There are no requirements in API-650 to even have a manhole. There is a table of standard MH details in the Standard, the smallest of which is 20 inch diameter.
Code/Standard requirements notwithstanding, do the right thing and provide at least a 24" manway. Anything smaller is a pain to get through, and some folks won't fit at all. The added cost of a decent size manway won't even be noticed a year after you have bought the tank - but the added size will be appreciated for the life of the tank.
Doesn't OSHA (or common sense) require 2 menas of egress from any confined space? I know that if I were in a tank and needed to get out because of a fire or other hazard, I would want to leave by the closest means and not have to cross hte hazard to get there. Also, ventilating the tank is easier with two manways.
You're getting carried away here. The posted question was limited to the API Standard, not OSHA, not what's right.
API-650 does not directly address OSHA issues, because not all tanks have to meet OSHA. This is an international standard, so not all API tanks are built in the USA.
The "common sense" part comes from the Purchaser's requirements. The Purchaser specifies the common sense stuff.
jte... Speaking as a short fat guy with a beard, I do not think a 24"MH is enough. I prefer a 30"MH and write all specs that way.
IFR's... I always spec two MHs in the shell (preferably 30"). Multiple roof MHs also, especially if tank will ve lined. The painters have to mount a fan on a roof MH for ventilation.
