Establishing Critical Exposure Temperature

[PVDean]

Outdoor tank with an ambient commodity. Am I forced to API650 lowest one-day mean temperature chart plus 15 degrees? I don't have metal temperatures per say but I could get site temperatures over the last 5-10 years? If I do get a value for the lowest temp over that span, do I add 15 degrees to it?

 

[vesselguy]

No, you're not "forced" to use the low 1-day temp chart in the standard. You might want to refer to the local building code to access data on the lowest site temperature. If you want to, you can +15F to the lowest temp.

I assume the purpose behind your question is you want to set the minimum metal temperature for the tank for the purpose of selecting material grade. If so, then either ask your client (if you are a fabricator) or consult with your metallurigist and process engineer (if you're on the EPC or client side) and set it based on cost and realistic low temp that will be experienced by the tank. i.e. if the tank will be heat traced, or insulated, or has a heating coil inside, then in a hypothetical48 hrs shut down, what would the metal temperature be?, etc.

If this is not what you're looking for then sorry for my long winded answer.

 

[PVDean]

I'm doing calculations for the re-rating of a pressure vessel (MAWP) based on MDMT/MAT. It's a 1965 vessel made from A-212 B firebox material. We don't have build records outside of calculations, so even though the vessel measures >2" thick we can't claim it was produced to fine grain practices. No proof of PWHT, and no temperature associated with the hydrotest. I'm am providing calculations per API579 to reduce the MDMT/MAT by the stress ratio approach per fig 3.7. The CET will govern how far down I need to reduce the ratio.

My RBMI people are placing this material on Curve A, and at 2 inches thick the MAT/MDMT is ~100F. Therefore the reduction will have to be substantial.

 

[vesselguy]

If you're doing a Code "vessel" then why were you refering to it as a "tank" and reference to API650???? Very confusing.

A212 is noted as replaced by ASTM A515 standard. If it is not A515-60 then yes you would be on Curve A of UCS66 and 100F is about right for your MAT without MAT reduction. This must be a really high strength and room temp brittle material. I'm not a material guy so you really should consult (hire) an experience engineer to advise on the material aspect. If its anything like P91 material then it will crack even in +15C hydrotest temp. Scary stuff.

Like you say, with reduction to something reasonable, say 0 F, you're down to almost 0.4 on your stress ratio (useless). Since you have next to no information about the material and fabrication record (heat treatment/toughness, etc.) I would assume the worst and do my calcs on that basis. State your assumptions in bold prints. Do your calcs for both cases (one for without temp reduction and one with) and give the facts to your client. Your recommendation likely to include high warmup temp before pressurization. I don't know what this thing is but the best advice to your client is build a new one. I don't understand company that would put their people at risk trying to reuse vessels outside its design life.

Please post your final resolution here. I'm interested in what you end up doing with this thing.

 

[PVDean]

It is a pressure vessel for sure, operating at ~2600 psi right now. A 'code' vessel, well...no proof of that either. Tank was a poor choice of words.

The API650 reference comes from a bit of research on "The lowest (coldest) one-day mean atmospheric temperature..." entry in API579 [3.3.3] in determining the CET.

My problem is that this catch was just discovered by our recertification/RBMI folks, and this calculation is going to be applied to many other vessels of similar construction. This tank can be reconfigured to a system with lower pressure duties (it's on an experimental test stand), but other impacts of this calculation may be met with opposition. The CET is the cornerstone of this evaluation, and I just want to know if that figure in API650 is the only way to go. API579 3.3.3 says "should consider the following...factors" with lowest one day mean temp being the first factor.

I'm sure I can find atmospheric data for our site, but I don't know what to do with it when I find it. The API650 method is 15 F above that figure, so that translation is what I'm after. Lowest one-day mean temperature is a tough bit of data to find if I wanted to get site specific, but absolute lowest site temp is easy. Would I add 20 F to that? Just the method of the CET calculation is what I want to nail down.

There are no warm ups in our world.

Thanks for the ideas.

 

[metengr]

PVDean;
I reviewed API 579 FFS-1 and for your determination of critical exposure temperature (CET), I believe you have guidance using 3.3.3 subparagraphs a) through e). You have not interpreted 3.1.3 correctly. The CET for pressure vessels is defined as the coldest (lowest) metal temperature derived from operational data or atmospheric conditions at the maximum credible coincident combination of pressure and supplemental loads that result in primary stresses greater than 8 Ksi. The statement regarding tanks is for tanks and not pressure vessels. All of the information is there to review and understand. Please go back and re-review it!

It is a pressure vessel for sure, operating at ~2600 psi right now. A 'code' vessel, well...no proof of that either.

What? Something is not right. What do you mean no proof? Do you have a nameplate? Is there a National Board registration number for this pressure vessel? Is the vessel currently insured? Was this pressure vessel issued an Operating Certificate?

You need to review the data report for this vessel for a proper rerate evaluation. If you don’t have this information, API 579 FFS will not be able to help you. The basis for evaluation against brittle fracture relies on the critical assumption that the vessel was constructed to ASME B&PV Code, Section VIII or other recognized code of construction. Do you know the current MDMT?

 

[PVDean]

I see that I do not get the 15 degrees over the isothermal lines of the API650 map, thanks. The reason I am going to that map is because I don't have metal temperatures and the commodities are at ambient. I will explore metal temperature drop over the duration of a test though, autorefrigeration may be possible if the pressure drop is significant.

There is no national board number for this vessel. There are stamps by two state Corps of Engineer's, area and district, on the design calculations from 1964. The calcs state that the vessel should be designed and fabricated to section VIII, welding procedures and operators per section IX, NDE...The calcs state that the design temperature is -20 to 150 F, but also lists the material as ASTM A-212 Grade B Firebox plate steel.

The vessel is basically being used as a gaseous nitrogen accumulator and it is held at ~2600 psi, at ambient. I understand the statement in 3.1.3 about primary stress being greater than 8 ksi, but the stress state of my vessel is held much higher then that. I have 16 ksi, Ri=24.5 in, at a normal state (only dropping it to evacuate the test chamber so to speak), so 3.1.3 appears (the third sentence) to drive me to the coldest atmospheric conditions. I can get an absolute low temp for the site per year, but lowest one-day mean average temp?

I appreciate your input.

 

[metengr]

What I would do is use the one-day lowest temperature for this site and assume operating pressure concident with this lowest temperature.

 

[vesselguy]

metengr is right on the approach. Don't take the CET and determination of the lowest one day temp so seriously and stop you from doing what is practical. At the end of the day, the concern is the metal skin temperature. What would be the lowest metal temperature the vessel is expected to experience. Sounds like you have experience so you can determine that. Your response already answered your own question: Autorefridgeration.