N2 Pad On An Atmospheric Tank

Hi All,

I have been going through different threads in this forum all of which have been very useful and good learning. I am glad I am a member here. I need help with a project. 

I am a newbie in Chemical Engineering and have been given a task at work to design a nitrogen blanket system in an atmospheric tank used to store non combustible liquid (Base oil). Nitrogen blanketing is required to avoid oxidation of the product causing discolouration and poor solubility. 

I have calculated inbreath and outbreath rates based on API 2000 standard. However, I am not quite sure what else I need to focus on. My questions are the following:-

How do I determine the pressure of nitrogen blanketing ? I understand that its typically less than 1 PSI, but how do i decide what pressure to use as set point? 

This product requires heating to a temperature of 60C, do I need to consider Thermal in breathing and out breathing for non volatile liquid? 

The product also need to be constantly recirculated in the tank - are there any specific considerations to be taken in this case? 

Would anybody help me find answers to my questions or explain what are the steps that i need to follow for a case like this? 

Hi Raj,

Vapor pressure of the base oil at 60 ^oC?

Design pressure of the tank?

Hi Fallah,

What I could find was the vapour pressure at 20C which is <0.5 pa. 

The tank design pressure is unknown. It is a12500 IG tank (atmospheric). I believe it is simply designed to hold only the liquid pressure in the tank. We store up to 33' which is 80% of its height. 

Raj666:

You are new to an engineering assignment and you are very fortunate to have captured the attention of one of our Forums’ very knowledgeable veteran engineer - Nasser Fallah.  You should pay the utmost attention to what he can comment on and recommend you do.

But first, I want to address what I recognize as a basic first step to take when applying a nitrogen blanket on a storage tank.  I have written what are probably hundreds of comments on the subject throughout the years and the one underlying reason for this is because I also suffered the toil and troubles of applying blanketing to tanks when I was in your position, 58 years ago.  In the time period since then, I’ve accumulated a lot of experience and the most important lesson I learned was: always know exactly what is the current maximum allowable working pressure of the vessel - or tank - that you have been assigned to blanket.  (This is exactly what Fallah asks in his first post.)  I cannot over-emphasize enough the importance of this basic data.  I also realize that it probably puts you in a difficult position.  I know, because I’ve been there many times.  Processing management has traditionally been blind or ignorant in the past in considering storage tanks as merely receptacles for storage and with no need for further engineering other than tolerating them since they add no value to the stored product.  Their reluctance to spend capital money to ensure safety, efficiency, and environmental correctness makes it tough on a young engineer who has to request a tank mechanical rating before a boss who probably was responsible for originally purchasing a non-specified or calculated tank with detailed information as to alloy, design pressure, MAWP, detailed drawings, etc., etc., makes the project a hard one to do in a correct and safe manner.  This may not be your case, but I’ll bet it’s close to it.  Why else would you not have complete, detailed, as-built basic data and a current MAWP (all storage tanks should be routinely checked, inspected, and rated as to existing MAWP at intervals depending on the products stored)?

I strongly recommend that the first step you have to accomplish is to identify a credible MAWP for the tank in question.  I don’t know what you mean by “12500 IG” - but I do know that Canada enforces a strict and well-defined safety code for process vessels.  They religiously follow ASME section VIII and are very strict about process safety.  Any engineer working in Canada would be well advised to be totally backed up by calculations, standards, codes, and all project safety information and steps regarding an engineering project.

You will need the tank MAWP to identify the safe venting of the tank - whether it be in outbreathing or due to nitrogen inert feed valve failure.  You need this to base your set points, proving that you are working within a safe range.  I don’t believe you can rationally argue that the tank “ought to be able to take a couple of inches of pressure” and leave it at that.  Should anything hazardous happen in the future - God forbid - you can be sure that safety officials will not accept that basis as admissible and your career will be in jeopardy.

If you can’t rate the tank with in-house resources, than obtain an outside mechanical engineer who has the experience and recognition of doing an acceptable rating job.  It may be an uphill battle to convince your boss, but fighting for what you believe is the right thing to do is part of what engineering is about.  In the end, we all have to respond to the society that allows us to practice.

I hope this advice can help you out.

Hello Art Montemayor,

This is great advice and I really appreciate it. I will approach my in-house resources and try if i can get the tank rated, if not external help for sure. I am glad that I am a member of such resourceful forum. 

12000 IG is in Imperial Gallons. 

Thanks.

Hi,

consider the document attached to support your work.

Breizh

Thanks Breizh.