The Sultana – Part 2: How do boilers go boom?

During my investigation of the Sultana explosion, I reviewed the direct testimony for the words and descriptions used by witnesses and experts to better understand the causes of the explosion and perhaps finds clues that others had missed.

Flawed theories

The testimony of R.G. Taylor, the boilermaker who repaired the leaking Sultana boiler in Vicksburg, Mississippi, included the following statement: “As long as there is a sufficiency of water in the boiler, there is no danger of explosion.” Knowing this now to be very much untrue, it led me to search for what engineers and boilermakers in 1865 understood about steam and explosions.

In the early years of steam use, boiler explosions were common yet misunderstood. Explosions were blamed on acts of God, spirits, and electricity generation and hydrogen formation in boilers. By the mid 1800’s, boiler explosions were occurring almost daily. Engineers recognized that they happened due to a sudden burst of steam, but did not understand why.

Boilermakers and engineers believed that the cause of most boiler explosions was low water. This theory hypothesized that low water allowed a boiler plate to be exposed to the fire, overheat and turn red hot. Then, when water struck the plate, it instantly turned to steam overpressurizing the boiler and resulting in an explosion.

In addition, they believed that much of the energy was in the steam. An excess of steam was often cited. The low water theory proved to be deficient.

A new understanding

Significant scientific investigations of steam and steam power took place in the early 1860’s through the 1870’s resulting in the science behind steam explosions quickly advancing. Robert Thurston, the first President of the American Society of Mechanical Engineers (ASME), published “Steam Boiler Explosions, In Theory and Practice” in 1887.

By that time, two things had become clear that weren’t obvious earlier.

  1. There is a tremendous amount of energy stored in a boiler under pressure; almost all of this energy is in the water.
  2. A sudden drop in pressure will cause a portion of water to instantly boil creating steam which expands rapidly. This expansive power is what creates catastrophic explosions.

The figure below is an early table that shows the amount of energy stored in a boiler. According to this data, the energy contained in a plain tubular boiler was sufficient to launch the boiler 5,372 feet. Yes, 5,372 feet – over a mile high!

The table also shows that the steam in the boiler stores less than three percent of the total energy in the boiler and that the water stores most of the energy. Therefore, a boiler full of water has more energy stored and available for explosive power than a boiler low on water.

boiler explosionsPutting it in perspective

Water in a pot on the stove boils at 212°F. As pressure increases, the water’s boiling temperature also increases. For example, at an operating boiler pressure of 145 pounds per square inch (psig), water boils at a temperature of 363°F.

If there is a break or leak in the boiler, the pressure inside immediately drops. With this drop in pressure, the temperature at which water boils also drops. The water temperature in the boiler will then be greater than the boiling temperature. Therefore, some of the water will flash to steam, expanding and re-pressurizing the boiler.

If the opening is small and the boiler is strong enough, the pressure will be contained and the water or steam will steadily leak out as the boiler remains intact. If however, the boiler cannot contain the pressure due to a weakened condition, the result will be an explosion as more and more water flashes to steam and rips the openings wider almost instantaneously. Overall, the weaker or more brittle the boiler, the larger the explosion will be.

This phenomena can be illustrated with a bottle and a can of soda as shown in the video below. In the first bottle, a small hole, analogous to a small boiler leak, is created. The pressure drop creates a sudden carbonation release, but since the bottle can contain it, the leak and the carbonation remain in equilibrium, and the bottle remains intact – slowly losing water and carbonation.

In the second bottle, a larger hole is made and the bottle cannot hold back the sudden increase in pressure from the expanding carbonation. Note how the soda turns white due to the large amount of carbonation releasing immediately.

In the third part of the video, a can is scored slightly along the side to weaken it before a leak is introduced. This concept is the same for steam boilers. A sudden drop in pressure creates the expanding steam that supplies the tremendous forces.

The next post will review the most commonly cited cause of the Sultana explosion.

 

© 2015 The Hartford Steam Boiler Inspection and Insurance Company. All rights reserved. This article is intended for information purposes only. HSB makes no warranties or representations as to the accuracy or completeness of the content of this article.

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