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A Few Storage Tank Related Questions

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#1 traineeprocessengg


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Posted 14 March 2019 - 11:52 AM

Hello everyone. I wanted to ask a few questions related to different types of storage tanks used in refinery and nitrogen blanketing.


Part 1: Types of storage tanks


Many websites say that:

1) Fixed roof is for diesel, kerosene, residual fuel oil, etc.

2) External floating roof is for crude oil, gasoline, naptha, etc.

3) Internal floating roof is for aviation fuel, etc.

4) Bullets are for propylene, propane, butane, etc.

5) Underground tanks are for naptha, superior kerosene oil, motor spirit, etc.


On what basis is this decided that a certain product is to be stored in a certain type of tank? Are bullets used for propylene, propane, butane, LPG, etc because large volume of those products are obtained from cracker and they have to be maintained at high pressure because their vapor pressure is also high? Also, my reasoning is that floating roof is probably used for volatile material with high vapor pressure at ambient temperature because of which large volumes could breathe out through the breather valve if they were stored in fixed roof. Is that correct? (Also do we use breather valves in floating roof storage tanks?) And my last question is that on what basis do we decide whether to use internal floating roof or external floating roof?


Part 2: Nitrogen blanketing


I could not understand how nitrogen blanketing actually works (see image attached). Do we push the vapors of the liquid stored out to the atmosphere? Wouldn't that be hazardous? Or do we push the air out of the empty storage tank during startup and then when the liquid is pumped in we take nitrogen out via that control valve that vents to atmosphere? And why is a siphon breaker sometimes provided on vent to atmosphere lines and what exactly is a siphon breaker?


One more small question, why do we ground storage tanks? I know it's to prevent the liquid stored from catching fire if flammable and exploding but where is the static electricity coming from and where does it get accumulated?


Sorry for my beginner level questions and thank you in advance. 



Image link: https://cdn1.imggmi....fe6427-full.png

Edited by traineeprocessengg, 14 March 2019 - 12:03 PM.

#2 traineeprocessengg


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Posted 14 March 2019 - 11:58 AM

An addition to the nitrogen blanketing part of my question in the opening post, 


suppose the control valve on the nitrogen inlet to tank is full open, and the nitrogen pressure is 7 barg. The pump that pumps the liquid out of the tank has maximum shutoff of 5 barg. So will the pump experience max shutoff through the MCF line? Because the nitrogen in the tank will exert pressure on the liquid surface, which in turn will get added to the pump's suction pressure. So what exactly will happen in this scenario? I'm unable to imagine it in my head. 

#3 Art Montemayor

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Posted 14 March 2019 - 01:37 PM   Best Answer



Your query is like questions raised by lay persons or journalists trying to inquire about technical process operations and not like an engineer would look at a process problem or application.  I am forced to revert back to engineering basics in order to address your questions:

  • I don’t care what the “other websites” state about the application of storage tanks.  The statements are totally out of line.  The statements don’t take into consideration the actual fluid stored and the conditions it is stored under.  These are the first and most important basic data you have to know - and you fail to state that.
  • The next important basic data is the size of the storage requirement.  What is the storage capacity in question?  You also fail to identify that.
  • The best engineering advice I can give you is to never, never ever generalize when you deal with an engineering query.  Be specific and identify all the required basic data before answering or commenting on a process operation.  Your failure to do this is why you are so perplexed about applying storage tanks.  Without specific basic data you will always be wandering around without fully understanding the application.
  • A fluid’s vapor pressure at the required storage pressure is often what determines the type of storage tank best suited for the application.  LPG and other liquefied petroleum gases are stored in pressure vessels (such as spheres or bullets) when their vapor pressures are within the range of conventional vessel design pressures above the fluid’s vapor pressure at ambient temperatures and the application suits ambient storage temperatures.  If the quantity is large and the need and economics suit the application, then cryogenic temperature storage at relative ambient pressures may be attractive.  Practical sense economics always reigns in engineering applications - not generalizations.
  • Your reasoning for using a floating roof tank is wrong.  If the fluid has a high vapor pressure, a pressure vessel may be called for.  Use your common sense: an API storage tank with a floating roof is limited to relatively low pressures.  The floating roof is to compensate for varying liquid levels that may cause a partial vacuum and allows for vapor space expansion - as well as for large inventories.  The nature and properties of the stored fluid is what determines the type of floating roof - as well as the economics involved.
  • Your nitrogen blanketing sketch is overly simplified and doesn’t give a full definition of how the system works.  That’s why you are confused and don’t understand it.  Download the workbook on nitrogen blanketing in our website and study the explanations I give for nitrogen blanketing and how it is controlled and operated.
  • Again a common sense item: when you pump fluid out of storage tank that is sealed or devoid of a source of positive pressure, you will generate a partial vacuum in your tank’s vapor space.  Everyone should know and understand this.  You cannot tolerate a partial vacuum in a low pressure storage tank without the danger of collapsing the entire structure - and its contents!  That’s why you apply a conservation vent - a relief device that opens to relieve a buildup of internal pressure and also opens to allow atmospheric air to enter the tank when there is a partial vacuum produced inside the tank.
  • A nitrogen blanket works as explained in my workbook: nitrogen is maintained in the tank’s vapor space with a slight pressure (5-15 inches of water column is normal) by a control valve that feeds the tank’s vapor space on pressure demand (as when the fluid is being pumped out).  When the tank receives more stored fluid subsequently, the vapor space pressure increases (since the amount of nitrogen previously introduced is compressed by the rising liquid level).  The conservation vent vents excess nitrogen to the atmosphere to prevent an over pressure.  The nitrogen control valve should fail closed and if it fails to open on the next fluid pump-out, the conservation vent opens to the atmosphere to allow air to break a partial vacuum in the tank.
  • Storage tanks are positively grounded to prevent the accumulation of static electricity - especially when storing flammable fluids.  Explanations for static electricity can be found in your electrical engineering courses or texts - or in the internet.  Wikipedia states: “A static electric charge can be created whenever two surfaces contact and separate, and at least one of the surfaces has a high resistance to electric current (and is therefore an electrical insulator)”.  You should have covered this in your engineering curriculum.

This post is already very long and I hope you have the answers you seek.  If you don’t have basic engineering courses or training, please state so and we can recommend study material.



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Posted 15 March 2019 - 06:18 AM


Art Montemayor

"If you don’t have basic engineering courses or training, please state so and we can recommend study material."



this is the point which I would like to stick on. So, please do recommend study materials which would help to gain more knowledge and also to have better understanding.


Thanks in advance

#5 breizh


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Posted 15 March 2019 - 06:49 AM

Hello there ,

To support your work a few documents together with a link about static electricity .




good luck


Edited by breizh, 15 March 2019 - 06:52 AM.

#6 Art Montemayor

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Posted 15 March 2019 - 05:06 PM

Trainee - or ABCDER (and whoever is interested):


Attached are some documents for study on the topic discussed.


Attached File  Storage Tank Pressure & Blanketing Control Rev1.xlsx   452.89KB   56 downloads

Attached File  Venting Aboveground Tanks.docx   303.42KB   51 downloads

Attached File  Design and Study of Floating Roofs for Oil Storage.pdf   996.26KB   37 downloads

Attached File  Comparisons Between Internal And External Floating Storage Tank.docx   166.01KB   39 downloads

Attached File  Design_Construction_Operation of Floating Roof Tank.pdf   5.77MB   41 downloads

Attached File  Floating Roof Tanks Explained.docx   1.06MB   42 downloads

Attached File  Improve Selection and Sizing of Storage Tanks.pdf   1.42MB   55 downloads

#7 samayaraj


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Posted 18 March 2019 - 01:40 AM


Thanks Mr. Art for sharing your valuable comments

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