Pipe Stress 101 – What is Thermal Expansion and How Does it Create Problems in Piping
This is the first in a series of articles about pipe stress analysis. This article will cover the basics of thermal expansion and contraction and why it’s so important in an industrial facility. Later articles will explore the various aspects of stress analysis, piping interaction with structures, and piping interaction with various types of equipment. First we need to understand what exactly thermal expansion and contraction is.
What is Thermal Expansion?
For this discussion, we will focus on thermal expansion and contraction as it relates to piping. And this is for good reason, as piping systems in an industrial plant or refinery can represent up to half of all man-hours to design and more than a third of the total cost of the facility. Because these pipes often transport hot fluids or gasses, thermal expansion and the resulting stresses are a major concern. What we will discover is that it is not so much the thermal growth of the pipes themselves that is a problem. The real problem lies in what the pipes are connected to and what restricts their thermal growth.
Large pieces of equipment make up another significant portion of the cost of a facility. They typically provide the other ingredient needed to make the thermal expansion of hot piping a concern, something stationary to resist the thermal growth. Pumps, Pressure Vessels, Heat Exchangers and Compressors, almost all large pieces of equipment are anchored in place. When a hot pipe routed to that piece of equipment thermally expands, the equipment resists this movement and the forces at the intersection between pipe and equipment (at the equipment nozzle) start to increase.
Let us take a step back, what exactly is thermal expansion? Well not to go too deep down the rabbit hole here, but as a substance is heated the kinetic energy of its molecules increases. As a general rule, the more the molecules move and vibrate in a substance the greater the average separation. For a solid, like metal, this translates into an increase in area and volume as the temperature changes. The more the temperature changes the greater the change in the kinetic energy of the molecules and the greater the change in area and volume. This rate of change is known as the coefficient of expansion and it is unique to each different metal.
A classic illustration of thermal expansion is the image of a warped set of railway tracks in the hot sun. Railway designers of old got around the problem of thermally expanding railroad tracks by leaving gaps in the sections of track. The solution isn’t quite as easy for piping since gaps would let the fluid run out! However, there are other ways we can add flexibility into the piping system, a topic we will discuss in more detail in later articles.
Accounting for the Movement Due to Thermal Expansion
How much will hot piping grow? A carbon steel pipe that is heated from an ambient temperature (let’s assume 70°F for discussion purposes) to 300°F will grow 1.9 inches for every 100 ft of length. A stainless steel pipe of the same temperature will grow 2.5 inches for every 100 ft of length.
That may not seem like a lot, and if the pipe were laying on a rack somewhere with both ends unattached, or even with one end unattached, the movement may not even be noticed. This, however, is rarely the case. It’s typically not the thermal expansion of the pipe itself that is the root of the problem so much as what the pipe is connected to. Or perhaps I should say, the problem is what resists the thermal expansion of the pipe.
The other side of the coin is thermal contraction. When the temperature falls below ambient, the opposite of thermal expansion happens. As a substance cools the kinetic energy of the molecules reduces and the substance tends to shrink. This is likely to be most evident in the case of piping which carry cryogenic fluids and can be -200°F to -300°F, or even lower. What happens to our piping from the above example? Carbon steel pipe at -200 F will contract by 1.9 inches per 100 ft of length. Similarly, stainless steel pipe at -200 F will contract by just over 2.5 inches per 100 ft of length.
So which pipes in a facility or plant are likely to expand or contract? High temperature lines are very common in refineries, chemical plants, and even food processing facilities. In fact it is rare to find a facility that doesn’t have at least steam and condensate lines, both of which can be quite hot. Cryogenic lines are less common, but the contraction from a cold line can play just as much havoc with your systems as a hot line. A good rule of thumb is that if a pipe is insulated then thermal expansion, or contraction, is likely to occur.
In the next article, we will take a closer look at why understanding the thermal growth of piping is so important in an industrial plant. We will also try to answer the question of why proper stress analysis and good engineering practice is so important to mitigate the risks of thermal expansion to piping and equipment.
Matrix Technologies is one of the largest independent process design, industrial automation engineering, and manufacturing operations management companies in North America. To learn more about thermal expansion and pipe stress analysis, contact Chris Mach, Senior Consultant or Brandon Grodi, Mechanical Department Manager.