external pressure calculation for pressure vessel 2

[rajeev383]

while calculating the thickness in case of external pressure we use graph from sec II part D for calculating the allowable stress.
In any case suppose if we require to calculate the allowable pressure in case of compression how can we take the allowable stress value for carbon steel in case of compression?

 

[TomBarsh]

As you mention "sec II part D" it's apparent you are referring to ASME Boiler and Pressure Vessel Code, and likely Section VIII application.

See VIII-I UG-23(b) for "the maximum allowable longitudinal compressive stress to be used in the design of ...". The method determines factor "A" and then you go into the charts of II-D for allowable stress.

 

[TGS4]

From a pure mechanical engineering perspective - it is important to understand that failure due to compression is completely different from failure due to pressure/tension.

Therefore, for internal pressure / tension, the failure mode is burst, which is governed by yield and ultimate strengths. For external pressure / compression, the failure mode is buckling, which is generally governed by the Young's Modulus, and geometric factors.

Once you understand these important distinctions, then the rules to be followed will make more sense.

 

[rajeev383]

Thanks for the replies gentle man
But my concern is still the same. Actually I have to design a long horizontal pressure vessel(18000mm) where thickness due to internal pressure is coming out to be 7 mm but in case of external pressure it is coming out to be 17 mm for which I need stiffeners. What I want to know is that while calculating the thickness required due to external pressure do we have allowable compressible stress like the allowable tensile stress as mentioned in sec II part D.
will the case of buckling come here TGS4 sir?

 

[TGS4]

If you are designing a vessel to ASME Section VIII, Division 1 then you should follow the rules provided therein. The answers you seek are there, if only you follow the rules.

 

[PVDEngg]

rajeev383: It seems like you are looking for exactly what TomBarsh has replied. VIII-Div.1 UG-23 provides guideline about both allowable tensile stress and allowable compressive stress. Refer UG-23(b)for allowable compressive stress. Rest all please correct me if I am wrong, this allowable compressive stress is what used to design vessel for external pressure, its one and the same thing based on context TSG4 explained. And these allowable compressive stresses are in chart form in PART-3 of Sec II-D.

Since, the question was completely replied already, this is just a small attempt to break down the replies to very basic level based on rewritten question by rajeev383.

rajeev3873: Hope this clarifies.

Thank you TSG4 & Tombarsh for your luminary responses on many queries.

 

[prex]

rajeev383, when you calculate a cylinder to external pressure per ASME VIII, there is no such thing as an allowable stress under compression. The allowable calculated per UG-23 is for longitudinal compression, whilst in a cylinder you are interested in circumferential compression.
You need to follow the procedure starting at UG-28 to make the check. Other codes have different procedures and could give you a formula for the allowable compressive stress (still strongly dependent on geometry of course).

prex
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[r6155]

also see Case 2286-5
Alternative Rules for Determining Allowable External
Pressure and Compressive Stresses for Cylinders, Cones,
Spheres, and Formed Heads
Section VIII, Division 1

Regards
r6155

 

[jte]

As info, CC 2286 is on rev. 6 as of a few months ago.