I am doing a stress analysis for tank nozzles using WRC 107. The nozzles are connected to a closed loop inside the tank. The nozzles have a much higher design pressure than the tank.

Which design pressure should I use to do the stress analysis on the nozzles in WRC107?

Thanks,

Piping Rule of Thumb (R.O.T.)
#1 There is a Rule for everything
#2 There is an exception for every Rule
#3 The Boss is still the Boss

Your problem:
You have what appears to be two nozzles on a tank. You want to analyze them. You have two sets of conditions.
This falls under Rule #1and you would normally use the most savior conditions.

In fact you have a high pressure/temperature Heating coil in the tank which has the supply and return piping going through the tank wall.
This falls under Rule #2

Think of it like this. There are no flanges on the coil inlet or outlet, there are just two long pipes that penetrate the tank wall. These penetrations only need to be analyzed for the conditions of the tank contents and reinforced (or not) as applicable.

Thank you Jop.

Yes, it is heating coil in the tank. But there are flanges on the coil inlet and outlet. So I can use the tank pressure and temperature to analyze the nozzles then.

Another question, since the nozzles are not centered in the vessel, how to put the nozzle vectors?