Nozzle thickness limitation in ASME?PD5500?

[jmgalpers]

Just a question on ASME regarding nozzle thicknesses - I have seen some company PV specs limit the wall thickness of a nozzle to be a certain number of times the thickness of the vessel shell say 3 times for example maximum.

I was wondering if there is a section in asme which limits this specifically physically or is it more a factor of economics such that when you are over a certain nozzle wall thickness, no extra credit is given in the reinforcement calculation ? ( but it is not a breach of the code to be over that area physically and resulting thickness ).

In addition, I heard a similar rule of thumb that PD 5500 limits it to 1.5 times. Just to check if others on the forum agree or again it is a rule of thumb.

Thanks

 

[TGS4]

I agree with such a limit, but not for the reasons that you list. My rationale is that once the ratio of the thickness of the nozzle to the thickness of the shell exceeds a certain value (somewhere between 1.5 and 3 at least), then the rules regarding nozzle reinforcement and local stresses caused by loads from the attached piping start to fall apart. Rather than having two relatively flexible components joined together, by having a very thick nozzle you have created a "hard-spot" or thickness-mismatch. This mismatch is going to create stresses by itself that are not typically calculated by non-FEA methodologies.

So, the limit is a good and appropriate one - IMHO.

 

[jmgalpers]

Thanks very much.

 

[MJCronin]

jmg...

There are other issues in deciding upon nozzle thicknesses.

I have seen company guidlines that call for all nozzles less than 2"NPS to be Schedule 80 or higher simply to ensure that there is no damage fron tat 250 lb fitter that is looking for a place to put his foot...

Additionally, for certain process conditions, (concentrated H2SO4) it is wise to make the nozzles thicker where the liquid increases in velocity (read "nozzles")and accelerated erosion occurs.

Not every tank/vessel is appropriate for any condition.......despite what is told you by the plant managers..


-My opinion only


-MJC