Tanks for petroleum products storage at atmospheric pressure are ruled by standard API 650. API is the American Petroleum Institute.
Tanks for petroleum products storage or liquefied gases at low pressure (less than 10 psig) are ruled by standard API 620.
Small tanks for petroleum products storage at atmospheric pressure are rules by standard API D12.
Tanks for water storage are ruled by standard AWWA D 100. AWWA is the American Water Works Association.
Tanks for gases or liquefied gases storage at high pressure (10 psig and above) are ruled by ASME Code VIII. ASME is the American Society of Mechanical Engineers.
In those standards you'll find plenty of information on welding conditions and details.
Those are the American standards related to storage tanks. Over the years they became so popular, not only in the USA but all over the world (here in Brazil, for example, they're the only used), that they began to be used for other liquids than petroleum products and water.
API standards, for example, are used for flammable (combustible) liquids, such as alcohol and even vegetable oils.
AWWA, on the other hand, is used for non flammable liquids, such as mineral acids and caustic soda solution.
Giovanni S. Crisi
Sao Paulo - Brazil

See API 650, Section 5-erection, paragraph 5.2.1.5
butt joints for plate thickness 1/2" or less, the reinforcement for vertical is 3/32", for horizontal is 1/8";
for plate over 1/2" to 1", incl., vertical 1/8", horizontal 3/16";
for plate greater than 1", vertical 3/16", horizontal 1/4"
Hope this will help.
Kind regards
H.Chang

Sine complete penetration and complete fusion for joining shell plate are addressed in paragraph 5.3.2.1, then the width of weld is depended on the joint preparation-the thickness of the plate, angle of the groove, type of groove, or the welding process use. Usually the thicker of the plate the wider of the finished weld. There is no limitation specified in API 650.
Kind regards
H.Chang