ASME Code VIII vs. API 650/620

[Jens12]

I'm working on a project for a oil refining company and the Project Management Consultant did a basic engineering and produced some equipment data sheets for tanks which store recovered oil and some kind of liquid organic waste and so on. As design code ASME Code VIII was specified. Now the contractor released new data sheets and indicates design code API 650/620. The tanks are under a nitrogen blanketing but almost with atmospheric pressure.
Are there any objections against API Code ? Is the qualty of tanks fabricated according API less than ASME ?

 

[DSB123]

Jens12,
I the design pressure of the nitrogen blanket is such that it falls within the limits of API 650 then there is no objection to using the API Code for storage. Obviously a Vessel to ASME VIII is better, quality wise, than a storage tank to API 650 but you should be selecting the correct Code for the desired equipment.

 

[JStephen]

Are they indicating 620 or 650? What shape are the tanks? How big?

API-650 is applicable to flat-bottom vertical tanks, with pressure up to 2.5 PSI. API-620 is applicable to tanks with a vertical axis of revolution, up to 15 PSI. Neither is applicable to horizontal tanks or leg-supported tanks.

If the tanks are flat-bottom and of any size, I doubt they COULD be built to ASME Section VIII.

We sometimes see Section VIII specified for design, but not necessarily as a construction code, for non-pressured tanks or low-pressure tanks. Look at how this was worded in your inquiry.

As a contractual issue, I'd suggest you review your bid documents and their quote to see what was actually proposed. You may also have legal requirements where you are that specify that tanks must be built to API or ASME standards. And if it's a tank that cannot be built to ASME code, then you'd expect API.

There are differences in the design, detailing, inspection, material requirements, etc., between the different codes/ standards. If the configuration, pressure, and application fall entirely within API-650 or API-620, you're probably better off using it. ASME Section VIII is just a mess compared to API-650 or API-620.

 

[JoeTank]

DSB123,
I certainly do not agree that the quality of an ASME VIII vessel is obviously higher than that of an API-650 or 620 tank. The perceived quality of an ASME Code-Stamped vessel is way over-rated.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[tmore22]

I am working on a project which will require nitrogen blanketing on an additive tank. I would like to use a horizonotal tank so we will not have to take out of service every 10 years for an API 653 inspection to scan the bottom. The thought is that if we can use horizontal tanks, we will be able to UT the bottom and get a good idea if there is any metal loss on the tank - which should suffice for a 653 inspection. However, most of our additive tanks are built to UL 142 standards (and are not N2 blanketed) and I am not sure if UL 142 tanks are sufficient for N2 blanketing. I know they are good up to 1 psi and I am planning on around 5" of water column for the blanket pressure.

Any help you all could provide would be greatly appreciated.

 

[JStephen]

I'm not that familiar with horizontal tank design. But probably the major difference you'd see is whether the tanks ends were formed heads or flat plates with stiffeners. Once you get that extra cost covered, you should be able to put in a fair bit of pressure with minimal effect on design. It would probably help if you held pressure below 15 PSI so that you didn't require code vessels.

You didn't say how big the tank is. Up to 30,000 gallons or so, you're probably best off buying a shop-built tank, whether vertical or horizontal. If the capacity is much more than that, you would probably want to go with a vertical flat-bottom tank. (Or could be a suspended bottom if it was that critical). Would it extend the time if you added more corrosion allowance on the bottom? 1/2" bottom plate wouldn't cost too much on a small tank.

 

[weldtek]

Steve,
You're joking, right?

 

[solinar]

weldtek,

Im sure you, as have many of us, have seen horribly constructed ASME pressure vessels and very well constructed API tanks.

Is an average ASME pressure vessel without any additional inspection usually higher quality construction than an API tank with the same inspection? Probably so. However, its all about the specifications and the non-mandatory (by code) inspection that is required by the owner.

I tend to agree with Stephen. The quality of a code stamped vessel is overated. Its a great place to start and can result in a quality piece of equipment if properly supplemented with good design and inspection, but I have seen far too many piles of jumk leave ASME code shops to be fooled into thinking that an ASME stamp means your are getting a well constructed piece of equipment.

 

[tmore22]

Thanks for the help guys.

The size of the tanks will be 12,000 gallons. I think I'll stick to my original thought of buying the shop built (UL 142) tanks. The cost is much less than an API or ASME tank and should work fine for a blanket pressure of less than 3/4 psi. After speaking with the tank manufacturer, he stated that for vertical 142 tanks, they pressure test up to 1.5 psi and for horizontal they take it up to 2-3 psi with flat plate ends.

 

[weldtek]

In my mind, there are several facts that contradict the notion that tanks built to API are automatically of equivalent quality to vessels built to the ASME Code. Assuming that there no customer specs, nor any customer inspection, which isn't often the case, but does happen, the Vessel built to ASME Code has a couple of things going for it. First, for ASME work, the fabricator has to have a quality manual governing his engineering, procurement, material control, fabrication, inspection, etc.,and that manual has been reviewed and his shop practices audited for compliance, by an independent team. Second, even without a customer assigned inspector, the ASME Code stamped vessel will have been followed throughout it's fabrication, and inspected by an Authorized Inspector. I'm just going to refer to one other example for those still in doubt. Compare the rules for radiography in both Codes, and in particular note the differences in the requirements if defects are noted.