API-620 does not have rules for vacuum conditions....does that mean that it is not allowed or that with proper venting and good engineering practice you can design it for vacuum? Are the API codes typically pretty black & white or is there a lot of grey area?...I'm not sure where to draw the line and I fear it will get me in trouble.
I don't have API-620 on hand, so I will give you a general answer. There is a statement in the Standard that says that a tank that meets API-620 is suitable for a vacuum of 1 inch or 1/2 ounce (or whatever the tiny value is) of negative pressure. It is a minimal number that matches the typical ratings for pressure relief valves.
That's it. Nothing else. If you design an API-620 tank to withstand a higher vacuum you are on your own. Sound engineering principles would have to be used.
Can a tank designed for a vacuum above the nominal limit set by API-620 still meet the Standard's requirements? Interesting question. I say yes, but that is a question you may wish to put to the API Subcommittee on Pressure Vessel & Tanks. It would spur an interesting debate I'm sure.
Every API-620 tank for LNG, LPG and Ammonia service that I have been associated with has been designed of a vacuum condition far above the nominal limit of API-620. Good engineering practice was used for the design.
You might be interested to know that API-650 has an appendix in development that will add rules for the design of tanks subjected to external pressure.
The LNG industry accepts European Standrd EN1473 for the definition of LNG storage tanks by containment type.
Europen Standard EN1472, 1997, Installation and equipemnt for liquefied natural gas - Design of onshore installation.
United State (US) regulation NFPA59A does not classify LNG tanks based on containment and tanks are clasified as "single" -or "double wall" as per API 620 Appendix Q.
NFPA59A, National Fire Protection Agency,Standard for the production, storage andhandling of liquefied Natural GAS LNG, 2001 edition.
American Petrolum Institute Standard 620, Design and construction of large welded low-pressure storage tanks, 10th edition, Febuary 2002.
In addition, LNG facilitied in the US and US protectroatermust following federal regulation 49CFR193 whick require theouter or secondary cotainer be sized.
49CFR193, Code of Fedueral Regulations, 49CFR Department of Transportation, Subtite B. Chapter I, Subchapter D-Pipeline Safety, Part 193-Liquified Ntural
Gas Gacilities-Federal Safety Standard.
I can't speak much to the mechanical design of storage tanks since that is not my field and I can't say I've had much experience in specifying storage tanks. But I've had a need to become familiar with the code requirements of these vessels in order to size relief devices for them. Here's what I've found
From API 650
3.2.4 External Pressure
This standard does not contain provisions for the design of tanks subject to partial internal vacuum; however, tanks that meet the minimum requirements of this standard may be subjected to a partial vacuum of 0.25 kPa (1 in. of water) of water pressure.
From API 620
5.10.5.2
This standard does not contain provisions for the design of cylindrical sidewalls that are subject to partial internal vacuum in tanks constructed for the storage of gases or vapors alone. However, cylindrical sidewalls of vertical tanks designed in accordance with these rules for storing liquids (with the thickness of upper courses not less than specified in 5.10.4 for the tank size involved and with increasing thickness from top to bottom as required for the combined gas and liquid loadings) may be safely subjected to a partial vacuum in the gas or vapor space not exceeding 1 ounce per square in. with the operating liquid level in the tank at any stage from full to empty. The vacuum relief valve or valves shall be set to open at a smaller partial vacuum so that the 1-ounce partial vacuum will not be exceeded when the inflow of air (or gas) through the valves is at the maximum specified rate.
As Steve points out, practically all of what I've seen for vacuum ratings exceeds the minimum allowed for in the codes. What seems to elude me is when reviewing the vessel documentation, I can't say that I remember seeing where a method is spelled out as to how they accomplish a design beyond the scope of the code. Typically what I find is that the tank was built to API 650 or API 620 with a stated pressure rating and stated vacuum rating. But that may be off the subject.
the other day and found they have a technical interpretation for API 620 that may be helpful. Search for the interpretation regarding storage of methanol with a pressure range of +/- 35 inches water.
