Sustained Loads in Piping Codes vs. Hoop Stress

[pflow]

I have a simple question and I have probably overlooked something.

The stress from sustained loads in the piping codes are computeted with: PD / 4t

I was wondering why this are not based on the more limiting hoop stress: PD / 2t ?

 

[BigInch]

PD/4t is not half the hoop stress, it is the AXIAL tension stress created by pressure acting on a pipe with capped ends, or a closed valve. Pressure * Pipe's x-sectional Inside Area / x-sectional Area of the Pipe wall. If you approximate that axial stress, it is equal to 1/2 the hoop stress, or D/4t, where D is the average wall thickness = OD- t.

Independent events are seldomly independent.

 

[pflow]

Thank you for the reply BigInch.

Could you clarify why the larger circumferential "hoop stress" is not used in the sustained load analysis?

When inspecting the formulas in the piping code for required wall thickness, it looks like they are based on the hoop stress.

 

[BigInch]

Hoop stress is pressure related and constant, therefore not changed by temperature expansion.
Hoop stress is one component of the design. Only the initial wall thickness is determined by hoop stress formula, the resulting stress must be less than the hoop stress allowable. But it is not finished with that. The combined stress considering both axial, hoop and bending stress is another check which also must be less than the allowable stress for stress combinations. The wall thickness is adequate only if it passes both checks. Wall thickness should still be checked for other load and resulting stresses, such as hydrostatic and local buckling stresses, and for whatever other loads may affect the thickness.

Independent events are seldomly independent.

 

[pflow]

BigInch,

Yes, I agree that with your statement "the combined stress considering both axial, hoop and bending stress is another check which also must be less than the allowable stress for stress combinations."

Early in a project, I performed a quick hand calculation with hoop stress and bending stress. Than when the piping was draw up, I followed up with a pipe stress analysis (CAEPIPE) at ambient temperature and noticed the sustained load stresses were about 1/2 of my hand calculation.

Any feedback on how to explain this?

 

[BigInch]

Already told you.

If you approximate the axial stress in a pressurized round pipe with no temperature change, it is numerically equal to 1/2 the hoop stress.

Independent events are seldomly independent.

 

[ione]

pflow,
I think what you are missing is the definition of stress due to sustained loads, that is the algebraic summations of the longitudinal (axial) pressure stress and longitudinal weight stress. If you take into account just pressure effect you get: SL = P*D/4t.
So in the definition there's nothing related to with circumferential (hoop) stress.

 

[DSB123]

BigInch,
You are slightly incorrect in what you say. "The combined stress considering both axial, hoop and bending stress is another check which also must be less than the allowable stress for stress combinations". For ASME B31.3 and B31.1 the axial pressure stress, direct stress(from external loads/ weight) and bending stress(due to weight/sustained loads) is checked against the allowable stress for sustained condition. The hoop stress does not come into the equation and as you say is only used for thickness calculations.

 

[BigInch]

Combining Stress including hoop stresses is mentioned in all these codes
B31.4 Paragraph 402.7 & A402.3.5
B31.8 Paragraph 833.4
B31.1 Vii-5.0
B31.3 Paragraph 302.3.6 - S_L due to sustained loads, such as pressure ..


Independent events are seldomly independent.

 

[pflow]

Thank you everyone for the replies.

I would like to try to apply everything that has been discussed in this thread to B31.3 Fig. 323.2.2B General Note (a)(1). Does this "nominal pressure stress" need to include axial, hoop, and bending stress or some of these and not the others?

 

[ione]

Nominal pressure stress = hoop stress

 

[DSB123]

BigInch,
In response to your last post I only mentioned B31.3 & B31.1 so do not see the consequence of referencing the other Codes in responding to my response. Also your last line "B31.3 Paragraph 302.3.6 - SL due to sustained loads, such as pressure .. " is correct but can you advise how the longitudinal stress SL has anything to do with the Hoop pressure stress? SL is calculated using the longitudinal pressure stress combined with other "longitudinal" stresses. This also applies to B31.1

 

[pflow]

ione, thank you for the reply. That is the kind of succinct statement I was looking for.

Is there a reference available for this "Nominal pressure stress = hoop stress"? Would just like to have some backup available if needed.

 

[ione]

CASTI Guidebook to ASME B31.3 - Process Piping

 

[BigInch]

The pressure component of axial stress in all pipe, is

1.) In Restrained segments is related to hoop stress by Poisson's ratio [ν],
axial stress due to pressure = P * D /2 /wt * [ν]

2.) In Unrestrained segments, the longitudinal pressure stress component
axial stress due to pressure = P x A_internal/ A_wall



Independent events are seldomly independent.