You can design whatever you wish as long as it meets a given standard and/or code.
You have mentioned AD2000 B8 in your heading. As far as I understand, this flange standard only contains concentric flanges.
Do you require to meet a standard? and what do you want this elliptical flange for? Design parameters (size, material etc.) A drawing or a ?sketch would help.
Per ISO-4126, only the term Safety Valve is used regardless of application or design.