Water heater BLEVE Explosion - Daniel Friedman BLEVEs Boiling Liquid Expansion Vapor Explosions

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Boiler & water heater tank explosion cause & prevention:

BLEVES This article discusses and defines BLEVE or boiling liquid expansion vapor explosions. We discuss the cause and prevention of explosions of heating boilers and hot water storage tanks, and the role of pressure / temperature relief valves on that equipment.

We explain how scale in a boiler or water heater can lead to failure or even an explosion, and we describe other conditions or mistakes that can cause a BLEVE explosion.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

What is a BLEVE or Boiling Liquid Expanding Vapor Explosion?

Insulated over relief valve (C) Daniel Friedman

Definition of BLEVE - Boiling Liquid Expaning Vapor Explosion:

A BLEVE is a sudden, explosive release of energy that occurs when a closed container (like a water heater tank) containing heated, pressurized and superheated liquid (water) suddenly ruptures.

The state change from water to steam releases an explosive amount energy to the surroundings.

Article Contents

Our page top photo of a 30-gallon water heater tank exploding through the roof of a test structure was snapped from a "Water heater rocket myth" a Mythbusters YouTube video (originally aired on the Discovery Channel). [Click to enlarge any image]

Detailed Definition of BLEVE, Energy Content

A BLEVE results from the sudden loss of containment of a liquid heated above its normal boiling point. The loss of containment is usually the result of a catastrophic failure of the container or vessel holding the superheated liquid. There are two contributors to the BLEVE blast wave:

(1) the compressed vapor in the container head space and

(2) the vapor flashing from the superheated liquid. The magnitude of the blast also depends on the degree of superheat, that is, the temperature increment above the normal boiling point.

As the degree of superheat increases, the fraction of liquid that flashes increases, thus increasing the severity of the blast.
  - Ogle 2012

Explanation of BLEVEs: The actual amount of energy released in a BLEVE depends on several variables of which two critical data are the pressure and temperature of water inside the container - in our case a water heater or boiler.

BLEVE energy also varies depending on the liquid that is being superheated. The energy release from a liquid propane tank BLEVE will be different from a water heater tank BLEVE.

Water in an open container will boil at 212°F (100°C or 373.2 K). But when we enclose and heat water in a water heater tank, cylinder, calorifier or similar appliance such as a heating boiler, increased pressure causes the water to become superheated

. If a closed container of superheated water ruptures, the superheated water changes state: it flashes suddenly from water to steam. This state change of water from liquid to vapor (steam) releases a tremendous amount of energy.

Definition of superheated water & latent energy: Water is considered superheated when its temperature is above 212°F (100°C). Superheated water contains latent energy that will be released if the pressure is suddenly released. It is this latent energy from superheated water that causes a Boiling Liquid Expansion Vapor Explosion or BLEVE).

Energy Release from a BLEVE

Just how much energy is released during a BLEVE explains why these explosions are so serious - so catastrophic when they occur.

gle (2012) points out that a number of investigators have described varying methods to calculate the explosive energy of a BLEVE, complicated further by a varying set of starting assumptions of temperature, pressure, head space of vapor, volume of flashed vapor, and more parameters that would give a plumber or homeowner a headache.

We offer some examples of possible energy released by a water heater BLEVE.

Watts gives this example of the amount of energy released in a water heater BLEVE explosion:

"... where water supply pressure to the water heater is 90psi, the boiling point is not reached until 331.2°F.

At only 50 psi, water flashes into steam and boils at 297°F.

This energy, if released by a water heater rupture, equals more than one pound of dynamite." - Watts (2011)

Watts and other experts and companies also point out that

When you heat water in a closed container, the thermal expansion pressure equals approximately 2 1⁄2% of volume for every 100°F rise.

- source: 52 Questions and their Answers [about] Hot Water [Heaters, Explosions & Water Heater Safety], [PDF] Watts Regulator Company (1973) retrived anew 2018/10/27, original source:

Tricks with Arithematic Explain Energy Released when Hot Water Flashes to Steam with an energy release equivalent to about 184 pounds of TNT

  1. Physicists explain that a phase change (aka state change) of water at 100 C to steam at 100 C will release 540 calories of energy per gram of water.
  2. One gallon of fresh water = about 3,785 grams of water. So if I've got this right, a 40 gallon water heater contains about 151,400 grams of water.
  3. And (151,400 grams of water x 540 calories/gram of water at 100 degC) = 81,756,000 calories of energy that will be released if that water flashes from water to steam
  4. 1 Calorie = about 1 gram of TNT in energy so we've got 81,756,000 grams of TNT or converting that to pounds of TNT
  5. We have (81,756,000 grams) x 0.00220462 pounds per gram = about 184 pounds of TNT !

    Really? Our estimate of 184 pounds of TNT as the explosive force of a 40 gallon water heater (or depending on where you live a calorifier or cylinder) needs further study. But indeed, 184 pounds of TNT = 0.092 tons (U.S.) of TNT or 384,928,000 Joules (J) of mechanical and thermal energy.

    Osprey, one of our reviewing physicists, estimated that 368,000,000 Joules of energy would be released by our 40 gallon water claorifier BLEVE. He calculated that energy as 0.09 US Tons of TNT, confirming, unless I've missed a decimal point, our original estimate.

Regardless of starting assumptions, all estimates of a water heater BLEVE show a very large amount of thermo-mechanical energy released in the explosion. That alone can explain why a water heater whose (rusted) bottom blows out becomes a rocket that can blast right through upper floors and roof of a building to sail some distance before crashing to earth.

Watch out: A BLEVE can destroy a building. It can kill people. That's why we emphasie the importance of properly-installed, properly-functioning RELIEF VALVES - TP VALVES.

Watch out: BLEVE explosions can occur with other pressurised containers of liquids, including liquid propane or LP gas.

One of the most-famous and most tragic, killing eleven firefighters, is the Kingman Arizona Propane tank car explosion that occurred in 1973, cited below. BLEVES are a particular risk on gas carrier ships.

Some Conditions that Increase the Risk of a BLEVE Explosion

Water heater pressure temperature safety valve (C) Daniel Friedman

Now let's explain why the energy release of a BLEVE can send a water heater tank or cylinder up through a building like a rocket.

Watch out: we have read that some homeowners frightened by the prospect of an exploding water heater or boiler replaced the entire piece of equipment.

If your heating boiler or water heater is otherwise in serviceable condition, but needs a pressure/temperature relief valve repair or replacement, repairing or replacing safety controls or devices on the equipment do not normally require that the entire heater be replaced.

Missing Pressure/Temperature Relief Valves on Heating Boilers or Water Heaters

Watch out: We have on occasion found water heater tanks and boilers whose relief valve was simply removed, perhaps because the valve was leaking. If you find this condition the heater or boiler should be should be shut off immediately and a professional plumber called to inspect and repair the equipment.

Install the proper Temperature / Pressure Relief Valve by Model, Type, and Relief Capacity

Safety Warning- a damaged, improperly-selected model, or modified pressure/temperature relief valve is dangerous and could lead to a serious BLEVE explosion should the water heater later overheat.

The model of relief valve installed should meet the specifications of the water heater or heating boiler or other equipment on which it is installed.

Typically the operating pressure range, temperature range, and the ability of the relief valve to relieve pressure or temperature at a specified rate are all controlled by the model of TP valve selected and must be matched to the size, operating pressure, operating temperature, and other features of the equipment being protected.

At HOT WATER PRESSURE EXPANSION RATE we discuss how we measure water pressure and how temperature changes affect water pressure in a closed water heater tank or heating boiler.

See WATER HEATER SAFETY for our complete list of water heater safety devices and water heater safety inspection advice.

Evidence of dangerous leaks and corrosion at water heater temperature/pressure safety valves

Leaky unsafe relief valve (C) Daniel Friedman

As our photo shows, mineral salts left behind as hot water evaporates from the mouth of a pressure or temperature relief valve can completely clog the spring that is intended to allow the relief valve to open under excess (unsafe) pressure.

The impaction of the relief valve spring with copper and calcium salts in this photo means that the valve is almost certainly not going to open should the heater's internal pressure become unsafe.

The drip shown at the mouth of this relief valve demonstrates that a relief valve can drip and leak for a long time without anyone observing this dangerous condition.

This relief valve needs to be replaced immediately and the cause for the valve's leakage also needs to be determined.

Covered Up or "Hidden-Discharge" Pressure/Temperature Relief Valves

Insulated over relief valve (C) Daniel Friedman

Covered-Over relief valves:

The relief valve on this water heater has been covered by owner-installed water heater insulation.

This is an unsafe condition as the operation of the temperature or pressure relief valve may be interfered with by the insulation and also because the valve cannot be inspected for evidence of leaks or failure.

Similarly, discharging a relief valve leakage or drip to a location where the leak or drip cannot be observed is a dangerous practice because the leak can go unnoticed, causing failure to recognize an unsafe condition.

When a relief valve continues to leak it may become damaged and then fail to operate when an unsafe pressure or temperature occurs.

Constricted, Under-sized, or Mis-Routed Relief Valve Discharge Tubes

Constricted relief valve (C) Daniel FriedmanThe discharge tube attached to the water heater pressure/temperature relief valve should be the same diameter as the opening of the valve itself.

In our photo someone has fastened a 1/2" pipe discharge tube onto a 3/4" diameter relief valve.

See RELIEF VALVE DISCHARGE TUBE for a complete catalog of proper vs. improper T&P valve drain line installation details.

Other Causes of Exploding Water Heaters or Boilers

Mineral Deposits and Heating May Weaken a Water Heater Tank Bottom, Contributing to BLEVE Explosion

Water heater BLEVE Explosion - Daniel FriedmanBLEVE explosions or boiling liquid vapor explosions can occur at both domestic water heaters (calorifiers or geysers) and at hot water heating boilers (hydronic heating systems). In a BLEVE explosion, the state change of superheated water from a hot liquid to a vapor (steam) form releases an enormous amount of energy.

We can understand the huge energy release involved in a BLEVE explosion by a quick review of the extra energy required to change the state of water from a liquid to a gas.

At SEER RATINGS & OTHER DEFINITIONS in our discussion of BTUs (British Thermal Units, a measure of energy), we point out that while only 180 BTUs of energy are needed to raise one pound of water at 32 degF to 1 pound of water at 212 degF, a much larger amount of energy, 970 BTUs, are needed to raise 1 pound of water at 212 degF to 1 pound of steam vapor at 212 degF.

Notice that the temperature has stayed the same! What has changed is the state of our pound of water: from a liquid form to a vapor form.

This figure is the latent heat of vaporization, the number of BTUs of energy used to raise one pound of water at 212 degF to one pound of steam vapor at the same temperature; in other words, the temperature is unchanged but the state of matter is changed from liquid to vapor

. State changes involve large amounts of energy.

We discuss the role of pressure/temperature relief valves in protecting against these hazards


Water heater tank explosions are rare thanks to the widespread requirement for and use of pressure and temperature relief safety valves.

But if the safety valve has been damaged, modified, or even omitted (as we saw on our neighbor's water heater), that condition, combined with overheating can cause a water tank to explode, creating a BLEVE - Boiling Liquid Expanding Vapor Explosion that releases tremendous force and causing extreme damage to a building.

Because repeated heating of the water tank bottom may combine with other conditions (such as corrosion or excessive heating due to mineral deposits on the tank bottom) to produce a weak water heater tank bottom, that is the part more likely to fail in an overheat and overpressure condition.

A failure at the water tank bottom may explain why a BLEVE can produce a water tank explosion that behaves like a rocket, sending the water tank skyrocketing up through a building.

BLEVE Explosions, History, Science, References & Research


Continue reading at HOT WATER PRESSURE EXPANSION RATE where we discuss how we measure water pressure and how temperature changes affect water pressure in a closed water heater tank or heating boiler or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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