Choosing the right steel for pressure accumulator shell

[JuzerNejm101]

I need to build a pressure accumulator which will be then used as a spring actually (upper side of the accumulator will be filled with nitrogen, and the lower won't be filled with oil, but will have a rod which will be holding movement of a machine part (rod) at a high frequency.) I'm having trouble choosing the right material for this purpose.
I'm not sure how much pressure I will require inside, so I took an above expected value of 60 bar. By inserting the values of pressure and a surface area in the formula p= F/A i get an value around 50 kN.
BUt then again using formulae for stresses from here (page 15-10):
www(point)navsea(point)navy(point)mil/nswc/carderock/src/mechrel/products/handbook/CHAPTER15RevB(point)pdf

I get really low results (the stresses (after converting units) end up to be between approx 2-7 N/mm2).
So I think I'm doing something wrong. Because these results indicate I can actually use any type of steel, which can't be right because I'm worrking with some rather high pressures / forces acting on the material in there.
The other thing is, my wall thickness isn't really constant and it's not cylindrical nor spherical as these formulae describe. It's following a curve of a membrane so it's more eliptical.
Can someone guide me to the right path? Thx.

 

[Navin A S]

You will need to use advanced calculations to determine the stresses in your pressure accumulator. You will need to take into account the shape of your accumulator, the maximum pressure it will be exposed to, the material properties, and any other factors that might influence the stress levels. You will also need to factor in any safety margins.For this type of application, you should use a material with high strength and good corrosion resistance. Commonly used materials for pressure accumulators include stainless steel, aluminum alloy, titanium, and composite materials. Each material will have different properties and must be carefully evaluated for its suitability in your application.It is also important to ensure that your accumulator is designed properly and that the seals are of the highest quality. If there is a failure in the accumulator, the consequences could be catastrophic.