By: Herman Strauss - SABS
|Herman Straus, SABS mechanical fluid
and pipes manager
Although this is a real nuisance and can cause substantial damage, this is not the type of burst geyser we are going to talk about today. Today’s article looks at the worst case scenario, the reason for all the regulations for geyser installations.
For this we have to start with a smallscience lesson. Primary school science teaches all matter has three states or phases, Namely: solid, liquid and gas. In other words: ice, water and steam. The single most important factor that determines the change of state is temperature. At approximately 0°C ice can change into water and at approximately 100°C water can change into steam.
What is important to note is how the altered phases react differently depending on atmospheric conditions. Take a syringe and fill it with water. Close the front end and try to push in the plunger. It is not possible to move because a liquid is not compressible. Now do the same but with no water in the syringe, only air. Look how far you can push the plunger in now. Even more importantly, what happens when you release the plunger? It moves back to where it started. Because there is no external force keeping it compressed, it will move back to the starting position.
Now back to the geyser. What happens inside a geyser? There is an electric element transferring heat energy into the water. There is also a thermostat to sense when the water reaches a set temperature and switches off the element. But, what happens when the thermostat fails to switch off the element? The water temperature continues to increase. It is fair to expect the water to start boiling when it reaches approximately 100°C. Only, that is not always the case. At Gauteng altitude, boiling water is cooler than at sea level where the atmospheric pressure is higher. Inside the geyser, the pressure is substantially higher. At 400 kPa, water will boil at about 144°C. Before this point is reached the TP valve would normally relieve the situation, but let us assume for a moment the valve is faulty, or was installed so the outlet of the valve has become blocked over time due to insect nests, for example. The element keeps feeding heat into the water and as the water heats up, it expands and causes the pressure to rise. After a while, the water pressure reaches 500 kPa where water only boils at 152°C. The cycle continues until something gives.
The question is: at what point will what give? In line with national standards, geysers are tested to twice their functional pressure. This means they should withstand about 800 kPa. At this point the water temperature could be in the region of 170 °C.
Assuming this is the point where the geyser ruptures. What happens now is just plain scary. As the water starts escaping from the geyser, the pressure drops drastically. With the pressure gone, the water is suddenly in a condition where the effective boiling point is only about 100°C while the water is far exceeding that. The result is water turning into gas in an instant. The correct terminology for this state of the H20 is called superheated steam.
Back to the experiment with the syringe: When the force used to contain gas under pressure is released, it expands rapidly until it normalises with the atmosphere. Instead of a bit of water leaking from the geyser, you actually have superheated steam escaping and expanding at a rapid rate and with immense force. This type of explosion is called a Boiling Liquid Expanding Vapour Explosion (BLEVE)
This happens with enormous energy. There are various models for calculating the explosive force of a BLEVE but we can look at a very basic rule. What is the potential energy available? From universally published data tables we find the energy of water at ambient conditions of 20°C and at sea level is about 0.296 KJ/Kg.K. At the point where the geyser ruptures and the water turns into superheated steam, the energy in the water is about 2839 KJ/kg.k
These values can be used mathematically (not for this article) to calculate the amount of energy released at the time of the explosion. For a 150-litre geyser this equates to about 200 million Kilo Joules of energy. This figure is difficult to comprehend. To try and relate that to some extent, imagine lifting 1 000 elephants one story above the house. Dropping them on the house is just about the same energy.
Fortunately, the standards and regulations for installation of geysers in South Africa provide for multiple fail safe mechanisms to keep the system safe. So to answer the question we posed right at the start of this article: If you regard the possibility of a burst geyser as an inconvenient nuisance you are likely to expose your customers to a real threat.