing. That’s because while they look static, pipes can be surprisingly dynamic. Changing loads can cause them to shift, get overstressed, fail, cause harm and do damage. And when the substances in those pipes are hazardous, there can be perilous consequences for the people and equipment in your plant.

If you’ve already experienced a piping-related problem, the process of pipe stress analysis can ensure you properly diagnose the issue and make a safe, long-term fix. If you’re building a factory from the ground up, it can help you design a piping system that’s as smart as it is efficient. In both cases, pipe stress analysis can help you ensure the safety of your plant environment and your workforce. This simple guide will help you approach that critical process thoughtfully and practically.

The importance of pipe flexibility

Pipe stress design is a balancing act between flexibility and rigidity. Depending on the loads placed on them, you may need to design your piping to be firm and elastic at the same time. Let’s start with the flexible part.

Thermal stresses are the primary reason your system needs flexibility. Pipes grow when their temperature rises above ambient conditions, and they shrink when it drops below. The same goes for the equipment those pipes are connected to. That growth has to go somewhere.

If pipes are not allowed to grow, or to move with growing equipment, they will push on themselves, their supports and the equipment they’re connected to until something gives way. Even a fraction of an inch of growth can generate tens or even hundreds of thousands of pounds of force if the pipes are completely locked in place.

The best way to address thermal growth is by adding flexibility to the system. Most often that comes in the form of flexible supports, expansion loops, and/or expansion joints. The key, as we mentioned earlier, is a balance of solutions that deliver the elasticity you need.

The importance of pipe rigidity

Flexibility isn’t the system’s only need, of course. Too much of it can make pipes vulnerable to things like wind and earthquake loading, transient forces like water hammer and steam hammer, and even vibration. That’s why parts of the system require rigidity.

Anchors are the main way piping systems can be made more rigid. Anchors break the system into isolated pieces by stopping any forces and movements from crossing them and impacting what’s on the other side. By isolating sections of the system, anchors make complex systems much easier to trouble-shoot when things go wrong. They also allow you to modify sections of piping without reengineering the entire system.

There are caveats, as always. Overuse of anchors can make thermal loads much worse. That’s why you need to locate them only in places with small thermal displacements.

Pipe stress design basics

Achieving a balance between pipe flexibility and rigidity isn’t any simpler than it sounds. But it can be accomplished with thoughtful design, routing and support. That starts with three basic system design rules:

Keep pipe stresses under their code-allowed limits to prevent pipe damage.
Keep forces at pipe-equipment connections under the limits recommended by the equipment manufacturer.
Keep large displacements from creating low spots in the pipe or causing the pipe to collide with nearby objects.

If you can meet these three criteria, you’ll likely have the balance of flexibility and rigidity your system needs. However, as with any balancing act, when you add to one side it can tip the other. That’s especially true in pipe stress design when you have environmental variables you can’t necessarily control (like how much structural steel you have nearby to handle support loads).

For all but the simplest piping systems, it’s a process you can’t eyeball or do by hand. The combination of supports and pipe routes is too vast, the number of bend and fitting options too extensive, and the balance too delicate to make hand calculations practical. Even if you have the intent and the commitment, you don’t likely have the man-hours and patience you’d need. That’s where the technology and experience of a consulting engineer come in.

Getting pipe stress analysis right

When it comes to pipe stress analysis, experience counts, and engineers who are familiar with the process, the codes and who have access to pipe stress analysis software are invaluable. Because the pipe stress analysis process is most often one of trial and error, stress analysis software like AutoPIPE by Bentley can dramatically reduce the time (and cost!) it takes to produce safe, efficient, compliant piping design.

Again, there are lots of competing goals in pipe stress analysis. Every load and component strength in the system must be accounted for. Thankfully, stress analysis software can do those calculations quickly, accurately for an infinite variety of pipe layouts. Alternative loads, supports and pipe properties can be added with a few button clicks. Most software also has a library of pipe fittings, valves and flanges it can call on to auto-calculate the “strength” of those components according to the various piping codes.

With the software performing those calculations, your consulting engineer is free to spend their time investigating potential causes for pipe failures and brainstorming solutions rather than crunching numbers. The best software can also generate raw results data, stress color plots, pipe movement illustrations, and animations to help explain your final conclusions. All of those are incredibly helpful when it comes time to make and defend your system design decisions and justify the time and expense.

Common piping design mistakes and fixes

Without careful analysis and design, problems can arise. Here are some common mistakes we often see in piping systems:

Pipe trapped between rigid connections – If you have two pieces of equipment that are anchored to the floor and you’re running a length of pipe between them that’s going to heat up, that growth has nowhere to go. That means either the pipe or the equipment is going to give, depending on which one is stronger. The way to alleviate that stress is not to take the straight route. Adding expansion loops (U-shaped bends) to the run will allow the pipe to flex and grow, turning your rigid rod into a giant spring and relieving the stress.
Growth around a corner when using pipe guides – Long straight sections of pipe are often held up by a series of guide supports made from pipe shoes with guiding clips on the sides that allow them to grow by moving forward and back without moving side to side. When you have a bend in that pipe, however, the clips before the bend will interfere with growth after the bend (and vice versa). To allow that growth, it’s often smart to cut those clips off near the corners, that way the pipe has the freedom there to move in both directions at the same time.
Expansion joints blowing apart – When you’re stuck with a straight pipe but don’t have room to incorporate an expansion loop to manage growth, an expansion joint can be a useful alternative. It’s essentially an accordion-like joint that gives the pipe the flexibility it needs. When that same pipe is under high pressure, however, it can also push the joint outward in both directions. If that pressure gets too high, it can blow the joint apart. Adding anchors before and after the section of growing pipe involved with the joint can eliminate that possibility and the potential danger that goes with it.
Support lift off – If you have a pipe that’s traveling vertically for a long distance, you need supports at the top and bottom to bear the weight of the pipe. If there is thermal growth in the pipe, , it’s going to tend to lift off of any supports you have at the top . Once that lift-off occurs, the pipe and bottom support can be damaged by the overload. The best solution is to avoid long vertical runs by. breaking those long runs up so that each piece grows a little bit, but not enough to lift off any supports and do any damage.
Equipment damaged by connection to a vertical pipe – When you have equipment that’s more sensitive than the pipes themselves, even a small vertical run can create misalignment and damage. These issues can be difficult to solve. That’s why it’s best to avoid connecting vertical pipe to sensitive equipment whenever you can. If you can’t, a cold spring (which relies on building the vertical section a little too short when it’s cold so that it lifts up less when it’s hot ) or spring cans (which tug up on the pipe to lift it off the delicate equipment) might provide a workable solution (though not an ideal one).

Proper planning

Clearly there are lots of challenges and pitfalls when it comes to pipe design. But the biggest is not planning ahead, making calculated decisions or properly protecting your system and equipment. Whether you’re talking about a single line of pipe and a few days of analysis or hundreds of lines and months or years of planning, don’t take it lightly.

Consult an experienced engineer and rely on the economy of software. Then prepare thoroughly, think critically and execute precisely. You’ll wind up with a safer, more efficient system that you can be proud of and will protect your plant, your process and your people.

Topics: Piping