Steam-out conditions 1

[MarioAlc]

Hi everyone

In our process data sheet for vessels we normally write a note indicating the alternate design conditions for steam-out.
Sometimes we write 0.5bara AND XºC

(XºC is just the plant steam temperature at atmospheric pressure)

However, sometimes I have seen "0.5 bara AT Design temperature" being X the design temperature of the steam.
I don't undesrtand the use of "AT" in this case. My opinion is that if you are below atmospheric pressure, your temperature can not be above 100ºC, it means, you can not have both half vacuum and high temperature at the same time.

The vacuum conditions arrives when the vents are closed or too small AND 100ºC are reached. Otherwise, it can not to go below atmospheric pressure, so I don't undestand why this note is so common: "half vacuum AT X ºC" being X a temperature much higher than 100ºC.

I undesrtand half vacuum AND XºC but not the use of "AT".

Thanks

Mario

 

[Latexman]

I don’t see a ?

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Latexman
Pats' Pub's Proprietor

 

[0707]

I don´t understand your doubt, steam out can´t have the vents of a vessel closed. Please clarify.

regards

luis

 

[MarioAlc]

My understanding is that the reason behind a FV conditions during steam out can only occur when the vents are closed by accident or it start raining during the steam out and the vents are not prepare to get enough air in.

My question is

why do we normally write a note "FV AT XºC"? By saying "AT" it seems the FV can be created at the same time we have 240 or 250ºC inside the vessel. I don't see how this is even possible and we still put "AT". The vacuum can only occur at 100ºC, there is not condensation above 100ºC.

I just want to know the opinion of process engineer regarding this very common note in vessel data sheets. If you check in the mechanical engineer forums you will see they talk about FV or half vacuum AT 280ºC.

I hope it is more clear now.

Thanks.

 

[Latexman]

Because a pressure and temperature must be specified. Just as they have to know the material of construction and thicknesses. These are some of the minimum requirements to do the strength calculations.

It is the Engineer’s responsibility to transmit the worst case conditions so there are no surprises, like a tank collapsed by vacuum, in the real world.

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[MarioAlc]

Sorry but it seems my question is not clear yet. I put an example.

Steam has a design pressure/temperature of 5 barg and 240ºC.

According my understanding, the note should be "vessel to be designed for steam out conditions: half vacuum AND 240ºC.

however, the note normally says "half vacuum AT 240ºC.

Maybe there is a reason to use the word "AT" that means the vacuum can occur at the same time as the 240ºC. As fas as I know, this is the use of "at" or "@" (= this specific pressure at the same moment of this specific temperature").

My point is that this will never happend, it is impossible to have vacuum above 100ºC. But engineers keep using "FV AT..." instead of "FV AND..."

Sorry for the missunderstanding, I hope it is clear now.

Thanks.

 

[Latexman]

The vacuum can occur extremely fast and some, or all of the metal, will not have time to cool. For example, someone could spray water on a part of the exterior of the vessel. On the opposite side, the metal does not cool. Or someone sprays cold water down into the center of a big vessel. It contacts steam and it condenses but no water cools the sides of the vessel.

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Latexman
Pats' Pub's Proprietor

 

[LittleInch]

I agree with latexman.

It is a realistic combination of pressure and material temperature.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[razookm]

@Latexman Well explained and very convincing. Normally we do not consider the design temperature of the steam supply but do consider the normal operating temperature for steam out conditions. This I think is a reasonable middle path without really overdesigning the equipment.

 

[georgeverghese]

Whilst you may argue the conditions for "half vacuum" (0.5bar abs), surely you have a case when you have full vacuum - this may in many cases be the more demanding mechanical design case from an external pressure perspective.

 

[MarioAlc]

Thank you very much for your answers!

 

[Latexman]

Agree with George. "Half vacuum" is mostly arbitrary. "Full vacuum" is a conservative, physical limit that is not unpractical.

Good Luck,
Latexman
Pats' Pub's Proprietor