7 replies to this topic

[ochoche]

Can fuel gas be used as blanket gas for petroleum tanks

[Art Montemayor]

The answer is yes, and it often is the "blanket" of choice in production areas - such as the "oil patch" - where it is usually the only gas available.

Although natural gas (& other light hydrocarbons) is not an inert, it is used specifically as a "blanket". In other words, the scope of its application does not involve creating an inert volume above the hazardous liquid; rather, it is applied as a means to keep air and other contaminants out. A slight pressure of blanket gas ensures that air will be kept out.

[somen]

With reference to the topic, I would like to also know that in my present project we are designing an oil tank of design pressure 40 mbarg/-6 mbarg which is having fuel gas blanketing apart from breather valve connection (pressue and vacuum) and emergency vent. The breather valve outlet is connected to atmospheric flare header.
In case of vacuum, air will come inside. Are there any chance that the fuel gas and air will form an explosive mixture in the tank? Any concern regarding routing the air and fuel gas mixture to the flare header?

[pasquale]

For the design of your storage tank you should follow the guideline of the API 2000 standard.

Pasquale

[pleckner]

somen: I don't quite get it. First you say that you are supplying a fuel gas blanket with a breather valve but then state that the breather will allow air in during vacuum conditions. The fuel gas blanket is there to keep the air out during in-breathing, your breather should not be allowing air into the tank during normal operation. I hope you mean that the breather is acting as a back up in case you loose your gas blanket during operation.

A fuel gas/air mixture may indeed be a hazard and must be considered. I would look elsewhere for back up protection, possibly a nitrogen source.

[proinwv]

I agree with Phil.

And, you should not be bringing in air, except under a failure situation where the PV vent serves to protect the tank in the case of blanketing failure.

There is more information on my website also.

Also, look at the posts on our Tank Blanketing Forum here on ChE.

[Leticia]

Hi,

I have a 3000 cubic meters tank in which we storage a product that needs blanketing with nitrogen. We have design the nitrogen supply system following API2000. The tank is keep at 10 mbar of pressure and there is a blanketing valve, the problem is that I don´t understand the nitrogen flow, when we pump the product at 45m3/h the flow of nitrogen is more or less 70m3/h, but when we pump at 90m3/h the flow of nitrogen rises until 250m3/h, what I mean is that It follows and exponential curve. Is it normal? Is my valve working well? Where can I look for information about it? Thanks!

Best regards,

Leticia

[Art Montemayor]

Letty:

First, this should be a totally different thread and it belongs in the Tank Blanketting & Venting Forum. This thread was started by - and belongs to - Ochoche and I believe we should respect that right. A different thread is called for in order to address your particular query. Please try to post this query in the Tank Blanketting & Venting Forum where it will be fully appreciated and acted upon. In the meantime .....

Go to the Relief Devices Forum and Post #58:

http://www.cheresour...x...

And download the Excel workbook that I’ve prepared for assisting and helping in the design of N₂ blanketing storage tanks as well as protecting them from over- and under-pressure.

There have been more than 18,500 persons viewing this thread and close to several hundred that have personally contacted me for a copy of this workbook as a help in their storage tank venting and blanketing applications. This workbook may not solve your problem by itself, but it may give you some guidelines that have helped in resolving a lot of similar problems.

For example, with a very large tank, such as you have, it is my opinion that you are operating under an inordinate blanketing pressure operating zone. You cannot be expected to do any decent controlling of blanket gas when you are trying to maintain just 4 inches of Water Column inside your vapor space. This is just not credible. The instruments required to do this in this size of a vessel have to be superior to anything I’ve seen in the field and also have to be maintained in accurate and repeatable operation 100% of the time – a condition that I don’t believe is happening. In other words, it is my opinion that the operating pressure of 4 inches H₂O on this tank is a naïve dream of the designer. You definitely need a much larger instrument Dead Band to operate successfully and with repeatability.

I’m not saying that your excessively small Dead Band is the cause of what you are witnessing. The lack of a relationship between the liquid displacement and the make-up nitrogen may be simply flow meter calibration errors or flaws. This must be verified locally. But the small Dead Band doesn’t help you ensure that you can control a stable operation in the blanketing pressure.

If you can prepare an Excel sketch showing your controls and flows as a P&ID, and upload it here, perhaps we can have a better understanding of what is happening to your storage tank blanketing. Are you consuming excessive quantities of Nitrogen? I suspect that you are.

I will await your post and response in the Tank Blanketting & Venting Forum.