arbitrary subst of ss for cs
arbitrary subst of ss for cs
(OP)
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)





RE: arbitrary subst of ss for cs
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?
RE: arbitrary subst of ss for cs
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.
RE: arbitrary subst of ss for cs
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: arbitrary subst of ss for cs
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.