TNT calculation of LPG explosion

[sumon emam]

If any LPG sphere explodes, how can be equivalent TNT calculated? Say, the capacity of sphere is 1100 m3.

[jokull]

This is a easy to use TNT calculator. It is explained how it works, therein.

[link Point to another website Only the registered members can access]

[f81aa]

Hi:

First, thanks to jokull for providing us a link to the TNT Equivalence Calculator, by The Institute of Makers of Explosives. The reader is informed that "TNT equivalence" is a normalization technique for equating properties of an explosive to TNT, the standard.

Second, sumon emam, it worries me your question "If any LPG sphere explodes, how can be equivalent TNT calculated? Say, the capacity of sphere is 1100 m3". I could not tell if you are referring to the catastrophic rupture of the sphere due to a BLEVE or any other cause, or maybe to a release of LPG that could lead to a vapor cloud explosion. In terms of the pressure effects on the surroundings, the results could be different. And the recommendations one gives might not be the best ones.

I´d like to share some information with you and other Forum members.

Many types of outcomes are possible for a release. This includes vapor cloud explosions (VCE) (Section 3.1), flash fires (Section 3.2), physical explosions (Section 3.3), boiling liquid expanding vapor explosions (BLEVE) and fireballs (Section 3.4), confined explosions (Section 3.5), and pool fires and jet fires (Section 3.6). Figure 3.1 provides a basis for logically describing accidental explosion and fire scenarios. The output of the bottom of this diagram are various incident outcomes with particular effects (e.g., vapor cloud explosion resulting in a shock wave).

My comment (f81aa): These sections and figures can be found in Consequence Analysis of Chemical Releases, published by CCPS. From this book I have extracted the information typed in this post. Links for downloading this book have been posted many times in EGPET. I have just uploaded this book and the link is:

[link Point to another website Only the registered members can access]
Vapor Cloud Explosions (VCE)

When a large amount of flammable vaporizing liquid or gas is rapidly released, a vapor cloud forms and disperses with the surrounding air. The release can occur from a storage tank, process, transport vessel, or pipeline. Figure 3.1 describes the various failure pathways under which this scenario can occur. If this cloud is ignited before the cloud is diluted below its lower flammability limit (LFL), a VCE or flash fire will occur. For CPQRA modeling the main consequence of a VCE is an overpressure that results while the main consequence of a flash fire is direct flame contact and thermal radiation. The resulting outcome, either a flash fire or a VCE depends on a number of parameters.

Four features must be present in order for a VCE to occur. First, the release material must be flammable. Second, a cloud of sufficient size must form prior to ignition, with ignition delays of from 1 to 5 min considered the most probable for generating vapor cloud explosions. Lenoir and Davenport (1992) analyzed historical data on ignition delays, and found delay times from 6 s to as long as 60 min. Third, a sufficient amount of the cloud must be within the flammable range. Fourth, sufficient confinement or turbulent mixing of "a portion of the vapor cloud" must be present.

My comment (f81aa): please pay attention to the text I just typed in bold letters. Not necessarily all 1100 m3 will go in the calculations so you could be making a mistake. It depends on the volume of your confinement.

The major difficulty presented to anyone involved in CPQRA is in selecting the proper outcomes based on the available information and determining the consequences. The consequences of concern in CPQRA studies for explosions in general are blast overpressure effects and projectile effects; for fires and fireballs the consequences of concern are thermal radiation effects. Each of these types of explosions and fires can be modeled to produce blast, projectile and/or thermal radiation effects appropriate for use in CPQRA studies and these techniques are described in the designated sections.

Some models used to determine consecuences are:

• TNT equivalency model
• TNO multi-energy model
• Modified Baker model (Baker-Strehlow method)

The TNT model is well established for high explosives, but when applied to flammable vapor clouds it requires the explosion yield, determined from past incidents. There are several physical differences between TNT detonations and VCE deflagrations that limit the theoretical validity. The TNO multi-energy method is directly correlated to incidents and has a defined efficiency term, but the user is required to specify a relative blast strength from 1 to 10. The Baker-Strehlow method uses flame speed data correlated with relative reactivity, obstacle density and geometry to replace the relative blast strength in the TNO method. Both methods produce relatively close results in examples worked.

All of the methods (except the TNT equivalency) require an estimate of the vapor concentration—this can be difficult to determine in a congested process area. The TNT equivalency model is easy to use. In the TNT approach a mass of fuel and a corresponding explosion efficiency must be selected. A weakness is the substantial physical difference between TNT detonations and VCE deflagrations. The TNO and Baker-Strehlow methods are based on interpretations of actual VCE incidents—these models require additional data on the plant geometry to determine the confinement volume. The TNO method requires an estimate of the blast strength while the Baker-Strehlow method requires an estimate of the flame speed.

Physical Explosion

When a vessel containing a pressurized gas ruptures, the resulting stored energy is released. This energy can produce a shock wave and accelerate vessel fragments. If the contents are flammable it is possible that ignition of the released gas could result in additional consequence effects.

A physical explosion relates to the catastrophic rupture of a pressurized gas filled vessel. Rupture could occur for the following reasons:

1. Failure of pressure regulating and pressure relief equipment (physical overpressurization)
2. Reduction in vessel thickness due to
a. corrosion
b. erosion
c. chemical attack
3. Reduction in vessel strength due to
a. overheating
b. material defects with subsequent development of fracture
c. chemical attack, e.g., stress corrosion kracking, pitting, embrittlement
d. fatigue induced weakening of the vessel
4. Internal runaway reaction.
5. Any other incident which results in loss of process containment.
Failure can occur at or near the operating pressure of the vessel (items 2 and 3 above), or at elevated pressure (items 1 and 4 above).

When the contents of the vessel are released both a shock wave and projectiles result. The effects are more similar to a detonation than a vapor cloud explosion (VCE). The extent of a shock wave depends on the phase of the vessel contents originally present.

There is a maximum amount of energy in a bursting vessel that can be released. This energy is allocated to the following:

• vessel stretch and tearing
• kinetic energy of fragments
• energy in shock wave
• "waste" energy (heating of surrounding air)

The relative distribution of these energy terms will change over the course of the explosion. Exactly what proportion of available energy will actually go into the production of shock waves is difficult to determine. Saville (1977) in the UK High Pressure Safety Code suggests that 80% of the available system energy becomes shock wave energy for brittle type failure. For the ejection of a major vessel section, 40% of the available system energy becomes shock wave energy. For both cases, the remainder of the energy goes to fragment kinetic energy.

In general, physical explosions from catastrophic vessel rupture will produce directional explosions. This occurs because failure usually occurs from krack propagation starting at one location. If the failure were brittle, resulting in a large number of fragments, the explosion would be less directional. However, the treatment of shock waves from this type of failure usually does not consider directionality.

Several methods relate directly to calculation of a TNT equivalent energy and use of shock wave correlations.

The main strength of these methods is that they are based mostly on experimental data. The weakness is that many of the approaches are empirical in nature, using correlations based on dimensional or dimensionless groups. Extrapolation outside of the range of the correlations provided may lead to erroneous results. For the purposes of this text, the range of validity may be assumed to be the range provided by the figures and tables. The energy of explosion methods assume that the explosion occurs from a point source, which is rarely the case in actual process equipment explosions.

BLEVE

A special case of a catastrophic rupture of a pressure vessel is a BLEVE. A boiling liquid expanding vapor explosion (BLEVE) occurs when there is a sudden loss of containment of a pressure vessel containing a superheated liquid or liquified gas. The primary cause of failure is usually an external flame impinging on the shell of a vessel above the liquid level, weakening the container and leading to sudden shell rupture. A pressure relief valve does not protect against this mode of failure, since the shell failure is likely to occur at a pressure below the set pressure of the relief system. It should be noted, however, that a BLEVE can occur due to any mechanism that results in the sudden failure of containment, including impact by an object, corrosion, manufacturing defects, internal overheating, etc. The sudden containment failure allows the superheated liquid to flash, typically increasing its volume over 200 times. This is sufficient to generate a pressure wave and fragments. If the released liquid is flammable, a fireball may result.

The calculation of BLEVE incidents is a stepwise procedure. The first step should be pressure and fragment determination, as this applies to all BLEVE incidents (whether for flammable materials or not).

Blast or pressure effects from BLEVEs are usually small, although they might be important in the near field (such as the BLEVE of a hot water heater in a room). These effects are of interest primarily for the prediction of domino effects on adjacent vessels. However, there are exceptions. Some BLEVEs of large quantities of nonflammable liquids (such as CO2) can result in energy releases of tons of TNT equivalent. The blast wave produced by a sudden release of a fluid depends on many factors (AIChE, 1994). This includes the type of fluid released, energy it can produce on expansion, rate of energy release, shape of the vessel, type of rupture, and the presence of reflecting surfaces in the surroundings. Materials below their normal boiling point cannot BLEVE.

Overpressure effects, if important, can also be obtained using any of different methods for the prediction of pressure effects.

Other books that could be useful, all published by CCPS and also posted many times in EGPET, are:

• Estimating the Flammable Mass of a Vapor Cloud
• Evaluating the Characteristics of Vapor Cloud Explosions, Flash Fires, and BLEVEs
• Wind Flow and Vapor Cloud Dispersion at Industrial and Urban Sites
• Use of Vapor Cloud Dispersion Models
• Understanding Atmospheric Dispersion of Accidental Releases
• Understanding Explosions

If download links are not valid, those books can be found in internet and by using P2P software such as eMule.

Finally, the printed version of Consequence Analysis of Chemical Releases comes with a CD containing worksheets that facilitate the calculations of the methods explained in the book. They can be downloaded from:

[link Point to another website Only the registered members can access]
Regards
P.S.: I have written krack and kracking to avoid that EGPET changes them to -----.

[sumon emam]

Dear f81aa, thanks for your kind reply. We have 2 LPG spheres and 4 Hydrogen storage vessels. I'm working with a program to evaluate any incident that will come from leakage of the mentioned gases or any explosion that may cause trouble to these vessels. When I made the question there was no good reference book to help me. But now I'm reading "Fires, Explosions, and Toxic Gas Dispersions_Effects Calculation and Risk Analysis" by Marc J. Assael and Konstantinos E. Kakosimos. It is a good book. I want to prepare effective safety measures that will ensure not to happen the catastrophic events as found in the books.

Help regarding this issue will be highly appreciated. You may share any report or preventive measures to avoid such events made by recognized refineries.

[abueno]

Hi, f81aa
All the information that you send is very useful, thanks for that, I have a question, I want to know the effects (overpressure, Therma radiation, fragments) in a BLEVE explosion of a tank of LPG (40%C4 60% C3) or mixtures of C3/C4, all the books calculate for C3 or C4, but not for mixtures, where I could found these information?

[f81aa]

Hi abueno:

I would use a software such as PHAST by DNV.

Regards

[abueno]

Hi f81aa

Thanks for the information, I will calculate with Phast

[sumon emam]

Hi f81aa,

do u have the following book:

Estimating The Flammable Mass Of A Vapor Cloud

[sumon emam]

Hi f81aa,

do u have the following book:

Estimating The Flammable Mass Of A Vapor Cloud

[abueno]

Hi f81aa:

I have read the post of sumon emam, thanks you two, I'll search this book as soon as possible

[f81aa]

Hi sumon emam:

I just uploaded it. The link is:

[link Point to another website Only the registered members can access]
Regards

[abueno]

Dear sumon emam

Thanks for your help and I downloaded it and I'll start working, I hope to have results soon