3 replies to this topic

[Ghasem.Bashiri]

Dear Process Experts
My Problem is steam out condition. Assume a vessel with 5 Barg and 100°C Design condition. Also I have a note as "Steam out condition" should be considered for this vessel.
As practice for steam out, Full vacuum is reported for vessel design pressure. But I need to know is it required to mention temperature also. Steam is available in the plant with 4.5 Barg and 170°C. Therefore vessel can be exposed to this temperature. Therefore is it required to consider steam as operation case (4.5 Barg and 170°C). Refer to attached curve from ASME, temperature has effect on external pressure design.
Is just full vacuum pressure without refer to temperature is sufficient? if not which temperature should be reported for full vacuum condition?
Ghasem Bashiri

Attached Files

•  ASME Section II.pdf   52.78KB   292 downloads

[Art Montemayor]

Ghasem Bashiri

I don’t believe you need to specify at what temperature the vacuum will be present – as long as the temperature is within the design stress limits of the steel (or other metal) used in the vessel’s fabrication. It that what you want to know?

You also ask: “is it required to consider steam as operation case (4.5 Barg and 170°C) ….?” I don’t understand your question or your concern.

I suspect we may be thinking of different “Steam-out” scenarios. The only steam-out scenario I know of is one that is done at atmospheric pressure while the vessel is opened up and sprayed with live steam to remove tars, hydrocarbons, and other impurities in the vessel to effect a clean out and make the vessel available for human inspection. Is that also what you mean?

[Ghasem.Bashiri]

Art;
Assume you want to start steam out condition!
With open vent, we will not have vacuum. Steam will be flashed to 1 Atm. Just steam cleaner in the home: Steam will be condensed and clean the surface.
First issue is that with vessel design temperature of 5 Barg and 100°C, is it required to consider ATM/150°C as alternative design condition for mechanical department checking?

Next: Vacuum will be happen in case of closed vent, steam out and then steam condensation. We expected that steam should be condensed with metal temperature between 100-150 C. Then vessel may be exposed to vacuum.
Now, Is it required to consider Vacuum with 140 or 150°C as steam out condition or just FV is sufficient. Refer to our mechanical department guide (attached pdf file in the topic) with more than 150°C we will have problem with external pressure design in the vessels.

Ghasem Bashiri

[Art Montemayor]

Ghasem:

The main point I want to alert you to is that you can have the creation of a partial vacuum within a vessel undergoing atmospheric “steam-out”, EVEN IF THE VESSEL’S VENT(S) ARE OPEN.

This is not a phenomena. This is pure physics. It has happened numerous times in the past and it will continue to occur if engineers are not aware of the basics taking place.

If the vessel vents are not sized and opened properly, a sudden cooling of the vessel walls – such as occurs during a rain storm – can cause a partial vacuum to be created inside the un-insulated vessel and generate a sonic flow of incoming atmospheric air if the vents are not large enough. Undersized vents will cause the sonic flow and delay the entrance of sufficient air to break the partial vacuum, resulting in a momentary negaive pressure created inside the vessel. This effect has happened numerous times - in API storage tanks particularly. Here in Texas (where this has ocurred) the effect is called “sucking in” in a tank. The resultant tank actually looks as if it were sucked in. Agreeably, this may not apply to you or to the application you are referring to, but my point is that you have to study and know the procedures and conditions under which you are going to introduce the steam into the vessel. My advice is to not take anything for granted.

I do not recommend that a pressure vessel be cleaned with steam pressure while it is 100% sealed off unless it has been rated and inspected for this type of service. It is obvious that the vessel would have to be designed for full vacuum conditions.

You are not clear in describing what it is that you propose to do. If you are asking if you can introduce steam – under pressure – into a pressure vessel, then you must give the steam conditions inside the vessel and the vessels full specification (a Datasheet). Usually, the answer is “yes” – but you must take precautions to protect against different scenarios that can appear, such as a partial vacuum scenario. As a process engineer you are responsible for clearly describing (to a mechanical engineer who will do the vessel rating) in detail the different credible scenarios that are possible for the subject pressure vessel.

You failed to attach your pdf file. I am attaching one of my safety files that illustrates what can happen in scenarios similar to yours. Look at the rail car incident.

Attached Files

•  Unbelievable Engineering Photos.xls   1.07MB   459 downloads