API-650 would generally be used if the pressure fell within its range of coverage.

I do not believe that you understand either the scope or the applicability of these two standards.

API 650 covers flat-bottomed tanks to a certain maximum pressure and operating temperature.

API 620 covers tanks to higher temperatures and pressures.

There may be some service conditions where either standard may be suitable.

What, in your particular usage of English, do you mean by "least restrictive" ?

Do you mean lowest cost ?  

   

Yes, I am not familiar with either of these Standards.

Least restrictive as in lower cost.

I should have found out more information about tank from Engineering Dept before posting.

My appologies...

MJCronin,
Thank you, I was able to obtain the answers I needed upon further study and inquires.

You are correct that 650 covers flat bottom tanks, and 620 also covers flat as well as other shapes.  620 max design temp is 50 degrees F higher than the 200 degrees F of 650.  620 covers up to 15 psi internal and 650 up to 2.5 psi (appendix F becomes applicable in case of internal pressure).  650 is vertical tanks only, 620 is vert and horizontal.

  

Note that Appendix M of API-650 goes up to 500 degrees, whereas API-620 is limited to 250.

API-620 may be applied to horizontal tanks, but doesn't actually include any design provisions for them.  Generally, horizontal tanks would be built to ASME code if pressurized or UL standards if not.

Right offhand, I can't think of any reason why one would use API-620 if the tank in question could be built under API-650.

All

"I can't think of any reason why one would use API-620 if the tank in question could be built under API-650."

I agree....Tanks designed to API 650 are designed to a very reasonable set of rules. Most flat bottomed tanks in the world are designed to 650 or the foreign equivalent.

On a side note, I would like to add that many horizontal tanks are designed to "almost ASME VIII" code.

Horizontals are designed, inspected, tested to Section VIII, but no code stamp. Design pressure is typically 10-14 psig.

IMHO, There is a hole in the API design rules about low-pressure horizontal tanks.

-MJC