Operations & Design for FR Atmospheric Tanks

[djack77494]

I need some expert opinions regarding API650 floating roof tanks. My current project expects to buy a number of these tanks, but I see a few problems I'm unsure about. First, the design temperature is up to 230 degrees F, which I think exceeds API650 limitations. Obviously high temperature atmospheric tanks are built, but I'm not sure how the regulations change with higher temperatures. Second, the client's process engineer asked me how a floating roof tank responds to fillings, emptyings, and cleanout operations. (Sorry this isn't glamorous stuff, but it is down-to-earth.) Specifically, he wanted to know how air was vented during initial (and subsequent)filling, how a complete emptying of the tank could be accomplished, and was steam cleaning possible. I'm always worried when dealing with pressure and "atmospheric" tanks, so this is a real challenge to me. The client is asking me what sort of internal/external pressures develop during these operations, and can the tank handle this. I need "An Idiot's Guide to Atmospheric Tank Operations and Design", but don't have the resources available here.

Other pertinent information:
Tanks will hold several types of heavy crude oil at 160 to 180 to possibly even 205 degrees F (Design = 230).
Several sizes being used from 25,000 bbl to 200,000 bbl (or 70 to 200 ft.).
Some of the tanks will have internal steam coils for temperature maintenance.
Site is pretty remote. Thanks.

 

[BillBirch]

If the oil is so heavy, I would presume that there would be little in the way of light fractions, so why use a floating roof tank? A fixed roof would be more attractive.

I once worked in an Aussie Lube refinery, using Saudi crude as feedstock, and the bottoms were quite thick. This had to be heated to the sort of temperature you mention, just to be able to pump it as boiler fuel. In that case, the bottoms tank was a fixed roof (although much smaller than you mention.

A useful feature that we had was a bayonet-type U-tube heater inserted into the tank by the outlet. This enabled thinning of the contained liquid only at the point where it was needed.

 

[djack77494]

Thanks for your comments, Bill. I think my predecessors chose floating roof designs because of the large tank diameters. Personally I don't know how one decides whether to go fixed, external floating roof, or internal floating roof, though I'd be curious to learn. The steam coils keep the crude's viscosity from getting too high. This is necessary because we have tank agitators that keep everything well mixed. Their motors would be overloaded if the tank contents cooled to near ambient.

 

[JStephen]

Appendix M includes provisions for up to 500 degrees F. There are some restrictions on floating-roof tanks at elevated temperatures.

A floating roof has vents that open when the roof lands on its legs. When an empty tank is filled, air/vapor will just vent out of these vents until the roof starts floating. Normally, the floating roof would be operated between a high and low level, with landings minimized for emissions reasons.

I'm not sure about the steam cleaning specifically, as that would depend on the details. The main thing to consider would be adequate vacuum relief vents in the floater or in any fixed roof so that condensation of the steam couldn't suck a tank in. Steam under pressure would never be used in an atmospheric tank, as the allowable pressures for large tanks is usually only slightly above the weight of the roof plate.

A lot of the products that require heating don't require floating roofs- just fixed roofs.

 

[IFRs]

You are describing a very common type of tank. Neither the service nor the temperature are exotic or unusual. The filling and emptying and steam questions are also very common. I would recommend that you retain the services of an expert who can help educate you directly. This forum can't possibly fullfill that need without pages and pages of conversations that will be an inefficient use of everyone's time. I'd volunteer if needed.

 

[djack77494]

JStephan, thank you for your comment about the relief vents for a floater. IFRs, I agree that a full discourse on tanks would be voluminous. Any suggestions on books/magazines for that material? In the meantime, I'd like to know when/why one would switch between fixed and floating roof tanks. Specifically, how important are tank size, temperature, fluid vapor pressure, etc. Also, I'd assume that fixed roofs are cheaper, but is that always true?

 

[LOUDOG]

You mentioned in your first post that you are planning buy a number of those tanks and you are looking for an expert. If that is the case, why don't you send out some RFQ's to some tank manufacturers, fill out the information as best as you can, and see what they come up with.

 

[djack77494]

Yep, I could do that. I wouldn't want to dissuade a manufacturer from coming up with the best solution by asking for (say) a fixed roof tank when a floating roof tank would be best. But I'm shocked that I have been totally unable to locate any definitive publications describing how these decisions are made. Don't they exist?

 

[djack77494]

I guess not.

 

[IFRs]

The decision tree is short: cost, cost, cost. First, determine if floating roofs are needed for environmental, contamination or conservation reasons. Then if a floating roof is needed price a fixed roof with a lightweight aluminum internal floater versus an external floating roof. If a floating roof is not needed then price fixed versus external floating roof. Try to estimate lifecycle costs - maintenance, painting, inspection, fire protection equipment costs, etc. If you are buying new tanks go directly to the aluminum internal floating roof manufacturers to bypass the steel tank vendor's bias which is often under-educated. Note: underside corrosion of a fixed roof can be an expensive proposition that a floating roof can minimize. I am involved in several 225 foot crude tanks with steam coils and air spargers. They have aluminum internal floating roofs under steel fixed cone roofs. I have seen all kinds over the years - the choice all depends on your particular conditions and costs - environmental, chemical, operations, maintenance, corporate outlook (long or short term), etc.