Per API 650, does the shell long seam have to be staggered from one plate to another or could be welded as a continuous weld from top to bottom of the tank?
Vertical seams need to be staggered a MINIMUM of 5 times the nominal thickness of your plate. You will find that this is typically increased. Lets say you have a smaller tank with 1/4" plates, your minimum offset is only 1.25" total. Having those intersecting joints so close together could lead to heat induced deformation of plates and residual stress in the joint.
valleytank: Is this requirement per API 650? I know the ASME VIII code requires them to be staggered but I can't find a requirement in the API 650 code.
If by "long seams" you mean the horizontal seams, no - they do not have to be staggered and are most often a complete ring.
If you mean the vertical seams, they are usually the shorter seams and yes - they need to be staggered as valleytank responded. The section in API 650 is 5.1.5.2.b
Velleytank and IFRs are correct, but there is one caveat. S.4.11.6 requires that shop assembled stainless steel plates have their vertical edges aligned. In the completed tank, the appearance of the T-joint could cause confusion for those unacquainted with this section of the standard.
Seems like in one of the old codes, seams were allowed to be aligned- but not sure if this was API-650. Anyway, if you're looking at an existing old tank, it may not have had that requirement.
