arbitrary subst of ss for cs

[rustbucketguy]

For an API 620 tank built early 1970's, is it conceivable that a reputable tank fabricator could have designed a tank for carbon steel and simply have substituted stainless steel? Anchor bolt chairs are not even close, vacuum capability is not even close to being reasonable, the tank cannot be protected from vacuum conditions even with the minimum spindle in a commercial vacuum breaker (0.28 oz/sq inch required vs 0.5 oz/sq inch available)

 

[JStephen]

It's hard to answer your question based on the information there.

I'm not sure when SS was added to the API-620 standard, or if it's been in there all along. But if the tank was built to API-620 prior to the inclusion of SS in the standard, then there's no telling how the details of design were worked.

API-620 tanks are not normally designed for vacuum other than what's inherent in the design, if that helps. However, I do see Tyco/Varec P/V valves with vacuum ratings of 0.23 oz/in^2, so it doesn't look like your 0.28 should be that big of a problem.

When you say "anchor bolt chairs are not even close", do you mean the design is completely inadequate, or are dimensions off, or what?

And are there actually problems with sucking this tank in or bending anchor chairs, or is it just being re-evaluated and found lacking?

 

[rustbucketguy]

Actually, we had an uplift incident and during the evaluation, we found that the anchor bolt system was compromised some time in the past. Part of that compromise was in the form of broken anchor bolts, stripped threads, and mangled chairs. When we modeled the tank in a software package, it refused to accept the 0.5 oz/sq inch vacuum and continued to refuse until we got down to 0.28 oz/sq inch. The software package came up with significantly more robust chairs, even after changing the quantity from 12 to 24. The original design called for 6 and it was changed to 12 prior to fabrication.

I cannot imagine anyone designing a tank using cs materials and then building with ss, but am not sure about 1971 era. Sure would explain a lot of our apparent design discrepancies. We have Protectoseal conservation vents and was not aware of Tyco line. Thanks for the input.

 

[JoeTank]

It is not apparent to me why you are so shocked about an owner asking for an API-620 tank made of SS material. No fabricator would simply substitute SS for CS. It would be crazy thing for the fabricator to do. That type of a decision is driven by the owner, not fabricator. API-620 now has SS material, but did not back in 1971. How big is the tank. were there vacuum conditions specified? What were the design conditions for the anchorage. Who designed the tank, fabricator or purchaser?

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[JStephen]

Take a close look at what the software package is doing. If it is evaluating the tank using current standards, there may be a big difference between what was required then and what is currently required.

The vacuum design on tanks is pretty approximate. For example, the ASME code is more conservative than normally used for non-code tanks. If you're using an ASME procedure to evaluate the tank, you're probably overly conserative on the vacuum. The current (4th) addendum of API-650 actually has a vacuum design method in it, but I'm not sure how it compares with other methods. In the past, codes didn't necessarily combine wind loads and pressure, and that tank may not have been designed for vacuum and wind combined, if indeed designed for any specific vacuum.

I haven't dealt with Protectoseal extensively, but if you haven't actually asked, it might be worth a call to see if they have lower vacuum settings than what their literature shows. Or they might direct you to a different style of valve.

As far as substituting SS for CS, it wouldn't normally be done like that, but that's not as bad an idea as you make it sound like. The allowables in API-620 are fairly low, and SS stresses are usually high compared to yield, so there's probably not a big mismatch in stress. There's some variation in Young's modulus, seems like, and that would have some effect on the wind stability. Anchor design wouldn't be much different with SS or CS.