Boiler & water heater tank explosion cause & prevention:

BLEVES This article discusses and defines BLEVE or Boiling Liquid Expanding Vapor Explosions, or as it appears in some texts, Boiling Liquid Expansion Vapor Explosion: cause, dangers, prevention, forces involved, warning signs.

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.

What is a BLEVE or Boiling Liquid Expanding Vapor Explosion?

Definition of BLEVE - Boiling Liquid Expanding 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.

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.

Ogle (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: http://media.wattswater.com/f-52qa.pdf

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 (that's 1 kilocalorie or 1000 calories) or 1C = 0.184 pounds of TNT.
   
   So our 81,756,000 calories of energy = 81,756 C (kilocalories) of energy. And as 1 C = 0.184 pounds of TNT we have (0.184) x (81,756) = 15 pounds of TNT.
   
   Thanks to the two readers cited below for cleaning up the math of this calculation.
   
   Really? Reader Frank explains that when we capitalize "calorie" to "Calorie" or just abbreviate it as "C" then 1 "C" = 1 kilocalorie or 1000 calories (lower case).
   
   Really? Reader Steve explains that 1 gram of TNT = 1 "C" or 1 kilocalorie or 0.184 pounds, NOT 184 pounds.
5. When our 40-gallon water heater BLEVE explosion occurs we have the explosive force of 15 pounds of TNT.
   
   Really? Our estimate of 15 pounds of TNT as the explosive force of a 40 gallon water heater (or depending on where you live a calorifier or cylinder) needed further study.
   
   15 pounds of TNT = 0.092 tons (U.S.) of TNT or about 384,928 Kilo-Joules (J) of mechanical and thermal energy or about 385,000,000 joules.
   
   The U.S. Department of Energy puts 1 pound of TNT as 2300 BTUs or 2.42 megajoules or 2,426,628 joules of energy. - source: www.oit.doe.gov/e3handbook/appenf.shtml
   
   Really? 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.

Regardless of starting assumptions, all estimates of a water heater BLEVE show a huge 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 temperature and pressure relief valves.

See details at 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

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.

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

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

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

Watch out: The 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

• Scale accumulation combined with the concentration of heat at the bottom of gas or oil fired water heaters may weaken the bottom of the water heater tank.
  
  Weakness at the tank bottom explain how a water tank becomes a rocket shooting up through a building during a BLEVE.
  
  See
  
  • WATER HEATER NOISE DIAGNOSIS, CURE
  • WATER HEATER FLUSH PROCEDURE
  • WATER HEATER SCALE DE-LIMING PROCEDURE
• Defeated water heater temperature limit controls - modified by someone, or removed or replaced with the wrong part.
  
  Normal water heater temperature controls will not permit the heater to reach a temperature higher than the equipment is designed-for.
• Improper water heater position or installation, such as
  • Installing a water heater in a horizontal position to cram it into a crawl area where it does not belong
  • Installing a water heater or boiler into a hole dug into soil in a crawl area or low-ceilinged basement, placing the equipment base in ground-contact
• Water heaters or heating boilers exposed to physical damage from flooding or freezing or exposed to mechanical damage during transport or installation or perhaps by impact by a vehicle such as at a water heater or boiler installed in a garage and without impact protection.

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

BLEVE 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.

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

• Abbasi, Tasneem, and S. A. Abbasi. "The boiling liquid expanding vapour explosion (BLEVE): Mechanism, consequence assessment, management." Journal of Hazardous materials 141, no. 3 (2007): 489-519.
  Abstract:
  
  Among the most devastating of accidents likely in chemical process industry is the boiling liquid expanding vapour explosion (BLEVE). It is accompanied by highly destructive blast waves and missiles. In most situations there is also a fireball or a toxic gas cloud.
  
  The damaging effect of BLEVEs is reflected in the fact that the 80-odd major BLEVEs that have occurred between 1940 and 2005 have claimed over a 1000 lives and have injured over 10,000 persons besides harming property worth billions of dollars. Release of toxic chemicals like chlorine and phosgene from BLEVEs have damaged large chunks of areas surrounding the BLEVE site.
  
  This paper presents an overview of the mechanism, the causes, the consequences, and the preventive strategies associated with BLEVEs.
• Ogle, Russell A., Juan C. Ramirez, and Suzanne A. Smyth, CALCULATING THE EXPLOSION ENERGY OF A BOILING LIQUID EXPANDING VAPOR EXPLOSION USING EXERGY ANALYSIS - [PDF] Process Safety Progress 31, no. 1 (2012): 51-54.
  Excerpts: Abstract:
  
  Following the work of Crowl on calculating the energy of explosions using exergy (thermodynamic availability), we examine the explosion energy of boiling liquid expanding vapor explosions (BLEVEs). A number of investigators have proposed schemes for calculating the maximum work that a BLEVE can perform on its surroundings.
  
  Their proposed evaluation schemes yield varying results because they depend significantly on the precise specification of the thermodynamic path taken by the vaporizing, expanding mass.
  
  We demonstrate how to evaluate the maximum theoretical work associated with a BLEVE as a function of the fluid’s thermodynamic properties and its equation of state.
  
  Finally, we give numerical examples to compare the availability of a BLEVE with some of the proposed schemes suggested by others.
  
  The advantage of using exergy analysis to evaluate the maximum work of a BLEVE is based on the rigorous and unambiguous definition for exergy.
  2011 American Institute of Chemical Engineers Process Saf Prog 31: 51–54, 2012 Keywords: BLEVE, explosion, energy, exergy, availability, work
  
  Conclusion:
  The energy of explosion is the maximum potential work that can be performed by the explosive system.
  
  There is some disagreement among previous investigators on how to calculate the explosion energy for a BLEVE. Following the work of Crowl, we have presented a method for calculating the energy of explosion using the concept of exergy. The method was illustrated for selected nonreactive fluids.
  
  The advantage of using exergy analysis to evaluate the explosion energy for a BLEVE is that exergy has a rigorous and unambiguous definition.
• Phase Changes, retrieved 2016/11/29, Hyperphysics, Carl R. (Rod) Nave Department of Physics and Astronomy Georgia State University Atlanta, Georgia 30302-5060 Email: RodNave@gsu.edu, original source: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase.html
• HOT WATER PRESSURE EXPANSION RATE
• Writing in response to our posted query at http://openstudy.com/
  Osprey (from http://perendis.webs.com/ both sites helping solve physics questions & problems ) wrote:
  
  I've briefly scanned the link. Here's my back of the envelope guess.
  Interesting looking question, albeit in gallons and pounds of expl TNT.
  Lots of mixed units here.
  
  Water density = 1000 kg/m3. water latent heat of vaporisation = 2.3 10^6 J/kg
  1 gallon = 4.5 ^-3 m3.
  40 gal ~ 0.16m3
  1m3 = 1000 kg
  0.16m3 = 160kg
  160kg x 2.3 x 10^6 J/kg = 368,000,000 J = 368 MJoules ???
  Could be some "pop" that.
  
  Equiv to 184 pounds of TNT ?
  Either way, I'm not sure I'd like to be hanging around there.
  
  Notes: The "ton of TNT" is a unit of energy defined by that convention to be 4.184 gigajoules and according to Kyles converteR, 184 tons of TNT would = 769,856,000,000 Joules (J)
  - http://www.kylesconverter.com/energy,-work,-and-heat/joules-to-tons-of-tnt
• Prugh, Richard W. "Quantitative Evaluation of" Bleve" Hazards." Journal of Fire Protection Engineering 3, no. 1 (1991): 9-24.
  Abstract:
  
  Rupture of a container in which a liquid is held above its atmospheric-pressure boiling point can result in explosive vaporization of a large fraction of the contents. This phe nomenon is called a Boiling-Liquid Expanding-Vapor Explosion ("BLEVE").
  
  A method for estimating the TNT equivalence of a BLEVE is proposed, with descriptions of possible causes and consequences of the explosion and (if the contents are flammable) the consequences of the resulting fireball.
• "The Disaster Story". Kingman Historic District. Retrieved 2016/12/01, original source: http://kingmanhistoricdistrict.com/points-of-interest/firefighters-memorial-park/the-disaster-story.htm
  Excerpt:
  
  On July 5, 1973, Kingman [Arizona] was the site of a catastrophic BLEVE (Boiling Liquid Expanding Vapor Explosion) which killed 11 firefighters. The explosion occurred following a fire that broke out as propane was being transferred from a railroad car to a storage tank.
  
  This explosion has become a classic incident studied in fire department training programs worldwide.
• In addition to the BLEVE photograph in the article above, and the Mythbusters video photo of an exploding water tank (page top), here is a SECOND PHOTO [image] of the Mythbusters test of (cited at REFERENCES )exploding a water heater tank.
• RELIEF VALVE, WATER HEATER we discuss temperature and pressure relief valves used on residential water heaters.
• RELIEF VALVE, TP VALVE, BOILER for a general discussion of temperature and pressure relief valves used as safety devices on heating boilers and other pressurized. Pressure relief valves (that sense pressure only, not temperature) are also required on pressurized tanks such as water tanks in buildings.
• SUPERHEAT DEFINITION
• WATER PUMP SAFETY - Water tank pressure relief valves are discussed
• WATER HEATER SAFETY complete list of water heater safety devices and water heater safety inspection advice.
• Watts, 52 QUESTIONS and THEIR ANSWERS [about] HOT WATER HEATERS, EXPLOSIONS & WATER HEATER SAFETY], [PDF] (1973) Watts Regulator Company, 815 Chestnut Street, North Andover, MA 01845-6098 USA Tel: 978.689.6066 retrived anew 2018/10/27, original source: http://media.wattswater.com/f-52qa.pdf
• REFERENCES other reports of water heater or boiler BLEVE explosions

Some of the FAQs discussed below are adapted from information provided by the Watts Regulator Company in a 1973 publication.[10]

Question: what causes a hot water storage tank to explode?

Why do hot water heaters explode?

Reply:

Excessive temperature is the root cause of hot water tank explosions, though damage to the water heater tank from scale, leaks, or corrosion as well as improper installation of the tank or its plumbing can play a major role in water heater tank explosions as well.

Watts distinguishes between the basic cause of water heater explosion (excessive temperature) and physical causes (corrosion or weaknesses in the tank).

Combined with the explosive effects of "superheated" water the result of a break in a hot water storage tank is not just that the tank bursts, it explodes, as we explain above in discussing BLEVEs - boiling liquid vapor explosions. Watts comments that heat itself also reduces that ability of a water heater tank to withstand pressure.[10]

Superheated water, that is water heated above 212 degrees F or 100 deg C, occurs when water is heated in a closed container.

A pressure cooker used on a stove-top provides superheated water to cook food faster than would otherwise occur. In a pressure cooker that hissing clanking device that rattles around during cooking is a simple weight that sits atop an opening in the pressure cooker's top that continuously releases pressure, heat, and steam - to prevent the pressure cooker from exploding.

When we heat water in a water heater storage tank, pressures in the system are contained by the storage tank and plumbing system piping.

If the hot water tank suddenly ruptures its contents are no longer held at high pressure by an intact tank, but rather are exposed to atmospheric pressure.

This sudden drop in pressure combines with water temperature above 212 degF to cause the hot water to change state - from liquid water to steam.

It is this state change (from water to steam) that releases an enormous amount of latent energy stored in the pressurized hot water, and it is that sudden energy release that constitutes the "explosion" of a hot water heater tank.

Watch out: in addition to the obvious ingredients in a water heater tank explosion: a faulty heater or heater control that superheats the water in the tank, a faulty pressure/temperature relief valve, or a blockage in the TP valve piping or discharge tube, it is also possible that water hammer can contribute to a BLEVE.

Water hammer can cause recurrent leaks at a TPR valve until it becomes clogged and unsafe - it then fails to protect against a BLEVE. We discuss this contribution in the FAQs found

at WATER HAMMER NOISE DIAGNOSE & CURE

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-03-24 - by (mod) - it's 15 pounds of TNT or 2,426,628 joules of energy when a water heater explodes

Thanks for commenting, Steve, I appreciate all the editing help we can get - especially from careful readers.

I have reviewed, edited and corrrected the article accordingly.

On 2020-03-24 by Steve H energy released in a BLEVE explosion

Whoops, didn't see Frank's comment! You need to correct the calculation.

Your energy calculation is incorrect. The caloric value of 1 gram of TNT is 1 'C' which is a kilocalorie. Your estimate is off by a factor of 1000; the correct equivalent mass of TNT is 0.184 pounds, not 184 pounds, whihc makes way more sense.

On 2018-12-12 - by (mod) - getting the math right on energy released in a BLEVE explosion

Thank you for the comment, Frank, I'll review the math. We didn't make these numbers up.

On 2018-12-10 by Frank Rizzo

Your 184 pounds of TNT is wrong because Calorie = 1000 calories. (How do imperial users convey the capital C when speaking?) Should've stuck to metric.

On 2018-11-20 - by (mod) -

James

More likely in the scenario that you're describing you or someone would have gotten scalding burns. The believe explosion occurs typically involving a vessel that has a sudden release of pressure. The classic example is the bottom blowing out of a water heater. In that case the water heater becomes a rocket that can shoot right up through the building structure and out through the building roof.

On 2018-11-16 by James Hone

I once had to remove a blind flange that was bolted directly on a gate valve. The size of the valve was 6"and it was holding back ±300 degree ±300 psi superheated water.

Gate valves are notorious for leaking. I was concerned the blind flange was going to have superheated water behind it and when I loosened the bolts I would lose the pressure and have an bleve explosion. Luckily the valve didn't leak, but I have always wondered how big a risk I took, how big the explosion could have been, etc. What do you think?

On 2016-11-29 - by (mod) -

Re-Posting

(Apr 17, 2015) PAUL said:
Great info easy to read and understand

Question: is it high pressure or high temperature that causes a water heater tank to explode?

I was told that really it's ultimately high pressure that causes a water tank to blow up. But you say it's temperature. Can you explain this?

Reply:

It is indeed high temperature that causes an actual "explosion" of a hot water tank, not the pressure itself.

Without high temperature, if a water tank bursts under pressure, water simply squirts out of the tank.

But if we take those same conditions and add high temperature (over 212F) that occurs in a normallyt pressurized hot water tank, that is, if we replace cold water with superheated water in the same tank, when the tank ruptures the superheated water, now exposed to the lower pressure of the atmosphere, flashes to steam - a state change that releases enough energy to cause an actual explosion.

Often water heaters that have suffered from age or high bottom temperatures or scale develope their weakest spot at the bottom of the water tank.

That explains why, when a water heater tank ruptures and its hot water suddenly flashes to steam, releasing the latent heat energy that was stored in the superheated water, the result is a BLEVE and the water tank becomes a rocket that is propelled upwards through the building.

Note: water that is not superheated, that is water under 212F or 100C, has no latent heat energy to cause an explosive expansion.

Question: What's the difference between a BLEVE and a controlled steam pressure energy release?

If we have to shut off the water supply as part of repairing a water heater tank that is already overheated, is that dangerous?

Reply: definition of a controlled steam pressure energy release - why it does not blow up

Watts gives the following explanation: [with some paraphrasing - Ed.]

Example: a hot water tank has 60 psi pressure and has been overheated to 240 degF. What happens if the water pressure is shut off for repairs or some other purpose?

When a faucet was opened to bleed the pressure, it would immediately drop down, but just to 10 psi, not to zero psi - the pressure at which water boils under 240 degF.

Therefore, because this is a pressure boileing point combination, the water will flash into steam. The steam forms instantly (the meaning of "flash") and therefore pressure in the system will fall from 60 psi and remain at 10 psi until the escaping steam has reduced the temperature in the tank.

As the water heater tank temperture drops, the pressure falls accordingly, to each boiling point/pressure along the way. Ultimately the pressure will fall to zero psi. At that point the temperature in the system will be the boiling point of water at atmospheric pressure (212F at sea level).

This condition is called a controlled steam pressure energy being dissipated - not a condition that causes a BLEVE explosion. By contrast, a BLEVE occurs when superheated water is suddenly exposed to atmosphereic pressure. [10]

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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 the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• BLEVE EXPLOSIONS
• HOT WATER PRESSURE EXPANSION RATE
• RELIEF VALVE, WATER HEATER.
• RELIEF VALVES - TP VALVES - home
• RELIEF VALVE, TP VALVE, BOILER
• RELIEF VALVE, WATER HEATER
• WATER HAMMER NOISE DIAGNOSE & CURE
• WATER HEATER PRESSURE-ONLY RELIEF VALVES
• WATER HEATER SAFETY
• WATER HEATER TEMPERATURE ADJUSTMENT CONTROLS

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