Thermal Expansion and Piping Stress: Part 2, Stress Analysis and Proper Engineering Prevent Problems
This is the second in a three-part series on thermal expansion and piping stress. The first article provided a primer on the basics of the thermal expansion and contraction found in industrial facilities. This 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.
Piping systems are often complex, with lines running long distances to myriad pumps and tanks, and requiring numerous valves and instrumentation connections. These pipes may carry hot liquids or steam, which causes thermal expansion that increases the pipe’s length. Pipes carrying cold material experience thermal contraction.
Stress Analysis Overview
Stress analysis is the calculation of the forces created by thermal expansion. The stress analysis results are used to design the system flexibility that prevents damage from thermal expansion. This includes determining the route for the piping and the type of supports to be used.
A detailed piping stress analysis based on the applicable piping codes is the key to the proper engineering design of a piping system. The calculations for piping stress, thermal expansion, and anticipated loading will guide the engineering decisions for the optimum pipe routing, piping materials, and piping supports.
It is very important that a stress analyzed line is installed as designed. If the route or the supports are modified, serious damage can occur.
Common failure modes for piping systems include brittle fracture, plastic deformation or instability, incremental collapse, and the peak stress limit caused by cyclic loading fatigue. Each failure mode is caused by a different type of stress that is defined in the piping codes.
Engineering Methods for Handling Thermal Expansion
Hot piping is typically not routed in long, straight runs. Instead, expansion loops are frequently used to absorb the thermal expansion. These loops are commonly located in the pipe racks between major pieces of equipment.
Near pumps, tanks, and other equipment, the piping system may contain a series of elbows to prevent pipe growth from damaging the equipment—and to prevent equipment growth from breaking the pipe.
Yes, the thermal expansion of equipment must also be accounted for in the stress analysis. Equipment will expand and grow away from its anchor point—where it is attached to the floor ,wall or foundation. A pump for hot service will be supported at the center of the casing. A vessel mounted at its base will grow up, while a vessel mounted at its midpoint will grow both up and down. The extra elbows on the piping help handle this equipment movement.
Special pipe supports are used to control the direction of pipe expansion.
- Anchors are welded in place to prevent pipe movement at that point. The piping will grow away from the anchor point in both directions.
- Guide supports are installed wherever the pipe may be prone to lateral movement. A guide allows the pipe to grow in the direction of flow, but prevents movement to the side.
- Spring supports are used when thermal expansion can cause the equipment or piping to grow vertically. Springs allow the pipe support to maintain contact across the entire range of temperatures. They are installed in the “cold” position. After installation, the “stops” are removed from the spring so that it is free to move up and down as the temperature changes
Insulated piping may rest on pipe shoes, which keep the hot pipe and its insulation from resting directly on the pipe rack steel. As the pipe expands, these shoes will also move. The engineering design must ensure that the shoes do not fall off the pipe rack steel as the pipe moves.
Next time, we’ll look at what you should be looking for to prevent thermal expansion problems in your facility.
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