Stress on knuckle region of an ellipsoidal head: Section VIII Div 1

[dnigos]

I have a drum with a localized corrosion issue on the knuckle region of the head. Tmin calculation on the knuckle region (under hoop compression) is required for FFS evaluation. To me (coming from a recent graduate without prior experience), this calculation can not be done with a single equation and might require a more complex analysis. Section VIII UG32 only gives you Tmin for the dome region (under tension). Tmin of the knuckle SHOULD be less than the Tmin for the dome.

Please Advice. Thanks.

 

[jte]

dnigos-

You are out of Section VIII territory and squarely into API 579 / ASME FFS-1 territory when dealing with degraded equipment which has been in service. Sounds like you are well past a Level 1 (essentially a Section VIII) evaluation, and I don't think a Level 2 evaluation exists for a knuckle. This leaves you with a Level 3 evaluation - basically, you need to get out your FEA package and model it. Nonlinear would be highly preferred in this case though linear may get you a usable answer.

Here's the part you really don't want to hear: To do a Level 3 evaluation, you need to be experienced in FFS work. Clearly, you haven't had a chance to get this type of experience yet. This leaves you with the option to shut down and repair (weld buildup or otherwise) or hire a consultant to do the FFS evaluation for you.

And UG-32 gives you the minimum required thickness for the entire head, not just the dome region... The compressive nature of the stress at the knuckle is recognized, though not explicitly stated in the code.

jt

 

[MJCronin]

dnigos..

The "knuckle region" drum corrosion failure is the specific reason why an organization must properly layup a boiler when it has been shutdown. Corrosion always occurs in very expensive places when a boiler is neglected.

But boiler corrosion protection, of course, assumes that top management knows what they are doing and has a competent maintenace staff. It also assumes that they somehow care about the next purchaser of the plant and equipment

With today's crop of bold swashbuckling MBAs buying and selling multi-million dollar companies, minor bothersome details like boiler protection are left to the minions...... or in your case, dumped on the new engineer

Is this what happened ?

Tell us the size of your drum, capacity and age of your boiler. Are there any maintenance records ?

-MJC