Sustained Loads in Piping Codes vs. Hoop Stress
Sustained Loads in Piping Codes vs. Hoop Stress
(OP)
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 ?
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 ?





RE: Sustained Loads in Piping Codes vs. Hoop Stress
Independent events are seldomly independent.
RE: Sustained Loads in Piping Codes vs. Hoop Stress
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.
RE: Sustained Loads in Piping Codes vs. Hoop Stress
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.
RE: Sustained Loads in Piping Codes vs. Hoop Stress
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?
RE: Sustained Loads in Piping Codes vs. Hoop Stress
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.
RE: Sustained Loads in Piping Codes vs. Hoop Stress
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.
RE: Sustained Loads in Piping Codes vs. Hoop Stress
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.
RE: Sustained Loads in Piping Codes vs. Hoop Stress
B31.4 Paragraph 402.7 & A402.3.5
B31.8 Paragraph 833.4
B31.1 Vii-5.0
B31.3 Paragraph 302.3.6 - SL due to sustained loads, such as pressure ..
Independent events are seldomly independent.
RE: Sustained Loads in Piping Codes vs. Hoop Stress
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?
RE: Sustained Loads in Piping Codes vs. Hoop Stress
RE: Sustained Loads in Piping Codes vs. Hoop Stress
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
RE: Sustained Loads in Piping Codes vs. Hoop Stress
Is there a reference available for this "Nominal pressure stress = hoop stress"? Would just like to have some backup available if needed.
RE: Sustained Loads in Piping Codes vs. Hoop Stress
RE: Sustained Loads in Piping Codes vs. Hoop Stress
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 Ainternal/ Awall
Independent events are seldomly independent.