API 620 - 3rd Ed. 1965. Allowable Partial vacuum

[Mech85]

In the tenth edition of API 620, 2002, the following is written in Section 5.10.5.2 under "External Pressure Limitations". It states that "cylindrical sidewalls of vertical tanks designed in accordance with these rules for storing liquids..............may be safely subjected to a partial vacuum in the gas or vapour space not exceeding 1 ounce per square inch with the operating liquid level in the tank at any stage from full to empty." One (1) ounce/ sq. inch equals 0.43 kPa.

An existing storage tank we are reviewing wasbuilt to API 620 in 1968 has a design pressure of 1.0 psig (approximately 6.9kPag). I am trying to ascertain the value of the maximum partial vacuum the tank was designed to in 1968. I’m not sure if 1 ounce per square inch was used then. I believe the third edition of API 620, 1965, was used for the original design of the tank.

I am trying to obtain a copy of the old edition through our technical library but no luck so far. Is there anybody who can advise what the allowable partial vacuum was for the early edition (1965) of API 620.

Thanks

 

[JoeTank]

I researched this a long time ago, but my best recall is that API-620 has always had a statement about the one ounce limit. Preferably you will verify that rather than take my word on it.

Joe Tank

 

[JStephen]

The statement doesn't necessarily mean that the tank is designed for any vacuum, just that it will incidentally allow that vacuum when the other design criteria are met. I would assume that this is equally applicable to older API-620 tanks, whether stated in the older code or not.

 

[Mech85]

I obtained a copy of the second edition (June 1963) and the third edition edition (Nov 1966) of API 620. For future reference, this is what they stated.

In each of the two editions, Section 3.10.5 External Pressure Limitations states "......cylindrical sidewalls of vertical tanks designed in accordance with these rules for storage of liquids..............may be safely subjected to a partial vacuum in the gas or vapour space not exceeding 1 oz per sq. inch (Note 2) with the operating liquid level in the tank at any stage from full to empty."

Note 2 stated that "The vacuum relief valve or valves shall be set to open at a smaller partial vacuum so that the 1 oz/sq. in. partial vacuum will not be exceeded when the inflow of air (or gas) through the valves is at its maximum specified rate"

So the allowable partial vacuum in the vapour space has been consistently set at 1 oz/sq.inch or approx 0.43kPa.