7 replies to this topic

[benoyjohn]

Dear all,

I wanted to know a little bit more on the " FULL VACUUM" we specify for pressure vessels we design.

I feel the "full vacuum" which we talk about is not really FULL VACUUM. This is because under real full vacuum conditions there shall be no molecules of the fluid inside the vessel. This will require a vessel of infinite wall thickness?

This means that there is another more practical, more realistic value of vacuum which is talked of as "full vacuum"

• What is this vacuum conditions?
• Can you give your thoughts on this?
• Also Please correct me if my understanding is wrong.

Regards
Benoy

With wishes for a MERRY XMAS and HAPPY NEW YEAR.

[Art Montemayor]

Benoy:

It is nice to hear from an old-time member (over 3 years) like you. You have always brought interesting queries and comments to these Forums.

You are to be admired with the logic and probing curiosity you show with your query. And you are absolutely correct when you state that the term "full vacuum" cannot really define FULL VACUUM as we understand a vacuum to mean. Because it is virtually impossible to remove the last molecule from a system – and subsequently keep all molecules from entering that same system – it is incorrect to claim a “full” vacuum.

What educated engineers and scientists mean when they state "full vacuum" is actually a PARTIAL VACUUM. This is unfortunate mis-information that is cultivated in our midst simply because of tradition and nothing else. The persons who design for partial vacuum know this, but persist in calling it by a misnomer only because of tradition and nothing else. You are absolutely correct if you correct us when we mis-state a condition.

However, the mechanical equations employed in ASME Section VIII do take into consideration the state of full vacuum and, as such, do take into consideration that the vessel could be under total, absolute vacuum conditions. Consequently, the identification of the rating on the vessel is semantically correct --- although it will never be achieved.

You state that full vacuum conditions would require a vessel of infinite wall thickness. This is not true, as I inferred in the previous paragraph which says that the equations for rating the vessel do assume an absolute vacuum. In other words, you can contain an absolute, full vacuum with a calculated vessel wall thickness – at least theoretically. The fact that you will never achieve an absolute, full vacuum is another subject.

I can illustrate the above by use of a practical example: Outer Space is as close as we can find a full vacuum in Nature. Nevertheless, we have successful built space ships and satellites that withstand this ambient without any trouble. The ships and vessels were designed and built using the relationships previously cited. So this is proof that the equations work as best as we can test them.

The real, correct term that engineers should employ is “partial vacuum” and this term should be followed with a definition of its state – the level of the vacuum (mm of mercury, or psia, etc., etc.)

I hope this helps explain your query.

¡Feliz Navidad y Prospero Ano Nuevo! (A Merry Christmas to all, and a Prosperous New Year!)

[benoyjohn]

Art,

Thank you very much.
I admire the effort you take in making such responses.

With Best Regards
Benoy

[Kashif.Ali]

Art,

Just a quick question If we are designing a vessel for steam out conditions of 0 barg @ 240 oC + full Vacuum, would it mean Full Vacuum will also be at 240 oC. How is this possible as the Vacuum is only created when steam condensation occurs (which means temperature would be lower)? Please correct me if I am wrong.

Regards

Kashif

[Art Montemayor]

Kashif:

The only thing you might be wrong about is that you specify the wrong temperature for the fluid you are considering. Allow me to explain it this way: when you measure a temperature, you are actually measuring the molecular activity of a fluid (in the vessel). The corresponding temperature of saturated steam in your vessel when it is at atmospheric pressure (0 barg) is 100 oC – not 240 oC. However, if you are using SUPERHEATED steam at 240 oC (and 0 barg) – then you have the conditions you have cited for your design. This is the steam-out condition (0 barg = 760 mm Hg, a very positive absolute pressure – not even a partial vacuum).

You are correct in asserting that it is not possible for the vessel to be under vacuum – partial or full – once the steam – saturated or superheated – is condensed, since the corresponding temperature of the condensate must relate to the vapor pressure it is exerting (the partial or full vacuum) and which is colder than the temperature of the saturated steam. The resulting vacuum in the vessel would cause the temperature to decrease to the corresponding temperature at the resulting vacuum (the vacuum reading being the vapor pressure exerted by the fluid – water, in this case).

But let me state that it is quite possible to achieve a steady partial vacuum and a temperature of 240 oC. This can easily be accomplished by simply changing the fluid in question. All I have to do is select a fluid whose vapor pressure is equal to the desired partial vacuum at 240 oC.

Additionally, although you didn’t ask the question, it is, in my opinion, practically possible to have the walls of a pressure vessel achieve a temperature of 240 oC and suddenly undergo a deep, partial vacuum. This is what happens in real-life situations out in the processing plant. It is an important detail that many theorists fail to take into consideration when contemplating the ultimate stresses applied to a vessel’s wall and the material of construction’s weakened condition at the time of the stresses. You can be steaming-out an un-insulated process vessel with superheated steam at 250 oC until suddenly, a rain shower drops cold, continuous rain on the vessel. If the appropriate, properly designed vents on the vessel are not open at this time it is very possible to create a pronounced, partial vacuum that approaches full vacuum condtions. This happens almost instantaneously and while some parts of the vessel – such as the top roof – may be cooled down to 100 oC, other sections of the vessel may still be closer to 240 oC because of an inherent lag in the heat transfer by conduction in the metal. The heat conduction by convection happens very rapidly and is the principal cause of the steam’s condensation and the resultant partial vacuum. This may be a reason for conservatively maintaining a vacuum design temperature that is elevated in order to consider the weakened stress in some parts of the vessel during instantaneous partial vacuum creation.

I hope I have addressed your concerns and questions.

[Technocrat]

The thickness of pressure vessel is calculated based on internal pressure Pi by the equation t = PiD/2fj, but in case of full vacuum t becomes zero, then how one can find the value of t?

[Ali66]

The thickness of pressure vessel is calculated based on internal pressure Pi by the equation t = PiD/2fj, but in case of full vacuum t becomes zero, then how one can find the value of t?

Dhirajkumar,

I suspect either there is a mistake in what you wrote or there is a flaw in the reasoning you gave.

Obviously we need to use the right equation within its defined range of application.
You didn't clearly define the terms in the equation and its intended range of validity.

Otherwise, are you saying that, for vacuum application, the lower the absolute pressure the thinner the wall thickness? Obviously you can easily answer, no!

In vacuum service, the lower the absolute pressure, the thicker the wall. This valid statement already contradicts what you implied.

Ali

[djack77494]

The thickness of pressure vessel is calculated based on internal pressure Pi by the equation t = PiD/2fj, but in case of full vacuum t becomes zero, then how one can find the value of t?

This statement again demonstrates the importance of clearly indicating WHAT pressure you are talking about. Using the Imperial pounds per square inch (psi) units, you may be talking about gauge pressure (i.e. relative to atmospheric), expressed as psig, differential pressure (i.e. difference between any two pressures), expressed as psid, or absolute pressure as psia. Substitute the pressure units of your choice (e.g. barg, bard, bara), but please ALWAYS keep the suffix.

Mr. Dhirajkumar equation is correct for dealing with the calculated metal wall thickness to withstand INTERNAL pressure. Vacuum inside a vessel can be thought of as EXTERNAL pressure, and Mr. Dhirajkumar's equation is NOT applicable for that situation.

The whole idea of specifying that a vessel should be designed for "Full Vacuum" is a well known and (in my opinion) sensible concept. It is often specified for vessels that might undergo steamout, as Art discussed. Though no one thinks that an absolute vacuum is attained, the vessel walls may need to withstand differential pressures that could approach 14.6 - 14.7 psid (External). It is not easy to specify a limit (or to say HOW close to full vacuum might be possible.) Furthermore, it is not important since even if through excruciatingly difficult calculations you could show (for example) that the minimum internal pressure would ONLY be -14.4 psig. So what. Engineering is ALL about being practical, which in this situation means skipping the calculation and rounding up (down) to full vacuum, almost certainly adding FULLY zero to the vessel's cost.

Doug