Required Thickness Calculation for Nozzles in API 650 Tanks

[Tomas.a]

Hello everyone,

We are having troubles determining the required nozzles thickness for API 650 tanks currently in operation.

In 5.3.2.3 of API 650 says “In no case shall the neck thickness provided be less than the nominal thickness shown in the table 5.6a”.

API 653 does not provide information on this calculation, so we are using Table 5.6a from API 650. This table specifies a design thickness of SCH XS for nozzles. To define the required thickness, we take the values from this table and subtract a 12.5% tolerance (following the approach of ASME VIII Div. 1, UG-45).

However, when analyzing the measured thicknesses of the nozzles in our tanks, we are finding a problem: almost none of them meet the defined thickness requirements. In most cases, the tanks have SCH STD nozzles.

Is there an alternative method to determine the required thickness for these nozzles?

Thanks in advance!

 

[JStephen]

For pressure vessel design, it is normal to calculate required nozzle thickness based on nozzle ID and pressure at the point. Due to the low pressures in a typical API tank, this will give thicknesses very close to zero, so it is not normally calculated for a new tank.
If a customer specifies external nozzle loadings, that may be a second criteria to establish nozzle thickness, but I would expect that required thickness would still be very small.
If you are checking corroded thickness, I would not assume that it must be the XS thickness less mill tolerance, as that is basically the "new" tank minimum. so you are saying you don't allow any corrosion loss whatsoever. But I don't have a copy of API-653 handy to research the requirements for nozzles. I assume the question is addressed somewhere in there. If they mention a "calculated" thickness, this would likely be based on the internal pressure requirements and be quite small.
If a new API-650 tank is built with "STD" schedule nozzles, that is a deviation from the standard. It would quite likely still meet all required strength and reinforcing requirements, though. Whether it is worthwhile to "fix" issues like this is a different discussion.
I don't know if API-650 has ever specified anything less than XS, you'd need to find the standard edition the tank was built under and see what it says.
Note that the "XS" requirement is specifically an API-650 requirement, and tanks built to AWWA or NFPA standards may vary. I don't remember what the requirements of API 12F or 12D are, they may allow standard weight.

 

[JStephen]

I was looking through an old version of API-653 that I found online, and Section 4.3.9.2 states, "Nozzle wall thickness shall be evaluated for pressure and all other loads." The numbering may have been revised, but there should be something similar or in more detail in the current version.
As stated there, though, this would be design for hoop stress (equations in ASME Section VIII or whatever) and for externally applied loads.

 

[HTURKAK]

Hello everyone,

We are having troubles determining the required nozzles thickness for API 650 tanks currently in operation.

Can you provide some info.( size and height of the tank, shell course thicknesses ,diameter of nozzles .. ) and maybe get better responds?
I would like to remind that , the reinforcing area includes the reinforcing plate, reserve thickness or extra material in the shell when wall thick. is greater then required thickness, and the nozzle neck thickness with in 4X tn of the outside surface of the shell.
This is not explicitly stated but you can observe that the nozzle thickness by looking Table 5.7.