AutoPIPE Cut-Short Load Combination 1

[AKBen]

I am analyzing an above grade, insulated, sales crude pipeline to B31.4 in AutoPIPE. The pipeline is about 14 miles long. There are Z-bends and anchors every 500 to 1000 feet. I am coldspringing(AutoPIPE calls it a Cut Short) the pipe. I am trying to determine the correct method to combine the cutshort with other load cases. I tried to create a Code Combination by setting the Cut Short to U2 (U1 is my snow), but the U2 does not show up in the User Defined Code Combination dialog. Does anyone know how to create a User Code Combination with the Cut Short?

 

[AKBen]

I just figured it out. The analysis must be run once with U2 checked, then U2 will show up in the User Defined Code Combinations.

 

[JohnBreen]

Also, remember that you cannot use cut short (cold spring) with code stress calculation as it is not pr=ermitted by the Code.

John

 

[AKBen]

Where is it not permitted in B31.4? I interpret 419.7.3 d to mean that the minimum temperature cannot be changed to the cold spring base temp.

 

[JohnBreen]

AKBen

Look at paragraph 419.7.3(d)

(D) Calculations of pipe stresses in loops, bends and offsets shall be based on the total range from minimum to maximum temperature normally expected, regardless of whether piping is cold sprung or not................

The committee is telling you that when you calculate the thermal stress RANGE you cannot "take credit" for cold spring. You must calculated the thermal stress RANGE over the full thermal range as though the cold spring does not exist. The stress RANGE is the SUM of the stresses from the installation temperature to the maximum expected temperature added to the stresses from the installation temperature to the minimum expected temperature.

This is a fundamental concept in ALL ASME B31 Pressure Piping Codes as cold spring has virtually no effect on the displacement stresses due to thermal expansion and contraction. The ONLY value for cold springing a piping system is to reduce the end reactions on the equipment to which the piping is attached. Obviously, cold spring has no effect upon sustained stresses due to weight (dead weight and live weight) and internal pressure. Paragraph 419.7.3(e) is telling you that you must check the loadings on the terminal equipment both at the maximum expected temperature and at the minimum expected temperature (out-of-service on a winter night). So if you cold spring a piping system you have to check to be sure the terminal equipment will not be overloaded at the minimum expected temperature (out-of-service on a winter night.

Bottom line - cold spring has no effect on the calculation of stresses and stress RANGES as defined in ANY OF THE B31 CODES FOR PRESSURE PIPING including B31.4.

Happy Holidays, John.