Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
AGE of WATER HEATERS
ALTERNATIVE HOT WATER SOURCES
ANODES & DIP TUBES on WATER HEATERS
ANTI SCALD VALVES
APPLIANCE DIAGNOSIS & REPAIR
APPLIANCE EFFICIENCY RATINGS
AQUASTAT CONTROL Functions
BACKDRAFTING HEATING EQUIPMENT
BACKFLOW PREVENTER, HEATER WATER FEEDER
CARBON MONOXIDE - CO
CHECK VALVES, WATER SUPPLY
CLOGGED SUPPLY PIPING
CROSS CONNECTIONS, PLUMBING
DEBRIS in WATER SUPPLY, Water Heater
DRAIN a WATER HEATER TANK
ELECTRIC WATER HEATERS
FLOODED WATER HEATER REPAIR
FREEZE-PROOF A BUILDING
GALVANIC SCALE & METAL CORROSION
GAS BURNER Flame & Noise Defects
GAS FIRED WATER HEATERS
GAS PIPING, VALVES, CONTROLS
HEATING COST SAVINGS METHODS
HOT WATER SUPPLY
HOT WATER IMPROVEMENTS
HOT WATER DELIVERY SPEED UP
HOT WATER EFFICIENCY IMPROVEMENT
HOT WATER PRESSURE EXPANSION RATE
HOT WATER PRESSURE LOSS
HOT WATER PROBLEM DIAGNOSIS
HOT WATER QUANTITY IMPROVEMENT
HYDROGEN SULFIDE GAS
INDIRECT FIRED WATER HEATERS
MANUALS & PARTS GUIDES - HVAC
MIXING / ANTI-SCALD VALVES
NO HEAT - NO HOT WATER: HEATER DIAGNOSIS
NOISE, WATER HEATER
ODORS IN WATER
PIPING IN buildings, Clogs Leaks Types
RELIEF VALVE LEAKS
RELIEF VALVES - Water Heaters
SCALE REMOVAL, WATER HEATERS
SEWER GAS ODORS
SOLAR HOT WATER HEATERS
TANKLESS WATER HEATERS
THERMAL EXPANSION of HOT WATER
THERMOSTATS, WATER HEATER
TIMERS for ELECTRIC WATER HEATERS
WATER HEATER ALTERNATIVES
WATER HEATER ANODES, DIP TUBES
WATER HEATER AIR INLET
WATER HEATER DEBRIS FLUSH
WATER HEATER DRAIN PROCEDURE
WATER HEATER EFFICIENCY
WATER HEATER FLUSH PROCEDURE
WATER HEATER NOISES
WATER HEATER PROBLEM DIAGNOSIS
WATER HEATER COMPARISONS, PROPERTIES
WATER HEATER SCALE
WATER HEATER SAFETY
WATER HEATERS for HOME HEATING USE?
WATER ODORS, CAUSE CURE
WATER PIPES, Clogs Leaks Types
WATER PRESSURE TOO HIGH: DANGERS
WATER PRESSURE LOSS DIAGNOSIS & REPAIR
WATER QUANTITY IMPROVEMENT
WATER SOFTENERS & CONDITIONERS
WINTERIZE A BUILDING
Boiler & water heater tank explosion cause & prevention: BLEVES This article discussesand 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.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
See WATER HEATER SAFETY for our complete list of water heater safety devices and water heater safety inspection advice.
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 (orignally aired on the Discovery Channel).
How 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.
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.
Other reports of water heater or boiler BLEVE explosions are at references below.
Some Conditions that Increase the Risk of A BLEVE Explosion
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
As our photo shows (left), 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
In our photo someone has fastened a 1/2" pipe discharge tube onto a 3/4" diameter relief valve.
Other Causes of Exploding Water Heaters or Boilers
At RELIEF VALVES - Water Heaterswe discuss temperature and pressure relief valves used on residential water heaters.
For a general discussion of temperature and pressure relief valves used as safety devices on heating boilers and other pressurized please see RELIEF VALVES - TP Valves on Boilers. Pressure relief valves (that sense pressure only, not temperature) are also required on pressurized tanks such as water tanks in buildings.
Water tank pressure relief valves are discussed at WATER PUMP SAFETY.
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.
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs) about boiler or water heater explosions & BLEVEs
Some of the FAQs discussed below are adapted from information provided by the Watts Regulator Company in a 1973 publication.
Question: what causes a hot water storage tank to explode?
Why do hot water heaters explode?
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.
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 constitues 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. We discuss this contribution in the FAQs found at WATER HAMMER NOISE DIAGNOSE & CURE
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?
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.]
Questions & answers or comments about the cause & prevention of BLEVE explosions at water heaters & boilers.
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website
HTML Comment Box is loading comments...
Technical Reviewers & References