Correct formula for hoop stress in a cylindrical pressure vessel

[rdij]

I am designing a pressure vessel from plastic and am confused on the proper formula to use based on my geometry (NOTE this does not need to pass ASME requirements).

If we assume a cylindrical vessel with the hemispherical ends the relevant hoop stress equations are below (assuming thin wall and internal pressure):

σ = PR/t (cylinder)
σ = PR/(2t) (sphere)

Logic would dictate I would use the equation for a cylinder to calculate the thickness of the walls.

However, it was recommended to me from another (definitely more experienced) to use Barlow's equation which is, σ = PR/(2t), a common formula used in piping design. Barlow's formula is equivalent to the equation for a sphere above. So....what gives? A pipe is a cylinder, but is there an assumption in Barlow's equation that the length of the pipe allows you to ignore the end caps? I am confused on which one I should use, any help and explanation would be greatly appreciated.

 

[Geoff13]

Barlow's formula is σ = PD/(2t) which is equal to the cylinder formula of σ = PR/t.

Technically Barlow's uses Do (outside diameter) and the cylinder formula use R (inside radius), but for thin wall vessels this is an insignificant difference.