Maximum external pressure

[Technopriest]

I was comparing the maximum allowable external pressures for a cylinder calculated with italian VSR design code and with ASME ed. 01 add. 02.

The result was amazing: with a 2500 mm Do, and t=10, VSR allows about 0,8 MPa external pressure (The vessel is a jacketed one, with 0,7 MPa in the jacket and vacuum inside). Using ASME design formulae I need about t = 24 mm to resist the same pressure.
(values related to the same material, ASME SA240 TP 316L)

I was wondering: It's my fault (I'm italian, I'm not too familiar with ASME) or it's ASME really this demanding against external pressure load?
Can someone check my comparison, perhaps using newer editions of ASME code?
(I'm pretty sure the VSR calculations are OK)
There are other references for external pressure vessel design?
Thanks in advance

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[TomBarsh]

A third parameter is missing: the length between lines of support. In ASME Code, the thickness required to support a given external pressure is a function of the geometric properties: nominal thickness, diameter, and distance between lines of support (rings, heads, etc); it is also a function of the material properties and temperature.

Similarly, the maximum allowable external pressure will be a function of the same variables.

To rate the 2500 mm Do x 10 mm thk cylinder for external pressure we need to know:

1- the length between lines of support,
2- material specification or properties,
3- temperature.

If you can provide the items 1 and 3, and an ASME listed material that is equivalent to your actual material we can determine the MAEP for the cylinder.

 

[Technopriest]

I have a design length of approx. 550 mm. The design temperature is 150 °C. The material required is ASME SA240 TP316L.
I'm trying to design the vessel as a jacketed one, coils are not allowed. The design length can be reduced (up to 300mm) dividing the jacket in three pieces.
Using VSR code, this vessel is about 10-12 mm thick; using ASME I am not able to reduce thickness below 18-20 mm- the difference is great, so I think I've missed something in ASME that allows thinner walls.. but I don't know what! I've used the rules provided in UG for cylinders under external pressure; perhaps there is an addendum or other optional part that allows for more advantageous wall thickness calculation under external pressure?


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[tjrush2000]

External Pressure, (Corroded & at 150.00°C) UG-28(c)
L/Do = 550.0000/2499.9999 = 0.2200
Do/t = 2499.9999/19.258006 = 129.8161
From table G: A = 0.004918
From table HA-4: B = 9884.8281

Pa = 4*B/(3*(Do/t))
= 4*9884.8281/(3*(2499.9999/19.258006))
= 101.5265 psi (700.0002 kPa)



Design thickness for external pressure Pa = 700.0002 kPa
= t + Corrosion = 19.258006 + 0.00 = 19.26 mm


Compare the formula and values. Just a note, but carbon steel thicknesses required are approximately 12 mm.

 

[prex]

Technopriest,
Using EN13445 you get an allowable external pressure of nearly 0.5 MPa with t=10 mm and of 1 Mpa with t=20 mm (roughly calculated values): hence this rule is more in agreement with ASME than your calculation.
I think you should carefully revise your VSR calculation: the hoop stress in wall with a thickness of 10 mm and a pressure of 0.8 MPa is [σ]=100 MPa and this is higher than the allowable stress for material at temperature f=86.6 MPa.
For sure this can't be acceptable!

prex

http://www.xcalcs.com

Online tools for structural design

 

[Technopriest]

With the latest update to VSR, maximum allowable stress for ASTM A240 TP316L at 150°C temperature is ~109,5 MPa, so it's within the limits for the 10mm thickness case.
I still think there is some problem, because even with the EN13445 calculations that prex just provided the external pressures are far greater than those allowed by ASME..

Reading the appendix for external pressure calculations (forgive me, I cannot recall the exact letter) I found that the "Cylinder under external pressure" for ASME considers the cylinder and both of it's bases- but in reality, my cylinder is under external pressure of 0,8 bar only on one base, and that base is not flat. It's perhaps this the origin of the difference?

Still wondering..
Thanks anyway to you all for your replies..

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[TGS4]

I would suggest that you re-perform your external pressure calculations using the methodology of Code Case 2286-1. The Code Case is intended to deal with any inherent excessive conservatism in the ASME methodology. Once you've done that, please report back to the forum.

 

[LSThill]

If you have CODEWARE: "COMPRESS PRESSURE VESSEL SOFTEARE" USE Div. 2 option for Code Case 2286-1. for Div. 1 Calculations.

lst

 

[prex]

Technopriest,
suppose you are using a 1% yield value, but this is not what you should do. Under EN13445 if you use the 1% yield in place of the 0.2% one, you must divide it by 1.3 to get the corresponding 0.2% based value, so you come back to some 85 MPa.
The difference of EN13445 to ASME is not that surprising: one should also compare the allowable out of roundness, that for thin cylinders may be relevant, and no, this has nothing to do with end caps, all codes assume a bare cylinder with lines of support at distance L.

prex

http://www.xcalcs.com

Online tools for structural design

 

[Technopriest]

Prex,
the data I have refers to allowable stresses for 0,2% yield value. For 1% yield value (material X2CrNiMo 17.12.2 EN10028, thikness up to 12mm) I read 136+ MPa (all data refers to 150°C temperature).
I repeat this data applies to VSR code; I have not used EN13445 since I have not a complete access to the relative norms.
I am trying to find Code Case suggested by TGS4, I hope that helps.
Again, many thanks to you all.

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