A storage water heater is a pressure vessel holding a large amount of water and a source of heat. In normal operation the temperature and the pressure are limited by several independent protections and a dangerous state cannot arise.
An explosion becomes possible only when the protections that should interrupt the heating and relieve excessive pressure fail to do their job, while the heating element keeps transferring energy into the water.
To understand what happens then, you have to understand the relation between temperature, pressure and the state of water.
Water does not always boil at 100 °C
The boiling temperature of water depends on pressure.
Water boils when its saturated vapour pressure reaches the pressure of the surroundings it is in. At normal atmospheric pressure that happens at approximately 100 °C.
If the pressure is higher, a higher temperature is needed for the water to boil.
Approximately:
- at atmospheric pressure water boils at 100 °C;
- at 1 bar gauge pressure at about 120 °C;
- at 3 bar gauge pressure at about 144 °C;
- at 5 bar gauge pressure at about 159 °C;
- at 7 bar gauge pressure at about 170 °C.
That is why water in a closed vessel under pressure can stay liquid even at temperatures considerably above 100 °C.
This fact is the key to understanding the explosion of a water heater.
What happens if the heating does not stop?
Let us suppose that the electrical temperature protections have failed and that the heating element keeps working.
The water keeps receiving thermal energy.
At first its temperature rises, and at the same time the water expands thermally. If the system is hydraulically closed and the expansion of the water cannot be taken up or relieved, then because water is barely compressible and the tank is relatively inelastic the pressure can rise sharply.
That is why heating a completely enclosed amount of liquid water can by itself create a very high pressure.
In a sound system that rise is limited by the safety valve, or by the other elements meant to take up thermal expansion.
For the explosion scenario we assume that the relief of excessive pressure no longer works either.
The heating element therefore keeps putting energy into a closed vessel.
The temperature and the pressure keep rising
As the water heats up, the state in the tank depends on its temperature, the pressure and the available volume.
If the saturation temperature corresponding to the existing pressure is reached, steam starts to form.
But a large space filled with steam does not have to exist before the tank bursts.
A large amount of energy can be stored in the hot pressurised liquid water.
Water at a temperature of 170 °C, for example, can at a pressure of about 7 bar above atmospheric still exist as a liquid in equilibrium with steam.
The same 170 °C at atmospheric pressure is a completely different state, at that pressure water cannot stay stably liquid at that temperature.
That difference becomes decisive the moment the tank gives way.
What happens to the tank itself?
The water pressure acts on the whole inner surface of the tank.
As the pressure rises, so do the mechanical stresses in the material of the tank and in its joints.
In a roughly cylindrical vessel the hoop, that is circumferential, stress in the wall is particularly important. In a simplified model of a thin-walled cylindrical vessel it grows approximately in proportion to the internal pressure and the diameter of the vessel, and decreases as the wall thickness increases.
In other words:
the higher the internal pressure, the greater the force with which the contents try to push the walls of the vessel apart.
A tank has a certain mechanical strength. If the pressure keeps rising, at some moment the weakest part of the structure gives way.
That can be a weld, a corroded part of the wall, a joint or another mechanically weakened place.
The place where it first bursts is not what decides whether the event will be violent.
What is decisive is the content of the tank at the moment it bursts.
When the tank bursts, the pressure falls suddenly
Let us suppose that the tank holds water at 170 °C under high pressure.
The tank gives way and opens towards the room.
The pressure inside the vessel then starts falling rapidly towards atmospheric.
The temperature of the water, however, cannot fall from 170 to 100 °C in the same instant.
So we get water that is still very hot but suddenly finds itself at a pressure at which it cannot remain liquid as a whole.
Part of the water therefore turns into steam almost instantly.
This phenomenon is called sudden, or flash, evaporation.
Why is flash evaporation so violent?
The difference between the volume of liquid water and of steam is enormous.
At atmospheric pressure one kilogram of liquid water takes up approximately one litre, while one kilogram of saturated steam at 100 °C takes up about 1.67 cubic metres.
So in the transition from liquid to steam the specific volume increases by a factor of more than a thousand.
In a sudden pressure drop not all the water in the tank evaporates, because evaporation needs a large amount of energy. Only the part for which there is enough stored thermal energy evaporates.
But even that part is enough to create an enormous amount of steam.
For water at about 170 °C and 7 bar gauge pressure, for example, a sudden release to atmospheric pressure can in theory turn about 13% of its mass into flash steam.
If the tank held approximately 80 kg of water, that would be more than 10 kg of newly formed steam.
At atmospheric pressure that amount of saturated steam corresponds to a volume of the order of tens of cubic metres.
That is not a volume that will form instantly and ideally in a closed room in a real event, the process of bursting, evaporation, cooling and mixing is extremely dynamic, but it shows how much energy can be stored in hot water under pressure.
The bursting of the tank is the start of a sudden decompression
That is why the explosion of a storage water heater should not be seen only as the result of „the pressure blowing the tank apart“.
Excessive pressure first brings the vessel to mechanical failure.
But once the vessel starts to open, another process begins:
the tank bursts → the pressure drops suddenly → part of the hot water flashes into steam → the steam expands rapidly → the vessel is torn open and breaks apart further.
The faster the vessel opens, the faster the pressure falls.
The faster the pressure falls, the greater the amount of hot water that finds itself at once above the boiling temperature corresponding to the new pressure.
That is why the process can develop very quickly and turn an initial mechanical failure into a violent break-up of the tank.
Where does the energy of the explosion actually come from?
The energy of the explosion is not created by the steam itself „out of nothing“.
It was put into the heater beforehand by the work of the electric heating element and stored as thermal energy of the water.
While the water is enclosed and under high pressure, it can hold that energy while staying liquid at a temperature considerably above 100 °C.
When the pressure suddenly disappears, the new conditions no longer allow all that water to stay liquid.
Part of the stored thermal energy is therefore used for the sudden evaporation of water, and the steam that forms expands very quickly.
It is precisely the superheated water under pressure that is the reservoir of energy which makes a violent course of events possible.
What all has to fail for that to happen?
A sound water heater must not reach the state described.
Before that there are several independent opportunities for the process to be stopped:
- the normal temperature regulation has to stop working;
- the additional cut-out of the element at excessive temperature has to fail;
- the element has to keep transferring energy;
- the system for relieving excessive pressure has to be faulty, blocked or cut off from the tank.
Only when several independent levels of protection are lost can the conditions for a dangerous rise of temperature and pressure be created.
That is why the explosion of a storage water heater is not the consequence of one ordinary fault, it is the final outcome of a chain of failures that allows a large amount of energy to build up in a closed vessel.
The essence of the process
The whole mechanism can be reduced to a few steps:
uncontrolled heating of the water ↓ a rise of temperature and pressure ↓ water under pressure stays liquid even at a temperature well above 100 °C ↓ the pressure exceeds the mechanical strength of the tank ↓ the tank gives way and a sudden decompression follows ↓ part of the hot water turns into steam instantly ↓ the steam that forms expands rapidly ↓ the tank breaks violently apart
That is why the explosion of a storage water heater is not simply about „a vessel bursting because of high pressure“.
The destructiveness of the event comes from the combination of the mechanical failure of a pressure vessel and the sudden evaporation of hot water after decompression.