Boiler Explosion - Principle

Principle

Many shell-type boilers carry a large bath of liquid water which is heated beyond the boiling point of water at atmospheric pressure. During normal operation, the liquid water remains in the bottom of the boiler due to gravity, steam bubbles rise through the liquid water and collect at the top for use.

If this boiler opens up to the atmosphere as a result of a break from over pressure or other such failure the contents are allowed to expand suddenly into the atmosphere. The rapid release of steam and water can provide a very potent blast, and cause great damage to surrounding property or personnel. Since the water in the boiler is at a higher temperature and pressure (enthalpy) than boiling water would be at atmospheric pressure, some of this liquid will flash into vapor as the pressure drops by the rapid formation of steam bubbles throughout the water.

The energy of this expanding steam and water is now performing work just as it would have done in the engine, with a force that can peel back the material around the break, severely distorting the shape of the plate which was formerly held in place by stays, or self-supported by its original cylindrical shape.

The action of the rapidly expanding steam bubbles will also perform work by throwing large "slugs" of water inside the boiler. A fast-moving mass of water carries a great deal of energy (from the expanding steam), and in collision with the shell of the boiler results in a violent destructive effect. This can greatly enlarge the original rupture, or tear the shell in two.

Many plumbers and steamfitters are aware of the phenomenon called "water hammer". A few ounce "slug" of water passing through a steam line and striking a 90 degree elbow can instantly fracture a fitting that is otherwise capable of handling several times the normal static pressure. It can then be understood that a few hundred, or even a few thousand pounds of water moving at the same velocity inside a boiler shell can easily blow out a tube sheet, collapse a firebox, even toss the entire boiler a surprising distance through reaction as the water exits the boiler, like the recoil of a heavy cannon firing a ball.

A steam locomotive operating at 350 psi (2.4 MPa) would have a temperature of about 225 °C, and a specific enthalpy of 963.7 kJ/kg. Since standard pressure saturated water has a specific enthalpy of just 418.91 kJ/kg, the difference between the two specific enthalpies, 544.8 kJ/kg, is the total energy expended in the explosion.

So in the case of a large locomotive which can hold as much as 10,000 kg of water at a high pressure and temperature state, this explosion would have an energy release equal to about 1160 kg of TNT.