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Allowable stress for tubing on external pressure

Allowable stress for tubing on external pressure

Allowable stress for tubing on external pressure

(OP)
Dear all,

I've got a very simple, yet stupid question, since I feel im making a terribly simple (dumb) but tricky mistake.

There's this piece of tubing, which is approx 1500 mm long, and doesn't has any tubesheets in between
(so L/D > 50 for the external pressure calc of ASME B&PV VIII Div 1 UG-28).

Design conditions are 500 deg C external temperature, design pressure is of no concern right now.
The internal pressure is ambient, the tube only feels external pressure.
Tube is simple A213 or A269 TP316 material.  

When determining step 3 from B&PV VIII Div 1 UG-28 (c)(2), and looking at the tensile stresses at
design temp (500 deg C) for the tube, I find ASME II Part D lists 463 MPa (at pag 489 of the 2010 edt).
I found this to be a huge number compared to e.g. ASME B31.3, which lists a different value (107 MPa at
pag 171 of the 2010 edt) for approximately the same conditions.


Can anyone explain the mistake Im making?

RE: Allowable stress for tubing on external pressure

Different codes, different assumptions.  Use the one that applies and don't worry about the other.

Patricia Lougheed

******

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RE: Allowable stress for tubing on external pressure

(OP)
That I do understand, yes, they're different, hence different assumptions.
But a tensile stress value @ 500 deg C which differs by a factor 4?

Thats's due to a different reason. Not just the ocde, I cant believe that.
I must by making a huge mistake, but Im not sure what ..

RE: Allowable stress for tubing on external pressure

The figures you quote are nonsense. Even the value for B31.3 seems high to me, could possibly be for 500°F instead of 500°C, and the value for ASME is close to the ultimate strength for the material and far beyond the yield strength (at room temperature!).
Do you have problems with the units of measurement?
 

prex
http://www.xcalcs.com : Online engineering calculations
http://www.megamag.it : Magnetic brakes and launchers for fun rides
http://www.levitans.com : Air bearing pads

RE: Allowable stress for tubing on external pressure

(OP)
I dont have problems with the units of measurement (I double checked that, what I reported before is correct):
- the B31.3 reference, which is 107 MPa @ 500 C. So no 500 F (I used approx 15.5 ksi @ 925 F)
- the reference I gave for II-D was incorrect though, it was the ultimate tensile strength,
so that's where I made the mistake (the mistake I was looking for, that is).

The right value in II-D I shouldve taken was for the max allowable stress value S (pag 73), which
is 107 MPa or 79.2 MPa @ 500 C for smls 316 SA312 pipe (depending on which conditions are met).

Please correct me if im wrong.

RE: Allowable stress for tubing on external pressure

OK. You should be able to use the higher value, especially for an external pressure calculation, but you could also go over this step and determine the yield at temperature from the external pressure charts.

prex
http://www.xcalcs.com : Online engineering calculations
http://www.megamag.it : Magnetic brakes and launchers for fun rides
http://www.levitans.com : Air bearing pads

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