All my previous articles in this website describe stress analysis methodology using Caesar II based on ASME B 31.3. But I received requests from few pipeline engineers to describe the methodology based on ASME B 31.4. So I am trying to explain few guidelines for performing stress analysis based on ASME B 31.4. Hope it will be helpful for you.
ASME B31.4 covers piping systems transporting liquids. The stress analysis of a pipeline is quite different from that of plant piping. The most fundamental difference between pipeline and plant piping is the very long length of the pipeline. A pipeline with kilometres in length has the potential of producing a very large amount of expansion. A reasonable estimate of the movement and its interaction with the end resistance force afforded by connecting piping and equipment are very important aspects in designing a pipeline. The salient points for stress analysis I feel are as follows:
- B 31.4 code addresses analysis of lines within temperature range starting from -20 degree centigrade till 120 degree centigrade.
- There are no Sh values similar to B 31.3. A pipeline normally runs for several kilometres without any fittings attached. Because of such simplicity, the stress in the majority portion of a pipeline is quite predictable. Taking advantage of this characteristic, the code’s allowable stress for a pipeline is greatly increased, as compared to that for plant piping. All allowable values are linked with Sy (Specified Minimum Yield Strength) as the allowable stress of a pipeline is mainly to protect the pipe from gross deformation. Whenever you select B 31.4 in Caesar II all Sh value fields become grey.
- The following equations are used to calculate various stress allowable:
o Expansion Allowable=(0.72) (Sy)
o Sustained Allowable=(0.75) (0.72) (Sy)
o Occasional Allowable=(0.8) (Sy)
o Operating Allowable=(0.9) (Sy)
- Pressure elongation of pipe line is also important along with expansion elongation and need to be taken care. Caesar II automatically does this whenever you select B 31.4 code.
- There is nothing like liberal stress in B 31.4
- The modelling procedure is similar. Whenever material is selected the Sy value automatically filled from Caesar database. However you have to input the Design multiplication factor (Fac) value additionally. Fac value indicates whether the pipe is restrained, such as long or buried, or unrestrained. Fac should be 1.0, 0.0, or 0.001.
o This value should be one for pipe under complete axial restraint. This value should be one when the pipe is fully restrained, such as buried for a long distance.
o The default value for Fac is 0.0.
o When Fac is 0.001, this indicates to CAESAR II that the pipe is buried but that the soil supports have been modelled. This causes the hoop stress component, rather than the longitudinal stress, to be added to the operating stresses if the axial stress is compressive.
- Some parts of the lines are buried or underground and some parts are aboveground. So you need to understand the soil pipe interaction for buried parts. Soil properties need to be taken from Civil/Geotechnical team while performing stress analysis of underground piping.
- For underground piping there should be some minimum depth of cover as per B 31.4 depending on location of pipeline.