The National Board of Boiler and Pressure Vessel Inspectors submits an annual incident report that offers statistics of the number of pressure equipment accidents suffered within the previous year in the U.S. and Canada. It uses documented Occupational Safety and Health Administration (OSHA) data to confirm the total number of accidents, injuries and deaths involving boilers and pressure vessels. Incident rates vary, but the recent 10-year period documents dozens of boiler-related accidents and hundreds of injuries. One persistent troubling fact is that the majority of boiler-related accidents result in fatalities. In spite of these statistics, great progress has been made over the last century as codes and standards have been embraced to ensure improved boiler construction, operations and maintenance. Although incidents still occur, they happen much less frequently than in the 19^th century when accidents were almost a daily occurrence.

Worldwide, boiler accidents are more ubiquitous than in North America. Most developed countries have adopted the American Society of Mechanical Engineers (ASME) Boiler Code, a de facto International Standard for the design, construction, operation, and inspection of boilers. Despite code espousal, however, accountability to such demands great commitment. Unfortunately, many countries don’t have the resources to conduct jurisdictional inspections at regular intervals, and among the least developed countries, boilers may go unchecked entirely.

Along with code compliance, operator training, effective maintenance and established safety protocols often fall by the wayside. Furthermore, the reporting of boiler incidents, accidents and related fatalities is poor or even non-existent in some areas of the world.

What Causes Boiler Explosions?

Statistics point to the fact that most boiler accidents result directly from human oversight or lack of knowledge (for example, low water condition, operator error or poor maintenance).

Two kinds of boiler explosions exist.

1) Pressure part failure – Here, some component of the pressure retaining structure is too weak to withstand the pressure to which it is being subjected. Strength deficiencies arise from manufacturing or fabrication defects, substandard repairs, in-service deterioration or overheating due to low water conditions. Water heaters and steam piping systems may fail in similar circumstances.

2) Furnace explosion - Fuel-rich mixtures introduce themselves when inadequate air is provided for the amount of fuel being burned. In the same way, poor atomization of fuel oil may cause localized accumulations of unburned fuel, possibly resulting in an explosion. Finally, an insufficient purge of furnace gases or entrained fuel may leave a combustible mixture ripe for ignition.

Why Catastrophic?

As water is heated and converted to steam, it can require up to 1,600 times more space. If it has an opportunity to escape from its confined pressure space within a boiler, such as during a failure, it tries to leave the area all at once. The power of all of that steam pressure being released almost instantaneously during a boiler explosion is equal to that caused by detonating explosives or gunpowder. The corresponding shock wave from such a blast can be tremendous.

Various estimates of the energy released from a boiler explosion have been submitted over the years. Clearly much depends on the size of the vessel and the pressure-temperature profile to which the liquid or gas is subjected. For example:

• A typical 30-gallon residential-grade hot-water tank at a temperature of 332°F and 90 psi flashes into explosive failure with enough force to propel the average car 125 feet into the air with a lifting velocity of 85 mph. This explosive force equals about 2.56 ounces of nitroglycerin.
• A steam locomotive operating with 2,642 gallons of water at 350 psi and temperature of about 437 °F would have an explosive energy release equal to about 2,557 pounds of TNT.

Historical Perspective

Ever since the 19^th century, steam power has been revered as a catalyst behind industry, mechanized transportation and electricity generation. Its danger has also been recognized when it comes to boiler explosions, which can sometimes cause great damage to people and property. Various circumstances can initiate such violent, thunderous blasts that result from the buildup and instantaneous release of pressure. These boiler explosions were not unusual in the early days of steam power when serious accidents occurred on steamships, on locomotives and in factories. Catastrophes occurred almost daily as thousands died. Notable examples from North America include:

• 1854: Fales & Gray Car Works of Hartford, Conn.: Boiler explosion kills 21 workers.
• 1858: Steamboat “Pennsylvania": Boiler explosion on Mississippi River and sinks near Memphis, Tenn., with more than 250 casualties.
• 1865: Steamboat “Sultana”: Destroyed by boiler explosion on Mississippi River north of Memphis with approximately 1,700 dead, resulting in the greatest maritime disaster in U.S. history.
• 1905: Grover Shoe Factory of Brockton, Mass.: Explosion levels a 4-story building and kills 58 workers.

Laws Written in Blood

These early tragedies gave birth to mindfulness, codes and legislation. As a result of the Fales & Gray explosion, a group of men thoughtful in science and mindful of steam safety founded the Polytechnic Club of Hartford. The Sultana steamboat disaster paved the way for the formation of the Hartford Steam Boiler Insurance Co. Forthcoming boiler laws and legislation were spurred by the Grover Shoe Factory catastrophe as Massachusetts established a Board of Boiler Rules, which published the first boiler laws in 1908.

The American Society of Mechanical Engineers (ASME) formed a Boiler Code Committee in 1911. This led to the subsequent Boiler & Pressure Vessel Code (BPVC) adoption of 1915. The BPVC was later incorporated into laws in most U.S. states and necessitated formation of the National Board of Boiler & Pressure Vessel Inspectors in 1919 to provide inspection and repair guidance through the National Board Inspection Code (NBIC). ASME then published the B31.1 Boiler External Piping Code of 1922.

Codes and laws are aimed to ensure safe boiler design, fabrication and repair. Regular third-party, in-service inspections are mandated. Both original construction and ongoing repairs must comply with strict engineering guidelines set by jurisdictional authority. Boilers are now designed with redundant feedwater pumps, fans, valves, water level instrumentation, pressure relief valves, fuel cutoffs and automated controls.

Although the incidence of boiler explosions has been greatly reduced by the promulgation of codes and laws, accidents continued with fatalities persisting. In addition to high-pressure boilers, heating boilers, water heaters and steam piping suffered failures. Cases include:

• 1980: Gate City Day Care Center, Atlanta, Georgia: Cast-iron heating boiler explodes killing one adult and four children (impetus for Georgia’s 1984 boiler and pressure vessel act).
• 1982: Star Elementary School, Spencer, Okla.: Water heater explosion leaves six children and one adult dead (motivated Oklahoma legislature to pass safety laws governing water heaters and heating boilers in addition to annual inspections).
• 1985: Mohave Generating Station, Laughlin, Nev.: Hot reheat pipe failure kills six workers (and caused power plants across North America to inspect high-energy piping systems).
• 1986: Brookhaven National Lab, Upton, New York: Steam main piping gasket blowout engulfs and kills two workers.
• 2007: Salem Harbor Generating Station, Salem, Mass.: Boiler tube rupture causes a furnace explosion that kills three workers.

These examples illustrate how these unbridled explosive forces and their associated shock waves can deliver remarkable heat and multidirectional debris with tremendous force. Overwhelming damage occurs to anything in its path. Entire buildings can be leveled, vehicles overturned and people hurled or scalded. As the boiler explodes with an instantaneous shock wave and hot water flashes to steam, death and destruction remain.

Thankfully, the disasters of yesteryear are now rare. The promulgation of stringent codes, equipment improvements and safety measures mitigate risk. Nevertheless, boiler explosions remain a clear and present danger as disaster can strike at any time. As human error and complacency are ever-present dangers, the criticality of operator training and established safety protocols cannot be overemphasized.