external pressures API 620 versus API 650 1

[carthago]

1/What are the maximum allowable External Pressure for API 620 and 650???

2-/API620 talks in its paragraph 5.10.5.2 talks about one ounce per square inch with the operating liquid at any level...

Who large is one ounce per square inch in psi or millibar ??

3-/in this API620 Is vacuum (noted I believe as (Pg) which is negative in this case) something total DIFFERENT than the external pressure ??

 

[JStephen]

16 ounces = 1 lb. So 1 ounce per square inch = 1/16 lb per square inch = 9 lb/square foot or 1.73 inches of water column.

I don't recall what the API-620 requirements for vacuum are. API-650 limits vacuum to 5 PSF or so. There is a new appendix (new in Addendum No. 4) that allows vacuum design to 1 or 2 PSI. I don't recall the Appendix number, but it's the last one in the book.

Large thin shells are fairly sensitive to external pressure, and so are limited to fairly low values unless they are intentionally beefed up. A smaller tank may be adequate for quite a bit higher vacuum just due to minimum thickness requirements.

Vacuum and external pressure are essentially the same thing. Vacuum implies atmospheric pressure outside and less than atmospheric pressure inside. External pressure implies atmospheric pressure inside and greater than atmospheric pressure outside. But in either case, the significant fact is that the absolute pressure outside is higher than the absolute pressure inside. Whether the vacuum is treated as a positive or negative number would just depend on the application and how the variables are defined for that particular equation.