What is Thermal Bowing?

The thermal bowing phenomenon occurs when a horizontal pipe is filled partially by hot or cold fluid (LNG). Many thermal bowing occurrences cause unexpected damage to the piping or supporting structure. Since thermal bowing occurs mostly at transient conditions, such as during startup, the bowing phenomenon may go unnoticed until the damages are discovered.

It can also occur when one side of the pipe is exposed to the sun and the other side is in the shade.

This effect takes place when the temperature difference between the top and bottom of the pipe section is significant. It called the thermal gradient. This thermal gradient causes pipe thermal strains that produce pipe curvature called thermal bowing.

Assumptions for thermal bowing

The following Assumptions are made for thermal bowing

  • Thermal strain distribution across the pipe section is linear
  • Applied only for horizontal pipes that meet “horizontal tolerance” criteria |DZ| / ( DX^2 + DY^2 + DZ^2 )^0.5 < Tolerance
  • Bowing is acting only in the vertical plane

Basics of Thermal Bowing

The thermal gradient can be different for each pipe element. And also it can be different in each operation mode.

Temperature Distribution in Thermal Bowing
Temperature Distribution in Thermal Bowing

The pipe curvature due to thermal bowing effect:

Pipe Curvature Due to Thermal Bowing

r – curvature radius
D – outside diameter of the pipe
a – thermal expansion coefficient at operating temperature

Ttop – the temperature at the top of the pipe
Tbottom – the temperature at the bottom of the pipe

Performing Thermal Bowing in Start-Prof

The thermal gradient (Ttop-Tbottom) should be specified in pipe properties.

Specifying Thermal Bowing in Start-Prof
Specifying Thermal Bowing in Start-Prof

Thermal boring effect should be switched on in Project Settings.

Switching on Thermal Bowing Effect
Switching on Thermal Bowing Effect

The bending moment produced in the restrained pipe due to thermal bowing effect:

Bending Moment Equation in Thermal Bowing

E – pipe elastic modulus at operating temperature
I – the moment of inertia

Operating temperature should be equal to (Ttop+Tbottom)/2

Some more Resources for You…

Stress Analysis using Start-Prof
Stress Analysis using Caesar II
Stress Analysis Basic Concepts
Piping Layout and Design

Alex Matveev

Alex Matveev is one of the authors of pipe stress analysis codes GOST 32388-2013 Process Piping, and GOST 55596-2013 District Heating Networks. He is also one of the developers of PASS/Start-Prof software, which is developed since 1965 and used in 95% of process, power, district heating, gas and oil transportation design companies in Russia and CIS countries; it is a standard de facto in that region

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