17 replies to this topic

[PE123]

Dear All,

I am in the midst of improving N2 blanketing system for a fuel oil tank (atmospheric tank).
Currently, there are 2 flame arrestors on the top of the tank. N2 is injected manually via a flow guage.

I am installing a pressure regulating control valve to control the tank pressure at 20 mmH20. In addition, 2 new sets of flame arrestors plus breather valves will be installed.

After doing a detailed sizing, the venting requirements are established. However, installing breather valve on top of the flame arrestors reduces the venting capacities of the equipment and is insufficient for the venting requirements.
(The tank nozzle is 3" -- > even the max capacity of the flame arrestor + breather valve is unable to meet the requirement).

As such, I am considering to just install the N2 pressure regulating valves and a new set of flame arrestors without the breather valves. Is this desirable ? Can someone advise me on this matter, assuming the N2 loss is tolerable ?

thanks
FSK

[proinwv]

If I read this correctly, even without the breather vents, you will not have sufficient breathing capacity. If this is correct you need to stop and do what is necessary to provide that capacity. Otherwise you risk the possibility of tank failure.

Even if that is not so, without breather vents you will discharge both N2 and fuel vapors continually. In fact you might not even be able to maintain a blanket pressure with a 3" flame arrestor.

You need to step back revise your plan. It may require installing proper nozzles on the tank, which I realize can be a major undertaking on a fuel oil vessel. But that objection will not substitute for a system that will not function properly.

[Art Montemayor]

FSK:

Paul, in my opinion, is precisely correct in advising you to halt your design for a while, sit back and think carefully about your basic needs, requirements, and goals.

You don't tell us what "fuel" you are storing, its vapor pressure, or its flash point. But nevertheless, I would caution you on the use of flame arrestors. I just finished a large project that initially involved the use of flame arrestors - this project is owned by some of the biggest energy companies in the world and we ran into difficulties:

• the API definitely has taken a hard stand against the use of flame arrestors - mainly because of the hazards involved with the inherent internals of such devices and the large pressure drop typical of such devices; I don't have the API RP document number at present - but it exists.
• the maintenance of such devices is very difficult and cumbersome - sometimes leading to neglect and creating a worse hazard than not having one;
• ultimately, it was decided to remove the flame arrestors because we really had no hard, actual proof that they would be effective.

As Paul indicates, you must establish a credible and practical pressure working range for the nitrogen blanketing system to work effectively in accordance with your tank pressure relieving devices. Without this feature in place, your efforts and plans will not succeed.

[proinwv]

Art, thanks for the information on flame arrestors too!

[Art Montemayor]

Paul:

I am presently with my grandchildren in Tucson, AZ and not at home. I will return home on Sunday and have made a mental note to add the specific API document that addresses the hazards of flame arrestors to this thread for your information as well as anyone else that is interested.

[fallah]

Art:

I think you mean API 2210 titled:

"Flame Arresters for Vents of Tanks Storing Petroleum Products"

One page of it is attached and the problem of using flame arrester is inside the red rectangle. Correct me if i am wrong.

Fallah

Attached Files

•  Page from API 2210.pdf   102.09KB   158 downloads

[proinwv]

Thanks to Art and Fallah for this information.

[PE123]

Dear Paul,

Without the breather valve (Not an open vent), the venting capacity (inbreathing/outbreathing is sufficient) is sufficient for venting capacity.
i.e. Capacity of (breather valve + flame arrestor) < Capacity of flame arrestor
However, 2 breather valves are required to be commissioned at all times.

From the actual operating experience of another fuel oil tank where breather valves(BV) and flame arrestors (FA) are installed (BV is installed on top of the FA), it is observed that there is no N2 injection into the tank during the daytime due to hot ambient temperature ( ~ 30 degC). N2 is only injected during the night time where the ambient temperature is colder. For this fuel oil tank, tank pressure is maintained at 20 mmH20 by N2 blanketing.

As such, by installing only the flame arrestors which itself has an inherent pressure drop. N2 or hydrocarbon vapors need to overcome a certain resistance to be vented to the atmosphere.
Basing on the above points, my opinion is that during daytime, there will be no N2 injection and minimal hydrocarbon vapor will be vented if there is no liquid movement.
During the night time, there will be N2 injection into the tank and N2 pressure can be effectively maintained at 20 mmH20 in the tank. Minimal losses of N2 to the atm is expected (A breather valve with flame arrestor can ELIMINATE N2 loss if the set pressure is not exceeded) .

On side note, I am considering to modify the existing 20" manhole to accomodate one more breather valve + flame arrestor to increase the venting capacity. Currently, there are two 20" manhole on the tank top.

Paul, could you give me your take again based on my reply above. I would greatly appreciate that.

Dear Art,

thanks for the advice. The fuel oil has flash point of more than 60 degC (Above 37.8 degC) Boiling point can vary depending on the process as fuel oil is byproduct of the steam cracking of naphtha. In general, it can be said that fuel oil is the most heavy byproduct with boiling ranges from 100 degC to 300 degC.
As mentioned above, the plant has existing flame arrestors and breather valves for fuel oil service that has worked well for us for the past 12 years which I am trying to replicate the N2 blanketing provision for another fuel oil tank. The pressure range for the N2 blanketing is as per existing system ( 20 mmH20). As such, there is confidence that this pressure setting can be practically achieved.

Thanks all for the reply and would love to hear more opinions on this matter.
I understand the importance of the conservative and credible design as miscalculation can lead to tank overpressure or vacuum. As such, i am exploring all means to hear more professional opinions.
thanks.

FSK

[proinwv]

First, the manhole cover sounds like a good place to add nozzles without having to do hot tank work. Be sure that there is adequate space for flow to and from the nozzle in the tank.

Where one usually must be concerned about breathing is not the diurnal situation. That is easy. In breathing due to product withdrawl or sudden cooling can be substantial. Out breathing due to pump in or even fire exposure also can be substantial.

But I do not understand your statement,

Without the breather valve (Not an open vent), the venting capacity (inbreathing/outbreathing is sufficient) is sufficient for venting capacity. i.e. Capacity of (breather valve + flame arrestor) < Capacity of flame arrestor However, 2 breather valves are required to be commissioned at all times.

I also would take very seriously the information provided by Art and Fallah about flame arrestors.

[PE123]

Hi Paul,

Thanks for pointing it out. The venting requirement has been throughly and conservatively calculated taking into account of the liquid movement and thermal effects with guidance from API-2000.

For fire exposure, there is an existing guage hatch. In addition, i have intention to install an emergency vent.

Any take on the effect of just installing flame arrestor without breather valves especially on the possible N2 losses to atm ?

W.r.t to my "statement", what is meant is the venting capacity becomes smaller if breather valve is installed on a top of a flame arrestor as compared to just flame arrestor only.

for e.g. (Using actual numbers obtained from vendor)

Inbreathing capacity (Flame arrestor alone) = 2059 m3/hr
Inbreathing capacity (Breather valve + flame arrestor ) = 911 m3/hr

thanks.
FSK

[Art Montemayor]

FSK:

API 2210 is the document I have mentioned. Thank you, Fallah.

The API discussion follows with more information - beyond the page that Fallah has supplied. The strongest argument that can be made against any flame arrestor is that the API adamantly refuses to sponsor or recommend their use. The language used in API 2210 is so strong against flame arrestors that our Hazop committee had to back off the recommendation of using them. We had to remove the flame arrestors. And, as I said, this project was one of very, very high profile and with severe political implications that involved the top energy companies in the western hemisphere.

Another item of interest when discussing "flame arrestors" is that of DETONATION arrestor. This is another type of flame arrestor that is meant to stop a detonation from propagating into the internals of the storage tank. This apparatus is similar to a flame arrestor, except that it has a multiplicity of flame arrestor cells that makes it into a very, very, huge vessel. Even the standard flame arrestor we were evaluating was weighing in at 1,850 lbs weight - we had to design an internal superstructure support for the 1.0 tonne of dead weight. This was for a rectangular tank with 3,000 ft3 of internal volume. You haven't given your tank's capacity, so I can't compare. All I can do is alert you to what you may have in front of you. Our fluid was not fuel oil; it was methanol.

When we aborted the flame arrestors, we did what Paul suggests: we mounted the Pressure relief and the vacuum relief valves on one of the 24" top manways. We needed 2 manways to allow for quick and efficient ventilation prior to confined space entry during inspections and repairs.

[breizh]

To all,
Thanks for this very interesting discussion .

fsk , you may consider to talk to vendor like Protectoseal ,

http://www.protectos...ameArrester.cfm

Breizh

[PE123]

Dear Art,

I will consider the application of the flame arrestor seriously as adviced.

To share more:

The fuel oil tank is ~ 7,000 m3 in working capacity with 2 flame arrestors installed at the top of the tank. This is the original design from 30 years ago. So far, there has been no major incidents.

The project at hand is to install nitrogen blanketing with breather valve and flame arrestors for this tank.

Best Regards
FSK

[proinwv]

FSK, if you do not place breather vents on the tank there will be, as you are aware, a continuous flow of N2 out of the tank.

If you can get the information from the FA vendor for flow vs delta P for 20mm H20 pressure then you have the flow rate. Otherwise some rough estimates of area and orifice coefficient will give you an indication of the flow rates. I believe that you will see some very large losses of N2. While I do not believe that you should do this, I would point out that the N2 nozzle should be placed a good distance from the discharge nozzle(s).

[PE123]

Hi Paul,

thanks for the advice !
I am in the midst of obtaining the N2 leak information from the vendor. The numbers do not look optimistic and i am considering the option of modifying the manhole for the installation of a bigger breather valve and flame arrestor.

thanks for your opinion.

FSK

[Nomanwer]

I have a Class I storahe tank of capacity of 100m3. I have given nitrogen blanketing on it. Do i need to dive foam protection to tank also?

[Art Montemayor]

Nomanwer:

Your post is a totally different topic as to what is being discussed in this thread.

Please originate your own, original thread on this Forum - with the appropriate title. Your query will be addressed much more efficiently and accurately in that manner. You are just confusing the content of this thread and nullifying the ability to respond to your request without interruptions.

I will delete your post here in a few days, after you have posted your own thread.

thank you.

[kkala]

Just a note on fuel oil tank blanketing, in addition to above posts. We had installed N2 blanketing system on daily fuel oil tanks for refinery boilers (nitrogen PCV, cone roof tank, capacity about 50 m3); stored fuel oil could be heavy, requiring heating above its flash point (minimum 66 oC). But flame arrestors were not installed.
Relevant duty specs were elaborated in 1998, 2008. Last time I heard of flame arrestors was in a seminar of 1982.