MECHANICS OF AN EXPLOSION

AN EXPLOSION has sometimes been referred to as a sudden random disassembly of an object. There is no question as to the suddenness of the occurrence - it takes place with frightening speed. The object affected, separates and is destroyed in a random pattern. The fact that the object itself, the building, equipment and people that were in the area, may also be disassembled by an explosion is well documented.

An explosion can also be a violent and noisy outburst which is caused by a build-up and release of pressure. The bursting of a boiler or pressure vessel, an explosion of a combustible gas mixture inside a building, or the detonation of a bomb, may cause virtually identical effects. A remote observer, comparing the visual effects, could not know the true cause or nature of these varied explosions.

There are several different types of explosions that can trigger vessel violence. Let's take an in-depth look at one type of explosion trigger mechanism, namely a vapor explosion, of a type that could occur in boilers or pressure vessels.

A vapor explosion occurs when a liquid phase transforms quickly into a gas phase, causing a rapid explosive pressure increase. With this type, we have a superheated liquid. The superheating may be classified into two groups; superheating under pressure, or superheating by rapid heat transfer. If there is a closed vessel containing a liquid that is heated above its boiling point, and part of the vessel breaks away due to any increase of inner pressure, the pressure will decrease for a short time until the equilibrium between the vapor pressure of the liquid, and its temperature, is out of the stable state. The liquid will still be in a superheated state and proceeds to vaporize explosively. Initiation of the explosion may he attributed to the almost instantaneous vaporization of liquid induced by the abrupt destruction of the equilibrium between the vapor and liquid phases.

A liquid that is superheated above its boiling point will remain stable only while the liquid surface boundary remain intact. When the pressure retaining boundary is ruptured, the pressurized vapor begins escaping at a very rapid rate through the crack or rupture. At the very instant when this sudden release or pressure drop occurs, the equilibrium between the vapor and liquid phases is abruptly lost and the superheated liquid tries to cool down to its boiling point. In this phase the latent heat of the liquid is released.

A rarefaction wave then travels through the vessel decreasing the temperature and pressure, while a fast gas flow is directed at the break in the pressure boundary. This in turn forms an internal compressional wave.

The internal compression wave accelerates liquid particles which impact violently on the vessel wall near the break or breech. The strength of this internal shock or hammer is determined by the size of the initial hole, the initial temperature, initial pressure and the volume of the gas phase.

Here then are the major steps of a vapor type explosion.

(See slide 1)

As the liquid in the closed vessel is heated from temperature T1 to T2 (above the liquid's boiling point) the vapor pressure rises from P1 to P2.

(See slide 2)

When the pressure retaining boundary is cracked or ruptured, the internal pressure drops quickly to P.

(See slide 3)

At the liquid temperature falls to T2, the liquid evaporates in a violent manner while creating a catastrophic shock to the vessel's pressure retaining shell. The dual liquid vapor transitional phase causes a water hammer effect on the internal surface of the vessel. Part of the escaping liquid will be in the form of liquid drops or mist particles. The combination of rapid evaporation and internal shock causes a vapor explosion of considerable magnitude.

It is important to remember that water flashing to steam expands to almost 1700 times its original volume within a few microseconds.

(Set slide 4)

The resultant shock wave created by the water hammer causes a brittle fracture of the vessel.
Check this Link for an Example of Brittle Fracture Due to Strain Rate Effect

(See slide 5)

The result is complete destruction of the vessel and creation of an external shock wave that contains potential for even greaser destruction of life. Let's look at a schematic diagram of these pressure changes in a typical vapor explosion.

(See slide 6)

Considering the time pressure diagram; point (1) represents the time required to elevate the temperature from T1 to T2 point (2) represents a short interval (milliseconds) when the internal pressure dropped due to initial leakage; point (3) represents a rapid rise to peak over-pressure; and point (4) represents the pressure drop following brittle fracture of the vessel. (5) represents complete dissipation of pressure.

Regardless of cause, all explosions have a blast wave which forms when pressurized vapor suddenly is released in a fluid medium (air or water) and a pressure spike travels outward at high speed. The speed it closely related to the velocity of sound, or about 1050 feet/second in air. This pressure build-up is so swift that it creates a sharp pulse called a shock wave. The velocity of this wave depends on the amount of excess pressure developing at the wave front, which is always higher than the speed of sound but drops to approximately the speed of sound as the shock strength falls to zero.

(See slide 7)

Much of the explosion energy is concentrated in the blast wave. The largest portion is in the form of compressional work. The balance is in the form of kinetic energy associated with the wind behind the shock. Most of the blast wave energy is transferred to successive layers of the air as the shock front advances. The frontal area then increases by the square of the distance, allowing the energy to be spread more thinly in the space involved.

(See slide 8)

As the air enters the shock front it is given strong, forward motion while being compressed. The generated wind velocity and shock pressure are closely related. The positive pressure region behind the shock front lasts only a few thousandths of a second. Traveling behind the shock front is a suction phase in which the pressure dips below atmospheric and the wind reverses direction. This may cause some minor damage. However, the majority of the destruction is attributed to the shock wave.

(See slide 9)

When the air blast wave strikes an interior building wall, the air surge is brought to a halt and piles up at the wall. The instantaneous pressure on the wall is higher than the incident shock pressure. With strong shocks, the excess pressure in the reflected wave may be as much as eight times the incident pressure.

(See slide 10)

The force of the impacting blast wave tears structural walls apart and blows large areas of the roof away before the pressure build-up can be dissipated. The amount of destruction depends on the total positive impulse, acting on a given pressure confinement area of the building under the limits of the pressure time curve.

In a water-to-steam explosion, the time cycle of one cycle in the formation of the incident shock wave may be completed in about 3 MSEC (1 MSEC 1/1000 sec) and can generate pressure in excess of 30,000 psi.

When steam power was in its infancy these were those who thought boiler explosions were caused by "witchcraft" or "black magic", not too unlike some of today's grossly misinformed nuclear power opponents. It becomes a duty and responsibility, however, of those of us in the engineering field to use our clear thinking laws of logic and the established rules of physics, to explain away the myths and let truth prevail. By improved ASME Code designs, materials and inspections by National Board Authorized Inspectors, a dramatic reduction in the number of vessel explosions has been realized since the ASME and National Board began their diligent efforts to provide maximum safety standards.

I would like to share a story with you depicting how the use of improper material can cause destructive brittle fracture explosions. It is often referred to as the longest crack ever recorded!

The setting was a 30 inch diameter gas pipeline running from New Mexico to California. The date was Good Friday, 1960. The material had a carbon content of .37 percent and was subject to brittle fracture. They were in the process of testing the line with 700 psi gas when a brittle cleavage fracture occurred at a sharp stress riser in one section of pipe. The ambient temperature was 70F. The pipeline cracked and blew out of the ground for a total distance of 8.3 miles. The crack was progressing in both directions at a speed slightly faster than the speed of sound or 1050 feet/second.

Now, that crack might have still been traveling today but fortunately it hit a bad circumferential weld and cork-screwed out. At the other end, the crack traveled until it hit an under-slung drip pot. This slowed it down allowing it to break out at the adjoining circumferential weld.

There are those who say no good can come of a bad weld. If it hadn't been for those had circumferential welds, that crack could still be running! This brings us to another interesting point; regardless of how BAD some thing may be, if we look hard enough we can find some GOOD, Now I will admit it's hard to find something good about the total destruction of 8.3 miles of 30 inch pipeline, but no one was killed or injured, and because it was in a remote desert region, the only property loss was to the pipeline itself. There were other good side effects, also. It was reported that the explosion killed five rattlesnakes and numerous desert mice, however, I think you will agree, these are easier, more efficient ways to kill snakes and rodents!

As you can see, the mechanics of an explosion is not governed by "witch-craft" or "black magic" but instead always occur in conformance to definite laws of physics. Now that we understand more about the basics of an explosion, we should be in a better position to control and avert these destructive occurrences.

Remember, safety is no accident!