Hydro test cover detail for localized test of new nozzles

[jomanano]

Design per ASME Code Section VIII, Div. 1. Adding a 16" diamter nozzle to a vertical vessel and want to do localized pressure test instead of entire vessel.
Any geometrical considerations to be made to meet Code or practical methods? Is there a minimum distance past the nozzle opening that test cover must include to properly simulate the stresses acting on the vessel shell as if the pressure test was conducted to the entire vessel?

 

[SnTMan]

jomanano, I think you could follow general Code guidelines (not explicitly stated as such) on seperation of structural discontinuities, based on the decay length 2.5*sqrt(R*t). You could probably decrease this length somewhat.

Bring your AI on board early.

Regards,

Mike

 

[jte]

jomanano-

If this were new construction, and the fabricator already hydro'd the vessel, but not stamped, then for whatever reason wanted to add a new connection and perform a local test... It would not be acceptable.

While the use of hydrotest caps is not uncommon, you already know that the state of stress will be different: You mention that you want to "properly simulate the stresses..."

Think about the derivation of hoop and longitudinal stress. Projected area times pressure gets you the force, cross sectional area of the steel gets you the area. How could you reasonably expect to develop the same order of magnitude of stress in the shell with a hydrotest cap?

Yes, it is done frequently. No, it does not come close to creating a similar state of stress in the assembly. But it does make the operators feel better. After all, "it's been tested!"

jt

 

[jomanano]

Thank you for the responses so far.
This is for an alteration of an existing vessel, not new construction.
Jomanano

 

[jte]

I understand that this is for an alteration. The point remains. The ASME does not allow this as the state of stress cannot be simulated. The NBIC does - but don't fool yourself into thinking that you are creating a similar stress field.

jt