Since this seems to have turned into a newbies vs. graybeards thread, let me weigh in on the side of the greybeards.

1. You do not CALCULATE allowable stresses for nozzles, for the most part. There are standards (Heat Exchange Institute for shell and tube heat exchangers, and API 610 for centrifugal pumps, for example) that provide guidelines, but they are for the protection of the equipment and often do not provide limits for any one nozzle. (In particular, the HEI standard gives a method to calculate allowable loads for each individual nozzle, and a method for combining them into an overall load that is also limited. Often the overall limit controls.)

2. Normally, the equipment vendor provides allowable laods at nozzles for his equipment. If your company did not require them to do so with their bid, shame on your equipment engineers. Tank nozzles can be analyzed using the Bijlaard methods of WRC 107, but this analysis is now considered to be of limited value, sincve FEA is available.

3. Engineering is a cooperative effort. No one is expected to do it all by himself, and those who attempt to do things outside their area of expertise should be discouraged. Pipe stress analysis is enough for you to bite off in one sitting, there's no need to try to become an equipment expert in the same time frame.

4. Pipe stress analysis is not something you should attempt to learn by yourself, even with the help of this discussion forum. If you have no experienced piping engineer to learn from, you should strenuously try to find a job where a competent mentor is available. The mere fact that you have chosen to ask such an elementary question (which stands on an erroneous assumption as Anindya has pointed out) suggests that you have no one to mentor you. The fact that you are outraged to have your lack of knowledge pointed out to you does not speak well of your maturity. One who asks for free advice should take great pains not to insult those who respond to his request!

5. A number of people from China and India appear to have hacked into CAESAR II and, because they have the software available are presenting themselves to the world as pipe stress analysts. If you are one of those scum, I spit on you. If you have a legitimate copy of the software, then you have a legitimate right to have your questions answered, although as I have pointed out, you have no right at all to have experienced engineers endorse your erroneous understanding of the profession.

6. There's quite a bit more to doing pipe stress analysis than putting numbers into CAESAR II so that the code compliance report says you passed - if it were otherwise I'd have gone on to something more challenging a long time ago.