8 replies to this topic

[Shailu Gadge]

Dear all,

i am working on venting calculation for the Jet-A1 fuel storage tank. it is floating roof with closed external roof. i have calculated for the auto bleeder valve and rim vent for floating roof. my question is like is there any need of PVRV or any PSV on the floating roof because we also providing the normal vent of on closed roof by following the NFPA that vent size should be at least equal to or bigger than the liquid inlet or liquid outlet connection, whichever is bigger.(we have a 32" oulet ( Bigger) so deciding to provide 2 Nos. 24" vent on roof. so is it sufficient to avail to solve this breathing and out breathing rates.

also suggest that is it ok to use this vents caz storing fluid is Jet-A1 and need to take high care to avoid the dust and all impurities around. or it is better to put PVRV instead of a vents to avoid this problem in future.

Please suggest a answer.Especially waiting for the reply from Ankur sir.

 

Thanks....

 

 

[fallah]

Hi,

 

Which standard governs the tank design?

 

If it is API 650, then refer to H.5.2.1 Internal Floating Roof Pressure-Vacuum (Bleeder) Vent in Eleventh Ed. of this standard...

[ankur2061]

Shailu,

 

What a coincidence this is? Just a week back I was studying a proposal for a Jet A-1 fuel storage terminal and I had some reference documents (Jet A-1 tank datasheets) to refer to for the various appurtenances mentioned in that document for Jet A-1 storage tanks. Although some of the data provided was questionable.

 

The auto bleeder (bleeder vent) on the floating roof and rim vent are clearly required as per the guidelines of API 650 for closed floating roof tanks as mentioned by Fallah. The question is whether your tanks is designed according to API 650.

 

Now the question comes to the requirment of a process vent and its location. Without doubt a process vent is required. Again as far as the location of this process vent, it should be provided on the fixed roof of the tank and not on the floating roof since the auto bleeder on the floating roof takes care of any venting requirement between the extremely small vapor space and the floating roof. This process vent could be either an open vent or in the form of a PVRV. 

 

Also do not forget to provide an emergency vent or blow-off hatch on the fixed roof for external fire case scenario.

 

An additional point that was discussed during the proposal study I was doing related to requirement of nitrogen blanketing of the space between the fixed roof and the floating for the Jet A-1 tanks. After due discussion it was concluded that nitrogen blanketing was not required. The following logic was provided for not providing any nitrogen blanketing:

 

1. Nitrogen blanketing in petroleum product tanks is provided to prevent a flammable mixture being formed with air which could cause a fire and / or explosion if the concentration of the hydrocarbon and air mixture falls between its LFL & UFL.

 

2. Because there is a floating roof with rim sealing along the periphery of the closed floating roof tanks, the leakage from the rim seal to the vapor space between the floating roof and the fixed roof is negligible. Thus the vapor space between the floating roof and the fixed roof has very little hydrocarbon vapor and more of air. The mixture is therefore very lean or in other words it is below the LFL of the hydrocarbon-air mixture and thus precludes the danger of a fire or explosion.

 

3. The extremely small vapor space between the floating roof and the liquid level has mostly hydrocarbon vapors and since the rim seal of the floating roof prevents any large scale ingress of air from the vapor space between the floating roof and the fixed roof to the small vapor space between the liquid level and floating roof, the vapors in the small vapor space are hydrocarbon rich vapors or in other words above the UFL of the air-hydrocarbon mixture thereby precluding any flammable or explosive environment.    

 

The above arguments provide a basis for not having any nitrogen blanketing in petroleum product being stored in a closed floating roof tank.

 

For those young chemical engineers who have little idea of how a closed floating roof tank looks like the attached sectional drawing of a closed floating roof tank should provide a good pictorial representation.

Attached Files

•  Closed_Floating_Roof_Tank.JPG   30.24KB   131 downloads

Edited by ankur2061, 18 February 2013 - 06:22 AM.

[fallah]

[quote]...Also do not forget to provide an emergency vent or blow-off hatch on the fixed roof for external fire case scenario.[unquote]

 

Ankur,

 

In addition to your correct point as above, for internal floating roof tanks without a gas blanketing system Emergency Vent is not specified to be a required facility on floating roof. In fact, in case of fire it is supposed the seal material on the floating roof would melt and likely provide adequate venting.

Edited by fallah, 18 February 2013 - 07:26 AM.

[ankur2061]

Fallah,

 

Thanks for this update on emergency vent or blow-off hatch. The reference document (tank datasheet) I had used for study mentioned about a emergency vent and it appears that this could be a questionable point for a closed floating roof tank.

 

Regards,

Ankur.

[linus]

Hello Ankur,

 

I have an internal floating roof with nitrogen blanketting at 300mmWC. Rim vents are required to be fitted with PRVs. These are normally specified to vent vapours at 20mm WC. In this case what would be the specifications of the PRV?

 

Regards,

 

LINUS

[ankur2061]

Linus,

 

Why do you have N2 blanketing in your IFR tank? In post #3 I had provided the logic for not having N2 blanketing for an IFR Tank.

 

The terminology for atmospheric storage tank vent devices is Pressure Vacuum Relief Valves (PVRVs) as I understand. Is this the same as PRV mentioned in your post? By specification of PRV, do you mean it's size? If yes, then it is the vendor who will provide the size. There are several reputed manufacturers of PVRVs such as Protectoseal, Groth & Protego. You should consult them. Some links are provided below:

 

http://www.protectos...acuumRelief.cfm

 

http://www.grothcorp...?task=view&id=2

 

http://www.protego.c...rmation-flyers/

 

Hope this helps.

 

Regards,

Ankur

[linus]

Thanks Ankur for your prompt reply.

 

Nitrogen blanketing requirement is decided by Process Licensor, and down the years almost every internal floating roof I encountered had a blanketing system.

 

I was referring to valves on rim vents of floating roof, not conservation vents on fixed roof. These vents expel the vapors in the rim space at a pressure of around 20mmWC, thereby preventing vapor pressure build up in the rim space which could damage the seals.

 

By specification, I did not mean the sizing. In external floating roofs these vents expel against atmospheric pressure while there would be 300mmWC blanketing pressure the internal roof vents have to work against. Differential pressure would not be the correct way of specifying as this would imply subjecting the rim space to 320mmWC.

 

Your insight on this would be most welcome,

 

 

Regards,

 

LINUS

[ankur2061]

Linus,

 

I have practically no idea about vent relief valves on rim seals. This would be more the domain of an experienced and reputed IFR tank fabricator. However, I do have serious doubts about the functionality of a vent valve set at 20 mm WC gauge and working against a backpressure of 300 mm WC gauge. Obviously the set pressure of such a vent valve has to be such that it can overcome the backpressure of 300 mm WC gauge.

 

I would suggest to contact an experienced fabricator of IFR tanks and discuss this problem with him in order to arrive at a satisfactory solution.

 

One more thing that I cannot comprehend is that low pressure and atmospheric storage tank design whether fixed roof or internal floating does not fall into the purview of a licensor. The design is an open art and I can't buy the logic that a process licensor can dictate terms on this matter unless the tank contents are extremely sensitive to oxidative degradation rather than causing a flammable mixture in the vapor space. I have already presented my arguments that in an IFR tank the possibility of having a flammable mixture in the vapor space between the floating roof and the fixed roof are almost non-existent. Have you tried discussing this with your process licensor?

 

Regards,

Ankur.