How Powerful? When Vessels Explode

“The United States is like a gigantic boiler. Once a fire is lit under it, there’s no limit to the power it can generate.”  — Winston Churchill

When it comes to process safety, we worry about three things: fires, explosions, and toxic releases. When it comes to worst case scenarios, the EPA requires that we worry about one of two things: toxic releases or explosions. When the EPA talks about explosions, it specifically means vapor cloud explosions—the explosions that result from a cloud of flammable vapor igniting.

But “explosion” is a pretty loose term. Process safety professionals usually accept that the definition of explosion is “an almost instantaneous expansion of volume of a given amount of matter, usually with the generation of high temperature and the release of high-pressure gases.” That phrase, “usually with the generation of high temperature”, reveals our bias for thinking of explosions resulting from combustion or decomposition reaction.

But not all explosions result from combustion or other decomposition reactions.

In fact, vessels containing only water, which is not flammable at all, can explode. There are enough images of boiler explosions, especially from the early days of the industrial revolution, to drive that fact home. So, how do we estimate the energy of these explosions? It’s not as though we can use an explosive yield factor with a heat of combustion or heat of reaction.

Mechanisms for Mechanical Explosions

To state the obvious, vessels explode when the pressure inside the vessel exceeds the pressure that the vessel can withstand. It doesn’t matter if the issue is that the pressure is abnormally high or if the vessel has deteriorated so that it can no longer withstand normal pressures. The vessel experiences a pressure high enough to cause it to fail and release the material contained within. This material, no longer contained, expands almost instantaneously to atmospheric pressure. That instantaneous expansion is the explosion.

Mechanical explosions occur when the pressure increase is not the result of combustion or other decomposition reaction. What can cause a mechanical increase in pressure? Generally, there are three types of causes.

The first type of cause is sometimes referred to as a hydrostatic explosion. The vessel is liquid full and pressurized liquid exceeds the vessel’s pressure limit, resulting in a catastrophic failure. While this type of explosion may result from heating, it does not require heating. It only requires a pressure source, like a pump—especially a positive displacement pump.

The second type of cause is often referred to as a pressure vessel explosion. This results when the vessel contains a gas or vapor, the pressure of which exceeds the vessel’s pressure limit. Again, this type of explosion may result from heating, especially as the heating raises the vapor pressure of a contained liquid or simply causes a gas to expand (the ideal gas law says that the pressure of a gas goes up in proportion to the temperature). However, an external high-pressure source may be the cause of the increased pressure.

The third type of cause is the boiling liquid expanding vapor explosion—the BLEVE. In a BLEVE, a liquid is heated to well above its normal boiling point, but remains liquid because it is contained under pressure. Then, a vessel failure releases the pressure that was holding the material as liquid and the liquid flashes off, almost instantly. The flashed vapor, at atmospheric pressure, has much, much more volume than the liquid from which it flashed. This “almost instantaneous expansion of volume of a given amount of matter” is the explosion. A BLEVE requires no flammable material. However, if the material is flammable, the resulting flammable vapor cloud is likely to ignite and result in a subsequent, more powerful, vapor cloud explosion.

Energy from Overpressure via Pressurized Liquid

Of the three types of causes, overpressure via pressurized liquid is the least powerful. That is why testing pressure vessels is done with pressurized water rather than pressurized air. The energy that a hydrostatic explosion releases is typically not from the liquid, which is essentially incompressible and so does not expand, but from the release of the elastic strain energy on the vessel itself. It is enough to shatter the vessel and throw high-velocity fragments in the vicinity.

The energy released by the exploding vessel from pressurized liquid is not a function of the liquid. Rather, it is a function of the vessel’s dimensions and material of construction.

Energy from Overpressure via Pressurized Gas

Overpressure via pressurized gas is much worse. The pressurized gas expands, almost instantaneously, which is the very definition of an explosion. The energy released in such an explosion depends on the volume of gas in the vessel (so just the headspace if the vessel is partially filled with liquid) and the burst pressure (P_burst) of the vessel. Typically, it is fair to assume that the pressure at which a vessel bursts is four times the maximum allowable working pressure (MAWP) of the vessel.

Crowl developed this equation, for calculating the energy released when pressurized gas causes a vessel to explode, based on thermodynamic availability:

E = P_burstV [ ln(P_burst/P_atm) – (1 – P_atm/P_burst) ]

Using Crowl’s equation requires that pressures be in common units of absolute pressure, and requires the use of an appropriate conversion factor to give the desired units of energy.

Energy from BLEVEs

A BLEVE occurs when a vessel containing liquid above its boiling point is suddenly open to atmosphere. This doesn’t have to be because internal pressure caused the vessel to fail. It could result, for example, from an improperly bolted entryway failing, opening the hatch. With nothing holding the liquid back, it flashes. Not completely, but enough to do damage.

A Comparison

While it is not possible in this blog to quantify the energy from an explosion due to pressurized liquid, it is possible to compare the energy released during an overpressure via pressurized gas and the energy released during a BLEVE. For the sake of comparison, let’s consider a 10 cubic foot vessel with an MAWP of 100 psig that is half full of water.

For the overpressure case, let’s assume that the vessel is inadvertently exposed to high pressure steam, so that the vessel bursts at 400 psig (414.7 psia).

It is only the gas volume—5 cubic feet—that matters. These are the values to plug into the thermodynamic availability equation:

V = 5 ft³
P_burst = 414.7 psia
P_atm = 14.7 psia

The result is 4925 psi-ft3. The conversion factor is 1 psi-ft³ = 0.18508 BTU, so the energy released by this explosion is 911 BTU, which is the equivalent to an explosion of about 0.46 pounds of TNT or about one stick of dynamite.

For the BLEVE case, let’s assume that the 10 cubic foot vessel is once again half full of liquid water, but that it is operating at 338°F, so that the steam pressure is 100 psig, the MAWP. A look at steam tables shows that there are 280 pounds of liquid water in the vessel and 1.3 pounds of steam in the headspace. The enthalpy of the liquid, at 309 BTU/lb, is 86,520 BTU, and the enthalpy of the steam, at 1190 BTU/lb, is 1547 BTU. The total initial enthalpy in the vessel, then, is 88,067 BTUs.

Now let’s assume that the BLEVE is the result of a hatch failing open, causing a sudden vaporization of the now superheated liquid. Not all of the liquid will vaporize—it will stop vaporizing once the liquid is cooled to the normal boiling point. At the normal boiling point of water—212°F—the heat of the liquid is 180.17 BTU/lb and the heat of the steam is 1150.5 BTU/lb.

The total enthalpy will still be 88,067 BTUs. An energy balance shows that the total amount of steam released in the BLEVE is 38.6 pounds and the energy released is 44,411 BTUs. This is the equivalent to an explosion of about 22 pounds of TNT or about 50 sticks of dynamite.

Even If It’s Just Water

In the process industries, any explosion, whether a chemical explosion resulting from combustion or decomposition, or a mechanical explosion resulting from hydrostatic pressure, gas pressure, or a BLEVE, is bad. Nothing is as bad as it might be, however, if we are prepared for it. One way to prepare for an explosion is to ensure that preventative measures are in place to reduce the likelihood of the explosion. Another, equally important way, is to install mitigating measures that reduce the impact of the explosion.

To reduce the impact of any catastrophe, however, we need to know what the impact is. We hope that this discussion has been helpful in understanding the impact of mechanical explosion, even if it’s just water.