Thermal Expansion Creates Piping Stress: Part 1, What You Need to Know to Prevent Problems
This is the first in a three-part series on thermal expansion and piping stress. This article provides a primer on the basics of the thermal expansion and contraction found in industrial facilities. The second article focuses on the importance of proper stress analysis and good engineering to mitigate the risks of thermal expansion to piping and equipment. The third article describes how to spot problems in your plant.
Part 1: A Quick Primer on Thermal Expansion and Contraction
Piping systems in industrial plants and refineries can represent more than a third of the cost of a new facility. Because these pipes often transport hot fluids, thermal expansion and the associated stresses must be carefully considered to avoid problems. The forces created by the thermal expansion can be large enough to drive piping through walls, deform structural steel shapes, damage pumps and valves, and even fracture the piping.
What Is Thermal Expansion?
An object or substance tends to change its shape, area, and volume as its temperature changes. Materials generally expand when heated, and contract when cooled. That’s because their molecular structure expands due to increased kinetic energy at a higher temperature – causing the molecules to move around more. (The contrary behavior of water will not be discussed here.)
For example, when you start your car, the engine generates heat that warms your radiator fluid. It expands and some liquid transfers into the overflow container because the fluid is warmer and needs more space.
As metals are heated, they expand based of their coefficient of expansion: Not all metals expand at the same rate. That’s how home thermostats worked before digital versions came along. A bimetallic strip had two metals with very different coefficients of expansion. This caused the strip to bend as the temperature changed.
When piping is installed, it has a certain length. When hot materials are pumped through, the pipe length will increase as the temperature of the pipe increases. Accounting for this thermal expansion is necessary to prevent catastrophic damage to piping, equipment, and structures.
The opposite of thermal expansion is thermal contraction. Thermal contraction occurs in piping when a cold liquid is pumped through, such as liquid nitrogen for a cryogenic service. The piping length will shrink as the temperature falls, and poor design can lead to damage, just as it does for thermal expansion.
Accounting for Movement Due to Thermal Expansion
Pipes often have attachments such as drain and vent valves, instrument connections, and small sampling pipes. If the expected thermal expansion is not accounted for, these attachments may be broken off in slow-motion collisions with nearby structures.
How much movement typically occurs in piping? Carbon steel will expand approximately ¾ inch per 100 feet for every 100° F change in temperature. A 200-foot line operating at 270° F will grow 3 inches from its installed condition at 70° F.
Stainless steel will expand approximately 1-1/8 inches per 100 feet for every 100° F change in temperature. A 200-foot long line operating at 270° F will grow 4 ½ inches from its installed condition at 70° F.
Which lines are likely to expand? High temperature lines are common in refineries, chemical plants, and even food processing facilities. Steam and condensate lines are found in almost all facilities.
A good rule-of-thumb to follow is this: If a pipe is insulated, thermal expansion will occur.
Next time, we’ll discuss the importance of proper stress analysis and good engineering 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 Bill Schuller, Senior Vice President of Engineering, at (419) 897-7200 x 450.