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# API 650 VS API 620: a story already seen

## API 650 VS API 620: a story already seen

(OP)
Hi,

I am approaching the world of tanks (I am familiar with pressure vessels, heat exchangers, columns calculated with both ASME and EN and the various API codes, TEMA etc.) and I would like to design a tank with the following characteristics provided by our client:

• ID: 3400 mm
• Height (straight portion): 5000 mm
• Operating pressure: atm
• Design pressure: full of liquid
• Operating temperature: 96°C
• Design temperature: 110°C
• Material: UNS 32760 (duplex)
• Corrosion allowance: 2 mm
• Code: API 650/API 620
• Fluid: Slurry
• Fluid density: 1387 kg/m3
• Type: vertical cylinder
• Bottom: flat - slightly inclined (approx. 2 deg.)
• Fluid density: 1387 kg/m3
As I am a beginner, I therefore have checked on the forum the threads on which code to use and I started to read the standards, but you know better than me that in the beginning you need to have some reference points, so I would need your help.

From an initial analysis, I have understood that the main differences between API 650 and API 620 are as follows:

• API 650 covers flat bottom tanks
• API 620 also covers flat as well as other shapes
• API 620 max design temp is 50 degrees F higher than the 200 degrees F of API 650. Appendix M of API 650 goes up to 500 °F (260°C)
• API 620 covers up to 15 psi internal and API 650 up to 2.5 psi (appendix F becomes applicable in case of internal pressure)
• API 650 is for vertical tanks only, API 620 is vert and horizontal
• Typically, API 620 is for very large diameters and for field-assembled tanks, API 650 for welded tanks
So, I have a few questions:

1. In Section 5.6.1.1 of API 650, for diameters < 15 m, the minimum plate thickness must be 5 mm; does this mean corroded or can it be less once corroded?
2. Can UNS 32760 be used even if it is not tabulated in Table 5.2a of API 650?
3. In case Yes, can I use allowable stresses from ASME II for UNS 32760?
4. Am I missing something or is it OK to design this tank with API 650?
5. Which standard would you use considering the design data I have listed?

Replies continue below

### RE: API 650 VS API 620: a story already seen

If the tank could be built under either, always use API-650, and that's what I'd use here.
I believe Annex X covers that material, see allowable stresses there. (Standard stainless steels are in Annex S).
Edit: Just realized pseudo-ell is pseudo-elliptical. This would preferably be a self-supporting cone roof, second best would be an umbrella roof, which is probably what is meant.
The thickness in 5.6.1.1 is as-built thickness, not corroded thickness.

### RE: API 650 VS API 620: a story already seen

In the world of tanks there is always a desire to use some sort of recognised code or standard, but in reality for this size of tank that they just get designed to general principles based on vendor design and experience.

there may be no official limit, but anything less than 10 to 15m diameter is really too small for an API 650 or 620 tank which is really intended to store large amounts of petroleum products.

If you've got loads of money then fine, but tank vendors will balk at the costs of design for something so small but with a significant design temperature. Ask them for two prices. One to API 650 and one to their own standard.

Why in gods name do you have a 2mm corrosion allowance for a Duplex tank??

What's a "slightly inclined" flat bottom? makes no sense to me.

If this is an atmospheric tank why the ellipcal top end?

I can understand with your background being uncomfortable with the lack of standards and design guides, but that's the tank world for you in custom design small cylindrical tanks.

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

### RE: API 650 VS API 620: a story already seen

The "slightly inclined" bottom can crown up in the center, sloping down to the shell, or can be a shovel-bottom, sloping to one side of the tank.

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