6 replies to this topic

[Raj Mehta]

Why would maximum liquid out of the tank would result in Inbreathing ? Whats happening, Can someone help me visualize the scenario ?

 

Soure: API 2000. 

 

Thanks.

[ankur2061]

Raj,

 

What happens to the volume or space of the tank which is occupied by fixed volume of liquid initially and after you have removed the same liquid volume from the tank what happens to that volume or space in the tank? If you can answer this simple question you have answered your own question.

 

Regards,

Ankur.

[Art Montemayor]

Raj:

 

You are either very mixed up with static fluids or you haven’t expressed yourself correctly.  The maximum liquid flow rate drained out of a storage tank is not the only instance where tank inbreathing occurs.  ANY liquid flow rate drained from the same tank causes the same inbreathing effect.

 

I cannot stress how important and essential this engineering knowledge is to a young engineer and I fully support the spirit and the method of Ankur’s basic response.  What Ankur is stressing is your concentration and understanding from a simple, common sense point of view: a liquid cannot be drained from the bottom of a vessel unless there is a 100% replacement of the voided space it leaves behind.  This I believe, was probably taught to you in Physics or Chemistry Lab in high school (but it wasn’t thoroughly discussed or explained).  What I am referring to is the classical way of creating a perfect vacuum: you take a glass test tube, fill it with mercury and quickly invert it while draining a slight amount of mercury.  This creates a perfect (not partial) vacuum in the test tube because you have drained a portion of the liquid without replacing the vapor pressure it once had – which was atmospheric.

 

This same effect is what causes a catastrophic mechanical failure in a storage tank (or any other process vessel) if the tank cannot withstand the internal, partial vacuum created while it is being drained – and this is very important whether the tank is an atmospheric one or one blanketed with inert gas.  You must furnish the replacement vapor pressure inside the tank as the draining fluid is trying to cause a partial vacuum.  If you study the mechanical construction and limitations of a storage tank you will discover that these vessels are notoriously very sensitive to either internal positive or negative vapor pressure.  This same effect can be extended to pressure vessels that may be subjected to vacuum conditions during their operation and is the main reason why chemical engineers are involved in determining and identifying if and when this can happen to a vessel and to make sure that the Maximum Allowable Vacuum Pressure is fully identified and specified for the fabrication of such vessels.  A vacuum failure can be just as disastrous as a pressure failure.  And that, I believe, is what Ankur is indicating for you to study, understand and use as future knowledge.

[Raj Mehta]

 a liquid cannot be drained from the bottom of a vessel unless there is a 100% replacement of the voided space it leaves behind.  This I believe, was probably taught to you in Physics or Chemistry Lab in high school (but it wasn’t thoroughly discussed or explained).  What I am referring to is the classical way of creating a perfect vacuum: you take a glass test tube, fill it with mercury and quickly invert it while draining a slight amount of mercury.  This creates a perfect (not partial) vacuum in the test tube because you have drained a portion of the liquid without replacing the vapor pressure it once had – which was atmospheric.

 

@ART sir: 

 

Every tank has some negative (minimum) design internal pressure. So the liquid will drain until it develops that amount of vacuum (assuming there is no provision for inbreathing, due to faulty design practice or a miss out). Beyond that point, no liquid will drain right ? 

 

PS: The assumption is an ideal case and is just made to understand the concept in a better way. 

[Raj Mehta]

Thank You very much Ankur sir and Art sir, for making me think the correct way & explaining the fundamental behind it. 

 

Sometimes, You end up thinking more deep and get all swirl up in the mind thinking of all various possibilities, except the most probable & logical one. I though had this answer in my mind but was not quite sure of it being the main reason governing it. 

 

Thank you once again to both of you. 

[Art Montemayor]

Raj:

 

No, no.  your statement, as written, is not correct: “Every tank has some negative (minimum) design internal pressure. So the liquid will drain until it develops that amount of vacuum (assuming there is no provision for inbreathing, due to faulty design practice or a miss out).  Beyond that point, no liquid will drain right?”

 

I have probably failed to efficiently explain what the inbreathing is and how it affects the tank and its operation.  Let me try again:

 

Any tank (or vessel) has a definite MAWP (Maximum Allowable Working Pressure) and MAWV (Maximum Allowable Working Vacuum).  These values may – or may not - be clearly identified by the designer and fabricator.  But they are still definite properties of that tank - and they won't just go away.  Since these are pressure limitations beyond which a tank cannot operate safely, it is your obligation as an engineer to clearly and accurately identify these limits not only for the sake of your design and fabrication, but for the safe and efficient operation of the tank itself once it is out in the field.  Two of the major functions of a storage tank are the filling and the pump-out (draining) operations.  I believe that you can clearly visualize and understand the operations when the tank is being filled: if you don’t have an adequate vent designed, the air (or inert gas blanket) will be compressed and increase in pressure – perhaps beyond the limits spelled out by the MAWP rating.  If this should happen, the tank could rupture due to excessive internal pressure generated by a filling operation without sufficient vent capacity to “out-breathe”.

 

Now, turn to the other tank operation – the pump-out (or drainage).  As you evacuate liquid from the tank by pumping it out or by manually draining it and allowing gravity to assist in draining the fluid, you could exert an excessive negative gauge pressure in the tank’s vapor space (or what mechanical engineers call “external pressure”) due to the tank’s vent being too small and thereby creating a constriction and not allowing sufficient atmospheric air to enter the tank and replace the volume of evacuated liquid.  The effect is called “in-breathing” – as opposed to “out-breathing”.  In effect, what you can therefore create is a situation where you have a partial vacuum in the tank’s vapor space and this created partial vacuum may be in excess of the MAWV.  I think we both would agree that this is not a situation that we would want to occur – especially if we or our plant personnel are in the immediate area of the affected tank.  The tank could possibly fail under the partial vacuum and a serious and catastrophic event could occur in the case of flammable or toxic chemicals.  This is exactly what we, as Chemical Engineers, are supposed to design against and to ensure that it never happens.  That is why it is so important for young engineers to be perfectly aware of the limitations and operational requirements of an operating storage tank.

 

I hope this additional attempt has been a better explanation of the venting design requirements of an liquid storage tank.

[Raj Mehta]

 Extremely sorry for the late reply.  

 

Superb explanation sir. I was just thinking if I would have had a professor like you or unlike any other members of this forum, I would have had much more concepts clarity on multiple topics.

 

But its never too late.  

 

Thanks.