Storage of crude oil with vapour pressure higher than 11 psia

I have recently heard the 11 psia limit as well. I am not sure from where this proceeds, since such a limit does not appear in API 650. For floating roofs, I would place the limitation based on the contact pressure of the floating roof such that the vapor pressure cannot lift the roof.

I think it might come from a Shell DEP, which has the status of "code" in some parts of the world. I haven't got a copy of that one yet but it was mentioned in passing...

So in other words you'd have no big issues in running with a vapour pressure of atmospheric plus the roof weight?

Thanks for reply.
LI

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

atmospheric vapor pressure (at highest design temperature) plus corroded roof weight is the practical limit for a floating roof. Whether the customer requesting that would find a contractor to build the tank to that is another story. Much of that may depend on local regulations or company policies.

I was thinking the 11 psi limit was in the federal regulations.

Section 63.646(a) of the United States Code determines the regulations for emissions control for volatile content storage. This is the section which limits the maximum monthly true vapor pressure (NOT Reid Vapor Pressure) to 11.1 psia for a floating roof to be used for emissions control.

Obviously it would also not apply if the tank were being built elsewhere than the US.

As fegenbush states, the 11.1 psi is a USA EPA rule. Although this does not apply outside the US, there is decent science behind it and as such deserves consideration. API does not care what liquid you put in your tank vapor pressure wise. If your true vapor pressure is over atmospheric you are by definition boiling and a floating roof is out of the question. You are close at 13.5 but still 1.2 psi away. I would not get too worked up about local boiling around the heating elements but I would try to minimize it with finned tubes, adequate circulation, etc. You will get pretty high losses through the floating roof, it's appurtenances and seals - check with your local EPA for limitations and applicability. Corrosion may also be an issue, for the tank roof structure and roof plate underside, especially if the crude contains sulfur. On the other hand, wax build up on the shell should be limited. Good luck, please keep us appraised of your progress!

Thanks for input. I'm trying to see if this 11 psia limit is an accepted limit within the industry or whether it is a bit conservative. Ifrs, thanks for the hint about the fined tubes to reduce surface temperature.

For other reasons we're trying to get upstream to increase the bubble point, but that's not guaranteed so need to consider all other options.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

I think that the last post in this thread is very informative....

API-650 is good to 2.5 psig..... API-620 is good from 2.5 to about 15 psig..

For smaller, more reasonable volumes, a horizontal, saddle supported tank of ASME VIII design should be considered for design pressures of 10-15 psig.

Above 15 psig design pressure, an ASME VIII(or international equivalent)tank must be specified..

MJCronin,

Thanks for input - very useful. In your experience is there a practical limit on the size of API 620 tanks? We've been quoted ~ 100 ft (35m) and 100,000 bbls. As we have an awfull lot of crude to store (2MM bbls), if we get restricted to 100,000 bbls it will mean an awful lot of tanks....

Even at 2.5 psig, does it significantly affect the design and max practical size for an API 650 tank?

I know enough about tanks to ask the questions, but not neccasarily to know all the answers, so thanks for you input and if there is any other useful advice out there I would appreciate it for dealing with volatile substances close to their bubble point. I need to maintain 65 C to avoid waxing deposition so can't cool below this without then running into lots of other problems and currently upstream are only giving me crude with a ~70 C boiling point....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

Note that the vapor pressure of 11 psi does not mean that this is an API 620 tank - API 650 is just fine. Different pressure.

IFRs,

I believe LittleInch is wanting to put a nitrogen gas blanket on the tank (since a floating roof will not be acceptable) to limit losses to evaporation.

LittleInch,

A tank rated for 2.5 psig will likely require a full slab foundation to avoid uplift. At larger diameters, this can become cost prohibitive since the slab thickness will also increase with diameter due to the bending moment. This relates to cylindrical tanks only.

LittleInch/fegnbush...

The questions you ask are very important, but exceed my limited tank farm knowledge.

Aside from the issue of uplift and foundation type, anchor bolt style and scheme must be considered.

Also to be considered are the massive foundation loads for these tanks and the costs associated with supporting these loads

Suggest that you contact (and contract with)a storage tank consultant.

Philip E. Meyers literally "wrote the book" on the fine points of storage tank design and tank farm operations.

He works for PEMY Consulting:
Please respect us....come back to this thread and let us know about your final decisions and why you made them..

Thanks for input - very useful.

I will advise when we complete this aspect, but it may be a few weeks.

IFRs - I understand the pressure aspects, my key question was to try and understand if this 11 psia "limit" was a universally accepted number or not, which has been reasonably well answered and what the possible slaternatives were if a FR tank wasn't seen as accptable, which from an emmissions point of view may well be the case, even though the tank is not in the US.

Thanks again for input.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

A new product was recently introduced by Trelleborg called "DryFoam" that can easily handle vapor pressures up to flashing and beyond. It was orginally developed for crude oil storage tanks in the arctic that has vapor pressure exceeding 14.2 psia plus it will prevent fires. DryFoam is essentially small composite beads about 1/8" diameter that simply floats on the oil at all times. At 6" thickness of beads, the DryFoam easily floats and supresses vapors up to ~99%. They can tolerate fires and have a coating on each bead that swells and insulates the ones beneath so that a torch directed on the surface of DryFoam floating on gasoline will not ignite the gasoline. The product has been tested on crude oil (Saudi light), gasoline, heptane, diesel, ethanol and even cryogenic LNG. DryFoam won't sink, it won't corrode, and can be applied very easily and quickly plus it will prevent fires. So as long as the liquid level stays above the outlet pipe then there is no concern of ingesting the beads. If they do get sucked into a pump then the pump simply stops being a pump as there is insufficient NPSH so it won't go on for long at all. I should also mention that since there are no legs or pontoons as with floating roofs, then the tank's effective working capacity is increased significantly at virtually no cost.

I believe that DryFoam will work well in the described application very well. If you are interested the guy at Trelleborg is Bob Kelly and he's in their Boston office.

One of the problems with a floating roof that is floating on products near the vapor pressure is the stability of the folating roof. Many have sunk due to localized flashing of the product that causes differential settlement or sinking of one side that allows the underlying liquid to overflow onto the top and flooding it.

Thanks, I'll look this up and see where it takes us.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

How does this bead system work with hi wax oils?

Independent events are seldomly independent.

Thanks for all the input, very useful. Resolution now seems to be that sense has prevailed and the upstream element is conditioning the crude with a lower vapour pressure such that FR tanks can now be used, but will pursue these beads a bit further. Sound quite interesting.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way