13 replies to this topic

[ricky]

Hi all,

I have a probably delicate problem (or it's my thought only?) about calculating the vent capacity of floating roof tanks.
As we all know, API 2000 does not state or distinguish basis of calculation for either fixed-roof or floating roof tanks. In many case studies, fixed-roof is used, not floating roof, so I don't have any similar information.

Technically speaking, based on API 2000, I think only thermal case should be considered. Liquid pumping in/out is probably does not need to be accounted for since the liquid surface will create bouyancy at the roof (bouyance force negated by weight of roof, creating zero force resultant).

Again, in my opinion:
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The difference between effect of pumping at fixed-roof and floating roof is the accumulated pressure (let's assume pressure side only) when the liquid is being filled in.
In fixed-roof tanks, the accumulated pressure shall be instantly vented to prevent (at extreme) deformation of roof. While in floating roof tanks, the accumulated pressure can be 'stored' (in other words does not be vented instantly like in fixed-roofs), then create 'bouyancy' and when reaches set pressure, vented immediately.

Easily, fixed-roofs can not cope more pressure accumulation than floating-roofs. So I think the design of floating-roofs has some amount of 'margin'. On the other hand, this 'margin' is used to compensate extremely large capacity of venting (if calculated by thermal + liquid displacement as per API 2000). Imagine, for example, floating-roof crude tanks with capacity of more than 500,000 barrels, operating at extreme temperature, say like in the desert.
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I would like to share thoughts for this case.

I'm sorry if my description above confuses you
I really am a junior process engineer and have great interests in relief / safety.

Thank you

PS:
I've already described the problem here

[Art Montemayor]

Rick:

Just exactly WHAT is it that you propose (or expect) to vent from a floating roof storage tank (either internal or external)?

The basic concept of a floating roof storage tank is that the stored liquid inventory is kept covered and protected by applying a floating roof that sits directly on the liquid and seals the liquid from the atmosphere by a seal between itself and the internal tank walls. The roof can be directly touching the top liquid level or pontoons can be used to allow the roof to float.

If any vapors or inerts exist in the space between the bottom of the floating roof and the top of the liquid level, they rise and fall with the liquid level. These vapors/inerts are not subjected to any "pressure". The pump-in or pump-out of liquid inventory also does nothing to increase any vapor pressure in the tank. The only pressure that varies is the hydrostatic head of the level height in the tank.

Please explain in detail or, preferably, in a sketch what you mean by a pressure increase or venting requirement in a floating roof tank.

P.S. I am erasing your previous post in another thread, since this will be your central thread on this topic.

[Nandi]

Art,

I also had a similar question and while I was looking for information I came across this thread. So instead of starting a new thread, I am replying to this one.

I am calculating venting required for a floating roof tank. Now API 2000 says total normal venting capacity should be atleast the sum of venting required for liquid movement and thermal effects. Does this apply to floating roof tanks only or we should not consider the effect of liquid movement in case of floating roof tanks? Please give me guidelines on how to size relief valve for floating roof tank.

Thanks,

Nandi

[skearse]

Art,

I also had a similar question and while I was looking for information I came across this thread. So instead of starting a new thread, I am replying to this one.

I am calculating venting required for a floating roof tank. Now API 2000 says total normal venting capacity should be atleast the sum of venting required for liquid movement and thermal effects. Does this apply to floating roof tanks only or we should not consider the effect of liquid movement in case of floating roof tanks? Please give me guidelines on how to size relief valve for floating roof tank.

Thanks,

Nandi

As Art alluded to, there's really no application of API 2000 to floating roof tanks.

[Nandi]

But we need to provide some protection again possible pressure and vacuum built-up when the roof is at landed position. I mean when the roof is at its minimum, the tank will act as a fixed roof (smaller size).

Also, section C.3.9 in API 650 talk about some Automatic Bleeder Vents.

Thanks,

Nandi

[Qalander (Chem)]

Although Our ART& Skrease guided you well,

I will ask that have you ever physically seen/visited inside of a floating roof tank while resting on its supporting legs;i.e. landed condition?

In case the answer is yes then your query is not appropriate.

However if the answer is No then I suggest you making a thorough visit and witness the designing from inside and outside.

Since most of the now a-days designs are

to stop vapors/gases exiting/escaping from inside(below) floating roof to outside atmosphere and

do not 100% seal or stop ingress(in-flow) of fluids into the space below floating roof

therefore vacuume possibility does not arise.

[S.AHMAD]

Dear members

1. The SCOPE of API2000 very clear that the std does not cover external floating roof tanks.

2. WHY? If we understand how floating roof works, then we will find the answer

3. Can fixed roof tank subject to overpressure? the answer is YES. Why? the roof is fixed!

4. Can external floating roof subject to overpressure? NO! WHY? the roof is floating! Can we imagine what will happen to the roof if the pressure underneath increases?

5. Hope the above give some insights.

Edited by S.AHMAD, 28 December 2011 - 08:59 PM.

[ChemicalJasper]

With respect, you can get overpressure under a floating roof. I myself find this article looking for advise on installing a vent valve on an external floating roof tank (gasoline tank) which has been domed and bent by unstabalised product entering the tank following an upstream upset and gassing off under the floating roof within the pontoon ring. In extreme cases this can (and has) lead to the sinking of floating roofs.

[kkala]

http://www.cheresour...er-api-std-2000 '> http://www.cheresour...er-api-std-2000 may be useful, referring to almost same topic.

[thorium90]

With respect, you can get overpressure under a floating roof. I myself find this article looking for advise on installing a vent valve on an external floating roof tank (gasoline tank) which has been domed and bent by unstabalised product entering the tank following an upstream upset and gassing off under the floating roof within the pontoon ring. In extreme cases this can (and has) lead to the sinking of floating roofs.

It could be possible the roof got misaligned due to the sudden surge...

[kkala]

Having asked refinery operators about vapor escape in floating roof tanks in the past, they clarified that vapor will escape into air through the free space between floating roof and cylindrical tank wall. There is a peripheral (elastomer) ring in that space (sealing ring), but sealing is not perfect.
Of course ChemicalJasper mentions a much more violent event, not known to have happened here; preventing the "upstream upset" may be a way out.

Edited by kkala, 24 January 2013 - 06:00 AM.

[Hamid_r_2003]

i believe there is no need to calculate the vent rate for floating roof tanks. The purpose is not clear. The floating roof tanks shall have no normal venting.

[kkala]

I would agree, as long as tanks function as floating roof tanks. But at low liquid levels, when the roof (extended) legs touch tank bottom, tanks function as fixed roof tanks needing vents (we install them). This status is realized when the tank is emptied for maintenance or filled again, not normally. See link mentioned in post no 9 of this thread, mainly post no 5 there.

[Hamid_r_2003]

Dear kkala,
i agree with you however i believe the minimum height is somehow function of downstream pump safety guard system. It means that when the tank roofs is about its minimum height, the pump will stop. In maintenance cases the tank will be emptied by its drain lines which seems to need no in-berating from outside due to the gravity flow of drains. I also believe these tanks will behave as their normal function in initial filling cases.