15 replies to this topic

[Sevail]

Dear All

in some towers or vessel normall pressure operation for example 5 bar but in data sheet addition of upper pressure as a design pressure we consider full Vacuum as a design pressure for tower or vessel.
i want to know based on which condition we consider full vaccum for it.

Regards

[gvdlans]

Dear All

in some towers or vessel normall pressure operation for example 5 bar but in data sheet addition of upper pressure as a design pressure we consider full Vacuum as a design pressure for tower or vessel.
i want to know based on which condition we consider full vaccum for it.

Regards

For example: steam-out of vessel and subsequent cooling of the vessel in isolated state. The steam will condense resulting in a vacuum. Also draining of the vessel e.g. after hydrotest without opening a vent will result in vacuum. Things like these did happen in the real world...

[fallah]

I don't think the vessels would be subjected to steam-out process in isolated state.Secondly,for resulting in a vacuum due to sudden condensation of steam during steam-out, it is not necessary having isolated state.Am i wrong?

[gvdlans]

When dealing with safety you consider things that can go wrong, such as operators forgetting to provide sufficient inbreathing possibilities. These scenarios are quite often considered during PHA studies such as HAZOPs.

I am not an expert in steam-out operations. Maybe someone else can comment on the practical issues.

[Zauberberg]

I would agree with Guido. Actually, my first experience within petroleum refinery had started by witnessing vessel collapse due to vacuum conditions created during equipment drainage. The operator has forgotten to open the valve on the vent line, and there was no additional protection against vacuum. The scene was very impressive: 5m tall vessel collapsed in just a few seconds.

Steaming is usually done by having vent line valves completely closed or just slightly open (that is my field experience). In such case, it is possible for vacuum to be created during steam-out conditions, especially if manual (operator) intervention is required to prevent excessive vacuum. Another occasion when excessive vaccum can be generated inside the vessel, is after equipment hydrostatic test: if subsequent drainage operation is not properly attended and monitored, it can easily happen that vessel will collapse unless it has been designed for full vacuum. In one refinery, the entire De-C4 tower was ruined because of operator mistake after the hydrostatic test has been finished - he opened the drain lines, but vent line remained closed. And the tower ended up like crushed can of beer, or Coke, whatever one prefers the most.

[Art Montemayor]

Guido & others:

You are absolutely correct in your first and second responses.

Not only is a steam-out operation done in an "isolated" state (this is normally done with the subject vessel isolated from the rest of the process and readied for total clean out and drying out prior to vessel entry. This is a normal requirement when a vessel is to be inspected and/or repaired. I do not understand Fallah's statement.

If anyone is interested is seeing the actual, real results of what can happen due to what Guido has described as a possibility, I can look for and post some of the photographs I have collected through the years of such results. I have also been present in some incidents where tanks have collapsed due to steam-out --- for the very reason(s) that Guido has explained.

[gvdlans]

Art,

I am definitely interested in those pictures! I may use them to "liven up" my HAZOP studies...

Similar stories are also described (incl. pictures) in the book "Chemical Process Safety", by Roy E. Sanders, chapter 3 "Focusing on water and steam: the ever-present and sometimes evil twins".

Guido

[Zauberberg]

Art,

Please upload those photos. I think the awareness of potential hazards and their consequences becomes much more stronger when it is presented in visual form.

If the files are too large for upload on the ChE forum, you can use some of the following online storage facilities:

http://www.engineeri...32/Default.aspx
http://rapidshare.com/

[fallah]

Guido & others:

You are absolutely correct in your first and second responses.

Not only is a steam-out operation done in an "isolated" state (this is normally done with the subject vessel isolated from the rest of the process and readied for total clean out and drying out prior to vessel entry. This is a normal requirement when a vessel is to be inspected and/or repaired. I do not understand Fallah's statement.

If anyone is interested is seeing the actual, real results of what can happen due to what Guido has described as a possibility, I can look for and post some of the photographs I have collected through the years of such results. I have also been present in some incidents where tanks have collapsed due to steam-out --- for the very reason(s) that Guido has explained.

Dear Art,
I accept your statement as:this is normally done with the subject vessel isolated from the rest of the process ,but as per Guido (operators forgetting to provide sufficient inbreathing possibilities) and Zauberberg (having vent line valves completely closed or just slightly open ) statements,i mean in steam-out operation fully isolation of subject vessel e.g. from atmosphere may not be mandatory.Of course,i accept that operation may usually be done in an isolated state even against atmosphere.Also i believe even with having inbreathing possibilities,the rate of condensation may be so higher than inbreathing that causes the subject vessel to be collapsed.
Regards

[JoeWong]

Dear all,

This is one of the struggle that i always have during HAZOP...

Do we design our vessel for vacuum due to

i) vessel subject to steam-out during maintenance
ii) vessel subject to water-wash (to avoid pyrophoric fire) during maintenance
iii) vessel subject to hydraulic test during construction period


I would think system shutdown, steam-out & water-wash prior to maintenance is part of the operator activities (although it is not frequent) and it is repeated. Inadvertently maloperation of operator is remain VALID. Thus SHOULD consider vacuum condition.

However, hydraulic test is just once in the life time and it is special event in case of re-hydraulic test, thus NEED NOT consider vacuum condition

See one of the photo here.


Sevail,
Back to original query, present of FREE WATER vapor (above ambient temperature) during normal operation, there is potential of plant shutdown and vessel subject to natural cooling. FREE WATER vapor collapse would lead to VACUUM condition in the vessel. Then the vessel shall be designed for VACUUM condition.

[Zauberberg]

Fallah,

I believe Art was refering to isolation from the rest of process equipment/system (which is not submitted to steaming), and not to isolation from atmosphere during steaming procedure. Imagine if the entire section is 100% closed/isolated, what would be the purpose of steaming? There would be no steam flow into the system once when pressure inside the vessel equalizes with the steam supply pressure.

[djack77494]

I also had the experience of standing besides an atmospheric tank that collapsed due to vacuum formation inside the tank, and I can testify that it is an impressive event. Even for pressure vessels, just because a vessel may be capable of withstanding a considerable positive pressure, that does not mean that it can withstand a much smaller negative pressure. Also, I think the scenario of draining a vessel is very much underrated. Everyone seems to be very aware of the potential for steamout operations to result in vacuum. Allowing a full vessel that does not have an open vent valve has very similar effects. Significant vacuum can be generated with potentially disasterous results.

[Qalander (Chem)]

Dears,

It is true that if for any reason Storage tank(s) are subjected to negative pressure (vacuum) collapse results;
Damages are subject to the extent of vacuum and the time period of exposure or responsive action to prevent.

I witnessed one Asphalt tank 40 ft diameter and equipped with free vent, bed steam heating coil, bed air aspirer for mixing and insulated.
During pump out (transfer) the free-vent & its bird protective mesh got clogged; thus negative pressure (vacuum).
Only shell top course and the roof were deformed partly as per early intervention.

Regards
Qalander

[Art Montemayor]

Guido / Zauber / all other interested engineers:

I am attaching one of the files I found in one of my hard drives that depicts and describes incidents involving storage tanks and one railcar (here in Houston).

As part of the WinZip package, I include photos of one of the most tragic and totally unacceptable mistakes that I can recall - the Motiva Sulfuric Tank tragedy that caused the unnecessary death of some workers.

I keep and collect these photos as reminders to myself of how terribly wrong engineering projects and assignments can go when one is not vigilant and always aware of what can and will happen if one does not do the absolutely correct procedure and action.

 Tank_Photos_of_Incidents.zip   828.4KB   130 downloads

[Qalander (Chem)]

Dear Art Thanks and Best Regards

Consequential events are explicity displayed for many of the usual apparently non-hazardous scenarios; some of the pictures were really hair raising indeed!
qalander

[djack77494]

Thank you Art for the great photos and explanations. I thought the railcar implosion was especially telling.