ASME B31.3 Pipe with internal pressure below atmospheric

[Alessandro Celant]

Hi everyone,

My company is verifying some piping classes for a future carbon capture and storage plant design.
Some parts of the plant (with sizes from 30" through 92") will have internal pressure slightly below atmospheric pressure (e.g. 0.90 bar) but even much lower atmospheric pressure (e.g. 0.2 bar) [being atmospheric external pressure = 1 bar).

I am navigating through ASME B31.3 Sect. 304.1.3 and all the complex sequence of calculations (ASME VIII + ASME II.D and UG-28), which thanks to this forum I successfully did a double logarithmic interpolation for finding A values.
The Pa value I obtain at the end of the calculation shall be compared with the P, which is the design external pressure (1 bar in my case).
If Pa > P, everything's good as it is because the maximum external allowable pressure will be higher than atmospheric pressure.
If Pa < P, then the wall thickness of the pipe shall be increased otherwise it buckles.

BUT. As far as I understand, the internal pressure of the piping line will never appear in the calculation.
This means that the calculation is only for full vacuum condition inside the line (P = 0 bar)?

How could I verify that my 30"-92" pipes/ducts will not buckle with internal pressure of say 0.2 bar (being the external still 1 bar)?

Thank you in advance for your cooperation
Regards
Alessandro

 

[david339933]

Are you using bar(a) internally and bar(g) externally?

 

[Cobra17]

If you are calculating the internal pressure at 0.2 bar and the external pressure at 1 bar, they are independent of each other in the calculations. Having no internal pressure while 1 bar external pressure is a more conservative calculation that having 0.2 bar internal pressure with 1 bar external.

 

[LittleInch]

TO avoid confusion you really need to use bara and barg... Or just use bara exclusively.

Not quite sure what you've done, but if you want a full vacuum, use internal pressure 1 bara and external pressure 2 bara, like Cobra17 says.

Below 36" any practical wall thickness shouldn't buckle, but up to 92" that could get interesting.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Alessandro Celant]

Thank you all for your replies.

I am using exclusively bara: I am considering 1 bara outside the pipe (i.e. atmospheric pressure) and 0.96/0.90/0.2 bara inside the pipe (examples given).
[In barg, of course it would have been 0 barg outside and -0.8 barg inside].

In any case, Pext > Pint.

I agree with Cobra17: calculating the maximum allowable external pressure considering full vacuum inside the pipe is however the most conservative way to verify if the pipe wall thickness is enough to sustain the atmospheric external pressure (1 bara) simply because it's the worst case scenario (ΔP = 1 bar).
And by the way I presume that this is the only way you can check it as per ASME B31.3 Sect. 304.1.3 and the consequent methodology: full vacuum inside the pipe, because the internal pressure is not included in the calculation iter.

By doing so, my Excel calculation sheet (100% mathematically correct) says that for Pext = 1 bara a 32" pipe ASTM A358 TP304 with WT of 7.92mm won't be enough, because the calculated maximum allowable external pressure is Pa = 0,0876 MPa (0,876 bara < 1 bara), hence it will buckle. But only in case of full vacuum condition.

Before warning everyone that the 32" pipe needs to be thicker everywhere (even in plant locations where the 32" pipe will have internal pressure just a little below external pressure, say 0.98 bara), I'd like to have your opinion on this matter.

I honestly think that if the 32" pipe will have an internal minimum pressure of 0.96 bara, no buckling could happen.
Different story if the minimum design pressure of this system (currently unknown) would really be FV.

 

[Alessandro Celant]

Just for the record of this thread, I probably got the meaning of "P" value, i.e. the external design pressure to which the calculated maximum allowable external pressure must be compared (greater than).

If Pint = 0 bara (full vacuum) --> P = 1 - Pint = 1 bara
If Pint = 0.2 bara --> P = 1 - Pint = 0.8 bara
If Pint = 0.96 bara --> P = 1 - Pint = 0.04 bara

So, in case of pressure inside the pipe of 0.96 bara (slightly below atmospheric external pressure), the pipe wall thickness will globally feel an external pressure of P = 0.04 bara.

I believe this is the correct way of understanding the procedure.

 

[Cobra17]

Not knowing how long you pipe is, I used L/Do of 50 (Max) and got a minimum thickness of 3.3347mm due to full vacuum (1 bar external pressure). I assumed no corrosion allowance, 10% RT, and 12.5% UT on the pipe wall of 7.92mm

 

[SPDL310]

This is kind of out of my wheel house so I'm interested in what other have to say on this issue, but a couple of thoughts that have come to my mind are the following:

1) commercially available pipe with out of round tolerances that are acceptable for internal pressure may not meet your needs for vacuum service as those imperfections may significantly reduce your allowable external pressure.

2) Fittings such as bent elbows may have reduced allowable external pressures.

3) Are traditional beam element piping stress tool (i.e. CAESAR etc.) able to accurately predict buckling in piping under vacuum service?

 

[FacEngrPE]

You should consider if the Round Industrial Duct Construction Standards; SMACNA applies to any of your large low and negative pressure pipe.

The document provides rules for using external stiffeners to prevent buckling.

Offers a standardized, engineered basis for design and construction of industrial ducts of Classes 1 to 5. Includes a spiral duct chapter for Classes 1 and 2, covers design pressures ranging from 30” wg negative to 50” wg positive, plus carbon and galvanized steel tables. Changes to the manual include expanded tables for stainless steel and aluminum, expanded tables for duct sizes up to 96 inches in diameter, introduction of a new Duct Class 5 for systems handling corrosives, and spiral lockseam pipe

.