5 replies to this topic

[afd]

I was studying a PI & D and observed that a steam vessel was designed for only pressure relief. I have normally seen steam vessels designed for both pressure and full vacuum.

I spoke to an mechanical guy and he replied that a pressure designed vessel and withstand vacuum also.
How far is it true?

AFD

[Art Montemayor]

AFD:

Good eye! In my opinion you are absolutely on sound and logical ground if you demand that the steam handling vessel be rated not only for its MAWP, but it should be designed to withstand Full Vacuum - especially if it is located outdoors (as I presume).

If you are saying that you have been told that a vessel designed to withstand pressure will also inherently withstand vacuum, then you are being told a flat-out lie or falsehood. Any vessel has to undergo a full vacuum design and rating if it is to withstand vacuum. And it costs a little more to make most vessels withstand full vacuum.

As a matter of curiosity, has this vessel been HaZopped?

[Nirav]

AFD,

I have slighty different opinion from what Art has said to your SPECIFIC query. I am not against with Art since whatever he said is very much TRUE in GENERAL.

Now, consider my points on your doubts and try to find the answer by yourself.

[1] What's "external" design pressure of vessel?
[2] What's the thickness of vessel based on "external" design pressure? consider it as "t-external"
[3] What would be thickness based on "internal" design pressure (full vacuum in your case)? consider it as "t-internal". You can calculate by yourself based on formula given in ASME Sec.VIII, Div.1

You MAY notice that "t-internal" (based on vacuum) is less than or equal to "t-external". That is the reason why your Mech. engineer might have told you that vessel will also withstand "full vacuum".

However, as a process engineer, we MUST write the requirement of "full vacuum" on process datasheet. In your case, it could be just matter of revision in documentation. It may not require any changes in actual mechancial fabrication aspect of the vessel.

One more important thing is to specify temperature you expect at the time of "full vacuum" condition. It could be different than design temp. at "external" design pressure. For example, consider vessle handling hydrocarbons during normal operation. The requirement of "steam out" conditions in such vessel would require full vacuum design condition at operating temperature of steam. But actual operating or design temperature of the vessel could be different than that.

I have had similar discussion on similar topic with our Mech.engr once upon a time. I'm not sure, but if my memory goes right, our conclusion was that for carbon steel vessel, if "external" design pressure is more than 3.5 barg, it would give higher thickness than what you get based on "full vacuum". Based on such "cut-off" value, your mech. engineer might have given you immediate responce that vessel will NOT get affected by "full vacuum" conditions. But he should have also explained you the reason.

I suggest you to look into your doubt from whatever I tried to explained.

One more request to you is please come back and tell us your final conclusion.

Thank you,

[benoyjohn]

Nirav,

To extend a little bit on that,

I believe 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 realistic value of vacuum which is talked of as "full vacuum"
What is this vacuum conditions?

Can you give your thoughts on this? Please correct me if my understanding is wrong.

Regards
Benoy

[afd]

Thank you Art for your insight and Nirav for your elaboration.

This vessel is a modification in our plant .Yes it is installed outside in the open. You said it and the answer I got also was " dont bother A HAZOP has been done and that must have taken care of the things".

As a process operator I understand that HAZOP is done on PI & D. and HAZOP is done by a team of engineers who also are prone to overlook things. Like Nirav says maybe the PI&D may not be marked but the vessel maybe designed for vacuum as well.Anyway Nirav I may not get the answer.

Finally, my two cents on Organizational Behavior. When operators point out something the engineers feel low to accept it or even to say I dont know. This is my experience. I still recall a poster outside my school principals room" none of us is as smart as all of us"

AFD

[afd]

The vessel in question has arrived at location and it is infact designed for full vaccuum also. The vessel nameplate shows as follows:

DESIGN PRESSURE : internal/external 40 barg/FV

Another vessel which has a steam jacket has arrived at site and the manufacturer data shows that the jacket is not designed for vaccuum. it is designed for a steam pressure of 30 barg and has a volume of 200 litres.

Does this jacket need to be designed for vacuum also or is the volume so small that it able to withstand a vaccuum?
AFD