Effect of internal pressure

[GOBLINTECHNIC]

When we calculate the longitude stress, radial stress ,hoop stress due to internal pressure
(use ASME B31.3).

They are sustain load.

But if the pressure is not constant and rise up and down for a period such as P1= 700 psi
P2= 1000 Psi

ASME B31.3 tell
(a) Operation. The sum of the longitudinal stresses SL
due to sustained loads, such as pressure and weight,
and of the stresses produced by occasional loads, such
as wind or earthquake, may be as much as 1.33 times
the basic allowable stress given in Appendix A.

How i can calculate the stress analysis ?

I have 3 choice in my opionion

1. Use P2 calculate for sustain load only. Not calculate the occasional load.
2. Use the P1 calculate for sustain load and use P2 calculate for occasional load.
3. Use the P1 calculate for sustain load and use P2-P1=1000-700= 300 psi calculate for occasional load.

 

[BigInch]

To fall within a sustained load, or an occasional load, the total time at which the system remains at a given pressure must be considered. You will find those times listed in B31.3 I think it is Section 403.

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[JohnBreen]

GOBLINTECHNIC

You must first logically establish what your design pressure will be and use that in pressure design for determining your required minimum wall thickness. Then you use your logically determined design pressure and your calculated wall thickness (rounded up to the next standard schedule) in your stress analysis.

Perhaps you are confusing pressure/temperature VARIATIONS with OCCASIONAL loadings? Pressure/temperature VARIATIONS may affect your design pressure and therefore affect your minimum required wall thickness. OCCASIONAL loadings are a separate issue.

Regarding variations in pressure design, the B31.3 Code provides limits on allowable pressure/temperature variations in pressure in paragraph 302.2.4. If the NORMAL OPERATING pressure of your piping system varies between 700 psig and 1000 psig then it must be designed for the higher pressure rating - 1000 psig at temperature is the minimum Design Condition. The Code allows for SHORT TERM pressure/temperature variations above design pressure. Note in paragraph 302.2.4 that the limit for allowable pressure VARIATIONS ABOVE DESIGN PRESSURE is 33 percent higher than the design pressure and that cannot exceed more than 10 hours for any occasion and no more than 100 hours per year. If the variation is greater than design pressure but is not greater than 20 percent of the design pressure it is still limited to no more than 50 hours for any occasion and no more than 500 hours per year. ALSO, the total number of design condition variations shall not exceed 1000 during the life of the piping system. Those criteria limit the system that has a design pressure of 700 psig to maximum of 933 psig for variations and for not more than 10 hours for any occasion and no more than 100 hours per year. If the pressure (at temperature) can exceed these limits (and IT IS excessive since your 1000 psig is greater than 933 psig) then you must design for a pressure rating of 1000 psig at temperature. In calculating the minimum required wall thickness you must use 1000 psig in the calculations of paragraph 304.1.1 (see also 302.3.5(a)).

Please download this document:

http://files.asme.org/asmeorg/Governance/Volunteer/CareerSeries/9676.pdf

Circumferential stress due to internal pressure (hoop stress) is addresses by the minimum wall calculation. See paragraph 304 and also 302.3.5(a). Also see page 105 of the downloaded document. These are primary stresses. Radial stresses only become important in high pressure design where relatively thick walled pipe is the norm.

Longitudinal stresses due to pressure are additive to the longitudinal bending stresses due to weight (dead weight and live weight). "Sustained Longitudinal Stress" is the sum of the longitudinal stresses due to pressure plus the longitudinal bending stresses due to weight (see B31.3 Code Case 178). These are primary stresses. Note that this is a static stress "state" (in contrast, displacement stress due to thermal expansion/contraction and other cyclic loadings is a stress RANGE). The limit (maximum allowable stress) for the sum of the longitudinal stresses due to pressure plus the longitudinal bending stresses due to weight is Sh. Sh is the allowable stress for the basic material AT OPERATING TEMPERATURE as found in Appendix A. For the equation that addresses longitudinal stresses due to pressure plus the longitudinal bending stresses due to weight see B31.3 Code Case 178. In your example, for calculating the sustained longitudinal stress in accordance with paragraph 302.3.5(c) and Code Case 178, you must use 1000 psig as the design pressure.

The Code limit for stresses due to short term OCCASIONAL loads (high wind, seismic events, internal pressure surges, SRV blow down, etc.) may be as high as 1.33 times the allowable stress at temperature as provided in Appendix A (paragraph 302.3.6). These are primary stresses.

In performing the stress analysis the piping system wall thickness will be that required for 1000 psig (as above) and you will use 1000 psig as the design internal pressure in calculating the SUSTAINED longitudinal stresses due to weight and pressure. The limit on these calculated SUSTAINED stresses (the maximum allowable stress) will be 1.0 times Sh at temperature from appendix A. You will perform a OCCASIONAL loading analysis with the appropriate occasional loads (high wind, seismic events, internal pressure surges, SRV blow down, etc. but not all these acting together) included and you will use 1000 psig as the design internal pressure in calculating the OCCASIONAL longitudinal stresses due to weight and pressure. The limit on these calculated OCCASIONAL stresses (the maximum allowable stress) will be 1.33 times Sh at temperature from appendix A.

Regarding your "three choices", I do not understand how you have come to that conclusion.

Regards, John.

 

[GOBLINTECHNIC]

Thank-you very much for your kindness Mr.John