Tank operated above its design temperature. 3

[devaxrayz]

Hi all,

We have a oil-water separation tank with design temperature of 200F, material is carbon steel. Due to changing in oil quality, we have to heat the oil first before entering this tank up to 270 F (70 F more than it design temperature).

I know that for most material (e.g. carbon steel), the allowable tension stress is almost the same for temperature up to 650 F. So basically, if you design a tank whether with a design temperature 200 F or 400 F, you will get the same tank thickness.

That is why i'm thinking that 270 F is still applicable to my tank.

I just need a second opinion on this....

thanks before for any opinion.

regards,

-RAYZ-

 

[epoisses]

Rayz, if you want to play the game by the rules, the tank will have to be rerated by the Equipment Inspection dept or whatever these people are called at your site. It has to be a formal and duly documented decision with all the concerned, not a deviation from a rule that is unofficially tolerated because the PE thinks it's OK. It's not only about the safety of this particular tank, it's a safety culture issue.

Your assessment is probably correct but it may be wrong, I'm just imagining there might be instruments around the tank that can withstand only 200 F or some other detail that you had not thought of.

 

[25362]

What happens to water at these temperatures ?

 

[epoisses]

:-D
jackpot 25362... I am not used to think in deg F

Rayz, what is the tank like? What is the water content?

 

[SeanB]

Well I hope this is a 3 phase separator tank because at 270 °F the water is going to be steam!

 

[djack77494]

Guys,
Remember that there may be things such as gaskets, seals, etc. that are also part of the vessel and its attachments. They may or may not be suitable for the revised conditions. It is not safe to assume that just because one component can take it that all components can. Even if you think you've found the weakest link. You must examine each component under the revised conditions to make that determination. (even the paint, lol)
Doug

 

[JStephen]

It depends on the circumstances. If this is an atmospheric tank, refer to API-650, Appendix M.

With the higher temperature, you have a slight reduction in allowable stress and modulus of elasticity. If the tank is fairly small (say, 250,000 gallons or so), the thickness design is simply based on minimum thickness anyway, and minor adjustments in the allowable don't change anything. If it's a large tank, you would need to look into possible lowering the level slightly or taking other steps to avoid the minor overstress that could occur.

Another effect with heated tanks is that if you heat the floor up, but not the shell, you get a radial expansion in the floor that can result in very high bending stresses and consequently fatigue at the bottom of the shell. API does not even require this to be addressed up to 200F, and gives a method of evaluation for larger tanks over 200F. It also allows some exemptions, for example, if a certain minimum liquid head is maintained in the tank. Read through Appendix M for details.

The posts above mention water turning to steam. If you inadvertently put some water in there with the hot oil, you can generate steam at a rate much larger than anticipated in the design of the vents. You'll want to look into this possibility and check sizing on emergency vents especially.

 

[MJCronin]

deva,

As I understand your situation, you wish to take an API-650 designed tank (OK to 200F) and somehow "turn it into" an API-620 design (at 270F)..???

If the tank were to be specified and designed from scratch, it would fall under the API-620 rules.......which, of course, would consider all of the wise and reasonable issues and comments listed by the people above.

The allowable stress level of the steel is not the only issue that must be considered for this tank.....That's why API developed the API-620 code in the first place !!!


-MJC

 

[IFRs]

Just go to API 650 appendix M, do the calcs and see what it looks like. 270 DegF is still within the limits of API 650.

 

[JStephen]

And 270 is not within the limits of API-620, which only goes to 250 degrees.

 

[devaxrayz]

Dear all,

thanks for many posting... sorry for late response (access limitation).

25362, yeah you got me right... heating up wont work... must think another way.

However JStephen and MJC, thanks very much for directing me to App. M of API 650 and API 620.

regards,

-Rayz-