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# Calculating External/Vacuum Pressure for API 620 tank

## Calculating External/Vacuum Pressure for API 620 tank

(OP)
I have been asked to turn almost 50 year old API 620 storage tanks in to ASME pressure vessels. I say this jokingly, but it is the essence of what I have been tasked with. That being said, I am not sure how I would go about calculating an external/vacuum pressures above the 1 oz/in^s limit given in the API 620. I have attempted using ASME Sect VII, div 1 rules as well as API 650 guidelines. I even tried 620 rules using a negative pressure just to see how the results would compare. All ended up with different results, none of which I am confident are correct.

I am looking for some advice from an experienced tank designed as to what is the best method to use for external/vacuum pressure calculations above 1 oz/in^2 for a 620 tank short of using finite element analysis. Most of the tanks I am dealing with are carbon steel, API 620 style, cone bottom tanks. I say "620 style" because there is no record of what code the tanks were originally built to if any at all.

### RE: Calculating External/Vacuum Pressure for API 620 tank

This looks like the equivalent of turning water into wine.

assume you actually mean API 650, 610 is a pump spec.

"ASME" pressure vessels imply pressure higher than 15psig.

What you're trying to do sounds impossible, dangerous and liable to collapse or rupture.

Large, thin steel tanks have virtually no capacity to resist external pressure.

I would like to say good luck, but what you say you're doing sounds like sheer lunacy.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Calculating External/Vacuum Pressure for API 620 tank

(OP)
I meant to say API 620.

I just have to give them the internal and external pressure the tanks can handle, which is not much. As far as external/vacuum pressure I have been using the 1 oz/in^2, but I was curious if there was some trusted publication that details a method for calculating higher external pressure for 620 tanks. After a few days of searching and asking around, it would seem a lost cause.

### RE: Calculating External/Vacuum Pressure for API 620 tank

I would use the procedure from API-650 Appendix V for external pressure. Use the cone-roof design for a cone bottom. Note that Appendix V is itself limited to 1.0 psi external pressure, if I remember right.
ASME should be more conservative, and perhaps less suited for thin-wall tanks.

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