8 replies to this topic

[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 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.
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 to any who can help,
Doug

[djack77494]

Anyone have a clue?

[Art Montemayor]

Doug:



I’ve never installed or operated a floating roof storage tank. I came close to it on two occasions, but came out of the situation with a fixed roof tank. On both occasions, CB&I was consulted and we issued a formal report on our review and recommendations. Some of the issues that came out were the same or intimately related to the concerns and questions that you and your client raise. The tanks I evaluated were Internal floating roof type, not external type. I presume you are dealing also with internal floating roof types.

The way a floating roof tank responds to fillings, emptyings, and cleanout operations is very important and critical for the operator to know. The quality of the fabrication and the way the tank is operated makes a lot of difference on whether the tank will succeed in keeping the product safely stored and available. Any imperfections or flaws in design and construction will cause any thing from upsets to total failure in operation. The roof can seize or hang up if it is not perfectly aligned and fitted to rise and fall with the liquid level, guided by the proper column design. Emptying the tank is not too much of a problem; however the cleanout operations – especially the steaming out – can be messy and laborious. The fact that the roof literally floats on the liquid makes it difficult to get underneath it and obtain thorough steam out and cleanliness. You have to raise the roof and prop it up safely in order to effect a thorough clean out. Depending on the actual, specific design of the tank and it various mechanisms, the operation could be different, difficult, risky, or normal.

I think the proposed designers and fabricators of the tanks are the best and the most authoritative people to approach and to address these important issue with. I have designed and applied API design to 180 oF crude oil and I can see various engineering problems that will have to be specifically addressed with your application(s). The tanks will “walk” with subsequent heat-ups and cool-downs. This will happen because you can’t anchor down the tanks 100%. You have to allow for circumferential expansion and for piping flexibility. I presume the tanks will be 100% insulated. Every aspect of tank operation – including pre-commissioning and start up through normal and emergency shutdowns has to be described and explained in detail through a report issued by the designer and fabricator to the client’s approval. Before I reached the decision stage on the type of tank to apply, I had designer’s study on recommendation and responses to our specific questions on operation. I was able to discuss and respond to the designer’s recommendations and decide on which type was the best for the application.

I hope this experience helps you out.

[Mazfar]

Sir,

How does a floating roof behave with fire. What are emergency vent requirement.

[Qalander (Chem)]

Doug Hello/Good Evening,

Somehow I missed this post initially or probably in somewhat pre-occupied state of mind.
If this is not already 'Past' then I may put forward(my Edi o syn crazy)!

Firstly few queries

1) Ambient temperature too low(below zero Celsius) for the designated product service i.e pour point very high?

2) If this is this some sort of crude oil like material with very high pour point such scenarios may be expected and possibly bed steam coil may help somewhat!

3) As regards de-gassing/gas-freeing similar practices as for highly flammable storage floating roof tanks as guided in NFPA30 and related service API docs should form a basis.

4) However plentiful Nitrogen Gas and Steam for ejection of gases out and internal purging

5) Multiple shell man-ways and floating roof Man-holes will also be necessitated i.e. diagonally opposite and four each tank for tanks above 100 ft dia to ensure dead areas minimization.

6) All the roof and rim vents will have to doubly safeguarded through flame arrestors

7) The floating roof legs will have to be careful designed for two (Normal and Clean-out) level setting with comparatively more clearance space between top edge (highest steam bed coil) point elevation and lower most floating roof bottom side members as is usually kept in normal without steam bed coil floating roof tanks.

Above is a little glimpse of my wild thoughts, I wonder how much useful or ridiculous you find these to be!

[djack77494]

Doug Hello/Good Evening,

Thank you for your response, Qalander. Also, thanks to Art for his earlier response. As in other exposures I've had with atmospheric tank design issues, the problem went away before the solution was found. I remain distressed over the sorry state of tank design and the lack of basic design guidelines for process, facilities, and project engineers. Like I said earlier, what I (and I imagine many others) really need is "An Idiot's Guide to Atmospheric Tank Operations and Design". Too bad it's not available.

[Nageswar Kotari]

Dear Jedi,
I have answers to some of your questions as an operation guy here.

The venting of air is possible from a floating roof tank. There will be multiple number of goose neck vents on top of tanks which will allow the air to vent out of the system.

Complete cleaning of tank is not possible. But there are new techniques now like a silo under the tank like a drain line running out of the tank

The moisture content in the tank is always monitored and as always the density of fluids in floating roof tanks will be lighter than that of the water only the bottom part of the fluid will be free flowed to a stand by tank and that tank filters water out from the tank. Some thing like this. Bottom part water resides and it will be drained off. When there is a hint of a fluid during this draining it gives the clue that water is drained off. The balance fluid will be pumped back to a tank nearby. With this system the fluid system is kept clean. I have no idea of entire evacuation of fluid for steam cleaning. But I will remember this question and will answer you once we do it here.

Sorry as this will not help you much but out of operation knowledge I tried to help you out.

Thanks
Nag

[Qalander (Chem)]

Dear Nag, Hello/Good Afternoon:

I don't know any reason why you should not share your valuable experience(s) for forum members awareness/knowledge updating indeed!

However I feel I should mention that:

in case of goose-neck or inverted U-shaped vents emitting flammable gases/vapours mixtures; the hazard from any heterogeeous mixture forming a self-ignition source/finding an ignition source should never be ignored or left un-addressed.

Since I have come across such an incident (during steam-out of Flare KOD for some unavoidable work on its leveltrol).
Although nothing catastrophic - or even slightly damaging - could occur (All Thanks to the Only Lord of the Universe indeed!).
Hope this helps/paves way Forward .

[Nageswar Kotari]

Thanks for sharing this information jedi. But our tanks are filled with diesel oil and gasoline. They have these goose neck. I think these gases are not self ignition type. they need a source to ignite.
How ever I cannot fully guarantee this as our tanks are only for storage purpose under room condition. But you have a high temperature there. Thanks again
Further to this can you post me the API 650 Appendix E of Sesimic analysis of tanks? Sorry if not possible for you.
Thanks
Nag

Dear Nag, Hello/Good Afternoon:

I don't know any reason why you should not share your valuable experience(s) for forum members awareness/knowledge updating indeed!

However I feel I should mention that:

in case of goose-neck or inverted U-shaped vents emitting flammable gases/vapours mixtures; the hazard from any heterogeeous mixture forming a self-ignition source/finding an ignition source should never be ignored or left un-addressed.

Since I have come across such an incident (during steam-out of Flare KOD for some unavoidable work on its leveltrol).
Although nothing catastrophic - or even slightly damaging - could occur (All Thanks to the Only Lord of the Universe indeed!).
Hope this helps/paves way Forward .