17 replies to this topic

[jprocess]

Dear All,
The term "BLEVE" is the abbreviation of Boiling Liquid Expansion Vapor Explosion.
For me it is hard to interpret this term.I have not any sense about the concept of BLEVE and when it is probable?But I only heard about it that is a scenario of failure for LPG storage tanks and should be avoided by means of depressuring devices or other methods.
And if we decide to provide a blowdown valve according to API 521 guidelines,the 15 min. depressuring time is low,good enough or high to protect the vessel against BLEVE?
Your valuable comments are appreciated.
Cheers.

[pleckner]

The danger is not during a controlled release through a pipeline to say a flare header. The danger is a rupture at the worst or a small leak at best from a vessel to the surrounding environment (atmosphere). It is here that the liquid will quickly evaporate, form a flammable cloud and if there is an ignition source nearby, ignite with catastrophic results.

[Art Montemayor]

Jprocess:

The subject of a BLEVE is very important from the point of view that it represents a unique and special type of safety hazard to human beings. It is just as Phil has described it and it encompasses the realm of what I call compressed, liquefied gases. It is in this atmosphere – or application – that the threat of a BLEVE becomes an on-going concern – specifically with respect to hydrocarbon liquefied gasses such as the LPGs, propylene, butylenes, cyclohexane, etc.

What concerns me a lot is for engineers – particularly young or recent grads – to undertake an interpretation of how to design around a perceived term or event without having direct or personal experience in doing so. The BLEVE has little or no relationship to a controlled API depressurization design or to a controlled release. The main factor to bear in mind is that the sinister and stealth power of a BLEVE lies in the release of LIQUID product that quickly (if not instantaneously) boils or evaporates into the vapor phase and combines with the surrounding atmosphere to form a combustible and explosive mixture. In the majority of hydrocarbon gases, these mixtures result as having a specific gravity greater than 1.0 – i.e., they are heavier than air and, consequently, cling to the ground surface and seek out low spots. This latter property is a sinister one in that it will “seek” out ignition sources and detonate with – as Phil so correctly stated – “catastrophic results”. This is bad stuff and bad news for any chemical engineer and fellow workers in a process plant environment.

Please read the attached .pdf file that I’ve attached regarding a tragic incident that I’m very familiar with and that happened 20 miles away from where I was working on Oct 23, 1989. The authors of this paper don’t deal with the engineering specifics as well as I would have wanted. I happen to know some of the details on the release and the subsequent cause. The release was a BLEVE – it involved liquefied hydrocarbon gases that were released as a vaporizing, heavier-than-air mixture and found a near-by ground-level boiler where ignition immediately took place.

Read the simple and incredibly foolish reason the release took place. Learn the importance of knowing and mastering the methods, mechanics, connections, and implications involved in selecting, designing, operating, and maintaining process equipment and its related instrumentation. A block valve was used to block off the deadly contents of a reactor while contractor maintenance personnel were working in the area. These are deadly ingredients for a BLEVE.

I hope this helps you to understand what a BLEVE is and what it represents to you and to those under your care and supervision.

 Phillips66_Explosion_10_23_89.pdf   174.74KB   213 downloads

[fallah]

Dear All

In a double wall LPG(propane) tank:
Inner wall: steel top open
Outer wall: concrete with dome roof
T=­ 42˚C
P=1 bara
Max. Ambient Temp.=48˚C
Can we say:Because the tank is atmospheric,the BLEVE phenomena will not occur at all? If so,which are the related references?

Regards

[fallah]

Dear All

In a double wall LPG(propane) tank:
Inner wall: steel top open
Outer wall: concrete with dome roof
T=­ 42˚C
P=1 bara
Max. Ambient Temp.=48˚C
Can we say:Because the tank is atmospheric,the BLEVE phenomena will not occur at all? If so,which are the related references?

Regards

With the correction of T=-42 C i am still waiting for your kind reply.

Best Regards

[Art Montemayor]

Fallah:

To answer your direct question: No, you cannot state that a BLEVE will not occur at all with respect to your type of tank and its construction characteristics. A BLEVE is not necessarily related to the type of tank or its construction. A BLEVE can occur due to a variety of reasons, accidents, failures, or simply bad operation. Please read the accident report that I attached to my post and Phil Leckner’s description of what constitutes a BLEVE.

I get the impression that you are inferring that since your tank is essentially at atmospheric pressure, it is safer than a pressurized LPG tank. If that is what you are thinking, then I believe that is wrong. In fact, I am of the opinion that you have a different and greater jeopardy in an atmospheric cryogenic or semi-cryogenic storage tank than you would in a pressurized tank. A line or nozzle rupture would create a catastrophic spill and BLEVE almost instantly in your setup - one that would generate a huge, enveloping cloud of combustible vapor that is much heavier than air.

We know absolutely nothing about the construction details of your tank, where it is located (proximity to infrastructure, humans, etc), its accessibility, or how it is piped and operated. Therefore, we can only speculate. You are in a better position to know the specific details and should know the relative dangers in a spill, a rupture, or a process upset. But the facts still support the capability of having a BLEVE. It’s just a matter of how well your system is designed and how well it is operated and maintained.

I have no idea what you mean by “the related references”. Can you elaborate?

I hope this helps you in answering your question.

[gvdlans]

Atmospheric (=refrigerated) storage of propane is generally considered more "inherently safe" than pressurized storage. In case of loss of containment you will get a boiling pool of propane, rate of vaporization is limited by heat transfer rate from the surroundings to the pool. Indeed you get a flammable vapour cloud that is heavier than air (not just because of the low temperatures, but because has a higher molecular weight than air...).

I do not agree with Art that a BLEVE can occur in this type of tank. A BLEVE refers to the rapid (explosive...) boiling of a liquid. This will occur when a pressurized propane storage tank ruptures but will not occur when an atmospheric storage tank ruptures.

There is a section in the book "Inherently Safer Chemical Processes" from the Center for Chemical Process Safety (CCPS) where refrigerated and pressurized storage are compared. Following are some quotes from that book:

"Material stored at or below its atmospheric pressure boiling point has no superheat. Therefore there will be no initial flash of liquid to vapor in case of a leak. Vaporization will be controlled by the evaporation rate from the pool formed by the leak. [...] Because the spilled material is cold, vaporization from the pool will be further reduced."

"Marshal et al (1995) provide a series of case studies which evaluate the benefits of refrigerated storage for six materials - ammonia, butadiene, chlorine ethylene oxide, propylene oxide, and vinyl chloride. They conclude that "refrigerated storage is generally safer than pressurized storage" for all the chemicals studied except ammonia. Ammonia was reported to be an exception "due to a density shift with temperature making it heavier than the surrounding air.""

Note that propane is also heavier than air at ambient temperatures because of it's molecular weight (44 g/mol for propane vs 29 g/mol for air).

Don't get me wrong, even though atmospheric storage can be considered more inherently safe than pressurized storage, it still remains a hazard, meaning that it can hurt people, the environment or damage properties...

[fallah]

Fallah:

To answer your direct question: No, you cannot state that a BLEVE will not occur at all with respect to your type of tank and its construction characteristics. A BLEVE is not necessarily related to the type of tank or its construction. A BLEVE can occur due to a variety of reasons, accidents, failures, or simply bad operation. Please read the accident report that I attached to my post and Phil Leckner’s description of what constitutes a BLEVE.

I get the impression that you are inferring that since your tank is essentially at atmospheric pressure, it is safer than a pressurized LPG tank. If that is what you are thinking, then I believe that is wrong. In fact, I am of the opinion that you have a different and greater jeopardy in an atmospheric cryogenic or semi-cryogenic storage tank than you would in a pressurized tank. A line or nozzle rupture would create a catastrophic spill and BLEVE almost instantly in your setup - one that would generate a huge, enveloping cloud of combustible vapor that is much heavier than air.

We know absolutely nothing about the construction details of your tank, where it is located (proximity to infrastructure, humans, etc), its accessibility, or how it is piped and operated. Therefore, we can only speculate. You are in a better position to know the specific details and should know the relative dangers in a spill, a rupture, or a process upset. But the facts still support the capability of having a BLEVE. It’s just a matter of how well your system is designed and how well it is operated and maintained.

I have no idea what you mean by “the related references”. Can you elaborate?

I hope this helps you in answering your question.

Dear Art

We are still waiting for your viewpoint regarding gvdlans facts.
Regards

[dailey]

I am going to answer in general about BLEVES. A BLEVE generally is caused by fire impinging upon non-cooled metal. The impingement of flame on bare metal reduces its strength, which results in metal fatigue and resultant failure at or in the general area of the flame impingement.

Providing the vessel or equipment maintains its position, so that the relief devices are in the vapor portion, the relief devices should be sized for fire in relieving the pressure generated by the fire. Fire impinging upon metal that can be cooled, such as vaporizing LPG within the equipment, should not be of great concern for the short duration, because the vaporizing LPG will cool the metal it is in contact with. Remember, as the liquid is vaporized, the level will decrease to a point at which the flame will impinge upon bare non-cooled metal. Resultant is covered in paragraph above.

If the equipment is tipped, so that the relief devices are under liquid, the pressure will build up to a rupture point, as the relief devices when sized were not designed to relieve the volumetric liquid volume of the resultant vapor generated in the equipment due to the fire.

There are several very good articles and pictures on BLEVE’s that can be "Google".

[pleckner]

@dailey,

What you describe is typcially not what we refer to as a BELEVE. As I state in my response above, a BELEVE is usually due to a rupture at the worst or a small leak at best from a vessel to the surrounding environment (atmosphere). It is here that the liquid originally under some pressure will depressure, quickly evaporate and form a flammable cloud and if there is an ignition source nearby, ignite with catastrophic results. The release is to the atomosphere, not to a relief header.

[djack77494]

I think the definition of BLEVE is a bit wider in that it would also encompass a subcooled liquid (or a liquified gas). In the event of a loss of containment, the liquid could be rapidly released and contacted with (much) hotter surfaces, resulting in the rapid generation of a large vapor cloud. Would I be mistaken in believing that this fits within the definition of BLEVE?

[Art Montemayor]

Doug:

What you state is exactly what I think I have tried to explain. Compressed liquefied gases are the majority of BLEVE culprits, in my opinion. If I have failed in explaining myself, I apologize.

But I certainly agree with you and fully backup what Phil is also saying. The danger is in the SPILL (or leak) of such a quickly vaporizable liquid - not only the fact that these substances are under pressure and can quickly overpressure their vessels in case of a fire. A nozzle or attached pipe rupture will present the same scenario as a vessel rupture.

Any vessel or tank containing these type of substances is a potential hazard if not designed, treated, maintained, and operated with diligent care and attention. Fire is only one top hazard. A mechanical rupture, a failed gasket, a blocked-in 100% filled pipe subject to thermal expansion, etc. etc., are other events to be fearful of.

[gvdlans]

I think the definition of BLEVE is a bit wider in that it would also encompass a subcooled liquid (or a liquified gas). In the event of a loss of containment, the liquid could be rapidly released and contacted with (much) hotter surfaces, resulting in the rapid generation of a large vapor cloud. Would I be mistaken in believing that this fits within the definition of BLEVE?

I agree that it comes to a matter of definition, but I do not agree that this fits within the definition of a BLEVE. If for example refrigerated propane having a temperature of -42 degrees Celsius is released and comes into contact with say a concrete slab of +40 degrees Celsius this will result in rapid cooling of the concrete to -42 degrees Celsius and indeed some vaporization of propane and the formation of a vapour cloud. This vaporization is not explosive, as it is in an Boiling Liquid Expanding Vapour Explosion. If the vapour cloud is subsequently ignited and there is sufficient confinement and/or congestion there will be a Vapour Cloud Explosion (VCE) which is different from a BLEVE. A BLEVE is a physical explosion where a VCE is a chemical explosion.

There will not be an explosive vaporization of propane since the heat of vaporization of propane is much larger than the sensible heat involved with the cooling of the concrete.

Again, I am not saying there are no dangers involved with refrigerated propane storage, including the abovementioned scenario resulting in a VCE.

If you think that the definition of a BLEVE is wider than what I describe, may I suggest you to correct the article that is currently on Wkipedia (http://en.wikipedia.org/wiki/BLEVE) and that reads a.o.:

"[BLEVE] is a type of explosion that can occur when a vessel containing a pressurized liquid is ruptured. "

[gvdlans]

I went through the details of the Phillips 1989 explosion in Houston TX, not only the document included in Art's response but also the following report: http://ncsp.tamu.edu.../phillips66.htm
My conclusion is that at least the main explosion cannot be classified as a BLEVE.

The accident followed the following scenario: it started with a release of extremely flammable process gases that occurred during regular maintenance operations on one of the plant's polyethylene reactors. More than 85,000 pounds of highly flammable gases were released through an open valve. A vapor cloud formed and traveled rapidly through the polyethylene plant. Within 90 to 120 seconds, the vapor cloud came into contact with an ignition source and exploded with the force of 2.4 tons of TNT.

Even though the accident involved the rapid vaporization of liquefied gases (mainly ethylene), the vaporization itself was not explosive and therefore cannot be called a Boiling Liquid Expanding Vapour Explosion. It is stated that the 85,000 pounds of flammable gases were released in a matter of seconds, but there was no overpressure formed by the release itself. The 23 fatalities (and asset damage) were the result of the Vapour Cloud Explosion (VCE) that occurred when the formed flammable vapour cloud ignited, 90 to 120 seconds after the release.

The report mentions two other explosions that followed the main explosion: the explosion of two isobutane storage tanks 10-15 minutes after the main explosion and another when a polyethylene reactor failed catastrophically 25-45 minutes into the incident. These may well have been BLEVEs but I cannot conclude that from the available descriptions.

As this accident (and many others, e.g. Flixborough, Beek, Texas City Refinery (2005)) clearly illustrates, a VCE can be at least as destructive as a BLEVE.

[afd]

BLEVE refers to the explosion caused by the rupture of a tank holding Hydrocarbons in liquid form by pressure. It occurs when the pressure inside the container increases and cannot be held by the container and it ruptures. The contents are voilently released and the material being flammable, an explosion or large fireball occurs. The pressure rise in the container is basically due to a nearby fire.
In 1984 PEMEX LPG terminal fire in Mexico city had BLEVES in which 500 people perished.

If the material is non flammable also there can be rupture due to increase in pressure then the incident is called VTR or voilent tank rupture.
Most of the gases are heavier than air and settle down and feed the flame
There are only 10 gases that rise in air The neumonic used to remember them is DAMN 3H CAE (damn 3H cows ate everything) Gases are Diborbane,Acetylene,Methane,Nitrogen,Neon,Hydrogen,Helium,Hydrogen CYanide,Carbon monoxide,Ammonia,Ethylene.

I am attaching a document I wrote on BLEVE for an assignment.

HTH

afd

Attached Files

•  unit_2_Q1.doc   23.5KB   74 downloads

[gvdlans]

afd,

What you describe in your post is called a Hot BLEVE, a BLEVE caused by an external fire. You can also have a Cold BLEVE, when a tank ruptures as the result of a mechanical impact. Following text is taken from this report: http://www.safetyboa...lorry_fires.pdf

In the transport of liquefied gases, a BLEVE can have two causes:
1. Fire or flames that come into contact w ith the tank. This will heat up the contents of the tank and increase the pressure (according to the vapour/liquid equilibrium). At the same time, the rising temperature may reduce the strength of the tank in places. The combination of increased pressure and (locally) reduced strength will eventually cause the wall to rupture. No details about this process are known.
2. Mechanical impact (for example, from a collision) causing the tank wall to rupture. The pressure at which the liquid is released may be lower than in the case of a fire.
To distinguish between these causes, the first is sometimes referred to as a hot and the second as a cold BLEVE.

What are the consequences of a BLEVE?

With regard to a BLEVE, three mechanisms are distinguished that can cause damage and injury:
1. First of all, the shock wave, which causes damage in particular near the source (physical explosion).
2. In the case of fire and flammable substances, this is followed by a fireball. The fireball is the decisive cause of damage, and produces a combination of heat radiation and oxygen depletion, across a considerable distance (depending on the size), which, especially in tunnels, is fatal to human beings.
3. The third mechanism is the violent fragmentation of the pressure tank. The fragments can be hurled away across substantial distances.

The Vessel shall equipment with multiple large PSV already, why you think blow down can help you?

BLEVE can be cause by the rupture of vessel or external fire.

We use LPG deluge system to protect LPG vessel from go BLEVE by fire

[hollerg]

Blowdown at a lower pressure and additional flow area place the vessel at a lower pressure state reducing the likelihood and damage associated with wall failure and is discussed by API documents