Hi,
what would be the LPG vapor minimum design pressure at the top of the sphere as per good engineering practice and which parameters are considered to fix it?
Vessels holding non refrigerated LPG are specified with an upper design pressure of approx. 20barg. Generally, commercial grade LPG has a sat vapor pressure of about 14barg at 38degC. You can make a good first approximation by looking up sat vap pressure curve for pure propane, and find the sat vap pressure at the max normal operating day time temperature at your location.