Assessing nozzle thicknesses to API653

[KiwiTanks]

I have been asked to look in to the assessment of BST nozzles to API653. The code basically says to evaluate the thickness with the 'as-built standard'. However, often these tanks were built to an unknown code.

I have therefore been evaluating to both API650 (essentially Sched 80 pipe) and API620 (basically Standard weight pipe), as they are both API codes. Previously my clients accepted that if the nozzles were below API620 thickness, then they were replaced with Sched 80 pipe. - No problem.

However recently I have struck a client wanting to evaluate the thickness rather than just use a figure from the codes.(ie they don't want to spend the $$ I think)

Can anyone point me in the right direction as to how to assess these nozzles? Is there a basic (even not-so basic) formula somewhere that I'm missing? Or am I OK to continue using the API620 thicknesses as minimums?

Also - What happens when you install a new nozzle with Sched 80 pipe, then take thickness readings and find that it's below the thickness in API650 Table 5-6 (even if it's only 0.2mm say?)

 

[JStephen]

You can calculate required wall thickness of nozzles based on internal pressure as t = PR/Stress. This is not normally calculated with API tank nozzles because with the low pressures involved, it will come out to be paper thin, and the minimum thickness will control. If you have applied loads on the nozzle, you can calculate thickness based on them, but in most such cases, the shell construction would limit loading rather than nozzle thickness.

I would be wary about recommending changes to a tank "built to an unknown code" based on API-653. Or to put it more correctly, the better recommendation might be to demolish the tank and build a new one to a known standard.

On the nozzle wall thickness, refer to ASTM A53 or similar pipe codes as applicable. Pipe wall thickness can vary up to 12.5% in some of the standards, and that would govern rather than the normal allowable shell underrun.

 

[JoeTank]

One can evaluate the existing nozzle wall by checking the thickness requirements for pressure and proper reinforcement of the opening. The reinforcement calculation rules are in both Standards. If piping loads are present, they must be evaluated also. Appendix P of API-650 will be useful for this. There are no simple formulas that address all of these issues. By the way, API-650 requires Schedule XS, not Sch 80. Aside from the piping load requiements, the nozzle wall can often be very thin, so you will need to consider some practical tmin for your evaluation. Not very many oil companies have establish a tmin for the shell nozzle, but those that have often use a 1/8" minimum or similar.
You say that the tanks are non-Code. Does that mean they do not comply with a recognized Code/Standard or does it mean that you have lost the nameplates or documentation on the tank?

Joe Tank

 

[KiwiTanks]

Hi Joe Tank,

Thanks for the tips. Just as I thought - no easy answer.

As for the non-code tanks - I mean that there are no records of what the tank was built to. ie no nameplates, no paperwork etc, and they were generally built before I was born...

Cheers
Dave