Seismic Load included in 3Sm Criterion ? 1

[kapilengtips]

Hi ,

Per Customer Specs, I have Following Nozzle Loads Category :

1) Weight+Pressure
2) Thermal
3) Seismic

Now while peforming WRC 107 Check for 3Sm criterion, should Seismic Loads be included or excluded from the nozzle loads?

Thanks,
Kapil

 

[jte]

Are you implying that you consider stresses on a vessel due to a seismically shaking pipe to be secondary? What is the self limiting nature? Are you sure you shouldn't consider these stresses to be primary?

I'd use the 20% increase allowance with a 1.5S criteria for 1.8S unless the external loading can be shown to be secondary.

It'll be interesting to see how others weigh in on this.

jt

 

[kapilengtips]

No, what I meant was while performing the allowable nozzle load calculation ( Nozzle Load = Weight + Pressure + Thermal + Seismic (SC-II Vessel)) does the 3*Sm criterion of Pl+Pb+Q needs to be satisfied.

According to my understanding, since 3*Sm criterion is used to avoid fatigue failure and since earthquake load will not result in fatigue, therefore for Load Combination = Weight + Pressure + Thermal + Seismic) need to satisfy only two criterion :

Pm : kSm
Pl+Pb : 1.5*k*Sm
where k =1.2

However for Load Combination = Weight + Pressure + Thermal
I need to satisfy all the three criterion since they are operational loads and can result in fatigue if 3*Sm criterion is not satisfied.

Pm : Sm
Pl+Pb : 1.5*Sm
Pl+Pb+Q : 3*Sm

Please correct me if I am wrong in my interpretation.

Thanks,

Kapil

 

[jte]

I believe you are correct - just be careful of how you define "thermal". Thermal itegral to the vessel will likely cause secondary stresses. External loads such as those caused by thermal expansion of piping are likely to cause primary stress. See Div. 2 table 5.6 under "nozzle" and pay close attention to the phrase including those attributable to restrained free end displacements of attached piping.

jt

 

[prex]

jte, I understand that kapilengtips is correctly classifying the local bending stress due to the interaction of nozzle rotation with vessel wall as a secondary stress (the corresponding membrane portion being local primary and the rest barely primary).
kapilengtips, I do not agree with your reasoning: failure due to ratcheting is also called 'low cycle fatigue', failure may happen in just a few cycles. I think that you need to consider seismic loads associated with all the rest.
Also, your k factor, that increases the allowables for abnormal or occasional loads, is not applicable with the 3S[sub]m[/sub] limit.
That because this is a 'physical' limit: it represents the double of the (minimum) yield strength and thus the range of elastic stresses after shakedown, and there is no safety margin on it (except for stress hardening).

prex

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[TGS4]

kapilengtips, you may want to familiarize yourself with Table 5.3 of ASME Section VIII, Division 2. The methods (and factors) that you are quoting are from prior editions (NOT CURRENT) of Division 2. For simplicity sake, these are the checks that need to be performed:
P+Ps+D
P+Ps+D+L+T
P+Ps+D+Ss
0.9P+Ps+D+(W or 0.7E)
0.9P+Ps+D+0.75(L+T)+0.75Ss
0.9P+Ps+D+0.75(W or 0.7E)+0.75L+0.75Ss

Where:
P is the design pressure
Ps is the status head of liquid or bulk materials (eg catalyst)
D is the dead weight
L is the appurtenance live load or effects of fluid momentum
E is the earthquake load
W is the wind load
Ss is the snow load
T is the self-restraining load case (such as thermal)

I have argued that piping "thermal' loads are, as jte explained, "restrained free thermal expansion", and should be classified as D, unless it can be positively demonstrated to be otherwise.

In all of these load combinations, per 5.2.2.4.e), Pm is limited to S, PL is limited to 1.5S, and PL+Pb is also limited to 1.5S. Per 5.5.6.1.a), P+Q is limited to Sps, where Sps is the greater or 3Sm or 2Sy.

Specific to your question, I would include earthquake loads in the Sps (the formerly-mentioned 3Sm) check.

A few other things to note:
1) Make sure that you are in the applicability range of the WRC-107 charts.
2) Be absolutely sure that your maximum stresses are at the cardinal-directions. Otherwise, WRC-107 doesn't check those locations, and you may miss the highest-stressed part in your nozzle.
3) If your nozzle has repads, make sure you understand and agree with the methodology that you are using. WRC-107 itself doesn't specifically mention repads at all.

For everyone else ,please note that the CURRENT edition of Division 2 does NOT permit any reduction (the old k-factor) for earthquake anymore. This is a SIGNIFICANT change! The ASCE code made this change several years ago, and ASME has finally caught up. Don't let yourself (or your corporate software) be caught unaware.

 

[jte]

Kapil-

Is this a Div. 2 vessel or a Div. 1 vessel using Div. 2 methodology?

jt