Monday, October 24, 2016


Explosion of an overheated boiler


We received quite a few inquiries regarding the prevention of the overheating of a boiler or furnace that could lead to an explosion. If you have baseboard heating system in your home, then you do have a water heating boiler system like the one we describe in this blog. We briefly address this issue as a service to the communities we serve.

An overheated boiler is where the heating unit reaches an exceedingly high temperature and pressure, and the high temperature and pressure safety switches will not operate. This condition, also known as “runaway boiler”, is very hazardous to building occupants and emergency responders, including the neighbors. It is caused by the concurrent failures of redundant safety controls that maintain safe temperature, pressure, gas supply (if the boiler is a gas unit), and water supply. 

When boilers or furnaces overheat, it can cause a failure of the safety controls that are attached or very close to the body of the boiler. The failure of the safety controls will most definitely cause an explosion of the boiler. See images above and below.

Exploded boiler on its side

There are several types of boilers and furnaces available: hot water boilers, steam boilers, warm air furnaces. These are briefly described below.

Basic Components of a Hot Water Boiler

The basic components of a hot water boiler are (refer to the attached schematic):

· A gas valve where the fuel comes in

· Water feed gate valve and the associated pressure relief valve (PRV)

· The water circulator pump

· The pressure relief valve

· The air vent

· The Aquastat control that is used to regulate the water temperature

· The expansion tank

· The drain valve

· The vent flue

· The burners

· The pressure and temperature gauges/control valves

The water heating boiler operates as follows:

The boiler is filled up with water and maintains 12 PSI (pounds per square inch) water pressure. The air is bled out of the system zones. The circulator pump will not move water through the baseboard loops if air is in the system. The expansion tank maintains system pressure and absorbs water expansion and contraction from heating and cooling.

How water boilers risk over-pressurization when there is a failure of the pressure relief valve and at least one other safety control. The boilers are connected to closed piping systems with the potential to build up excessive pressures when water temperatures rise without control beyond the safe operating range which is up to 250 degrees F. The typical maximum operating pressure of a residential boiler is 20 PSI. For an industrial boiler, the max operating pressure is 50 PSI. Pressures that continue to rise above the above safety ratings can result in a violent explosion when the boiler vessel fails.

Based on our forensic investigations, the boiler failure incidents occurred when safety controls failed to operate as designed due to mechanical failure (i.e., aquastat shorting out, damaged wiring, or failed gas valve). Other causes of failure include human intervention events, including: removing or adding a pressure relief valve to stop water leaks; wiring errors; opening a gas valve in bypass (manual) mode.

A “Dry Fire Boiler” condition can occur when burners fail to shut off and the relief valve opens to release excessive pressure. The remaining water in the system can boil away if water supply to the PRV is shut off or the PRV is stuck closed. Without water, the boiler can reach extreme temperatures. See image below of an overheated firebox of a boiler that was heating without water.

You should never introduce water in an overheated boiler. If cooler water is introduced, it flashes into steam when it comes into contact with the heated surfaces inside the boiler. This “flash steam” significantly increases the pressure in the boiler as the steam energy is released. This reaction can result in a cracked boiler or violent explosion.

Owners of Flooded Homes Beware: It is very important to note that the heating controls exposed to flood water can fail after being dried and placed back in operation. Any controls exposed to flood water must always be replaced.


Did you know?



Steam Boiler

The basic components of a steam boiler are as follows:

· The combination gas valve, regulator and shut-off cock

· The low water cutoff switch

· The water level sight glass

· The pressure limit control

· The temperature and pressure relief valve

· The exhaust vent damper

· The electronic ignition

· The access door to the burners/pilot

Steam boilers can become over-pressurized when there is a failure of the pressure relief valve and at least one other control. Although the steam boiler is connected to an open piping system with radiators at the endpoints, the radiator air vents close at normal operating temperatures. Excessive pressure will build up if the gas valve or safety controls fail. Boiler pressures that exceed 15 PSI due to a failure of the PRV can result in a violent explosion when the boiler vessel fails.

Investigated boiler failure incidents took place when safety controls failed to operate as designed due to mechanical failure (i.e., blocked siphon tube (“pigtail”), failed gas valve, defective low level cutoff (LWCO) or though human intervention. Some examples of human intervention are: removing and adding a PRV to stop water leaks, wiring errors, opening a gas valve in bypass (manual mode).

As we said earlier, the leading cause of boiler failure is the low water condition caused by failure of the LWCO device. The following factors can lead to a breakdown or malfunctioning of the LWCO.

Float Type LWCO w/ Manual Reset (Steam or Water Boiler)

· Mineral deposits (calcium, etc.) typically contained in the water cause fouling of the device.

· Improper or faulty wiring, either permanent or during testing are causing malfunctioning of the LWCO.

· Normal wear and tear of the device can lead to failure.

· Luck or proper maintenance such as purging.

· Loss of Water due to leaks in piping, steam traps, etc.

LWCO switch stuck in the up position. This led to a boiler overheating, introduction of water into the overheated boiler, steam flashing and subsequent explosion

Warm Air Furnace

If you have a typical HVAC system, then you have a warm air furnace as part of the system. The basic components of a warm air furnace include:

· The gas valve

· The gas burners

· The blower and blower motor

· The furnace chamber

· The blower fan and limit control

· The gas shut off valve

· The heat exchanger

· The pilot assembly of hot surface ignitor

· The thermocouple or flame sensor

A warm air furnace is at risk of overheating when gas control failures produce temperatures exceeding the maximum design. An overheated furnace is a fire hazard and may crack, allowing carbon monoxide to enter the building through the duct work. For HVAC installations, if an A/C coil fails due to soft solder melting when high temperatures or fire compromises the system, potentially lethal toxic gas can be created. As with the other boilers, any controls exposed to flood water must always be replaced.

In a boiler explosion incident, the homeowner was experiencing trouble with the boiler ignition system, and switched the automatic gas valve to manual operation. The constant flow of gas into the boiler led to a significant increase in temperature and pressure that caused an overheating (runaway) condition. Fifteen minutes after the heating unit was turned on, the boiler exploded.

Pieces of the boiler scattered in the basement

Automatic gas valve was switched “ON” to manual operation, feeding the boiler with a steady flow of gas. This led to the overheating and subsequent explosion of the boiler. This particular boiler had no means to relieve pressure after overheating.

Several months prior to the explosion, the customer had a combination pressure reducing (water feed) valve and pressure relief valve control replaced. When the servicing company performed the repair, the combination valve was replaced with only a pressure reducing (water feed) valve. As a result, the boiler no longer had a relief valve to relieve excess pressure. Whenever a combination valve is replaced with a single pressure reducing (water feed) valve, a separate relief valve must also be installed. Boilers are equipped with designated tap holes for inserting a separate pressure relief valve.

In another incident, a homeowner had a thermal expansion tank installed on the supply line to the hot water boiler. The new expansion tank was erroneously connected to the outlet of the pressure relief valve, plugging it. The boiler had no way of relieving pressure and created a “runaway” condition.

Note the expansion tank erroneously connected to the outlet of the PRV valve. This was the cause of boiler explosion.

Based on the thousands of boiler maintenance, repairs and failure investigations we conducted over the years, we can tell you with certainty that the leading cause of heating boiler failure is low water. Most of the time, the low water fuel cutout safety switch has failed and this led to the overheating and failure of the boilers. There are always early sign of boiler failure, if you know where to look and how to interpret them. For example, small leaks could appear in the boiler vessel or the piping. These water leaks may not look serious, but they should never be ignored as they will most certainly lead into greater problems. Most of the time, some low cost items, such as a pressure relief valve, are defective or are old and need replacement. If they are not replaced, significant losses could occur.

If an insurer has refused to insurer your boiler, then this should be a good clue that it is time to replace it or repair it. Insurers, like The Hartford Steam Boiler Inspection and Insurance Company, Travelers, and others have a very good handle on boiler safety. When they stay away from a piece of equipment, please take the hint.

Above is a photo of a valve that was installed incorrectly. In fact, the inlet and outlet were stamped on the valve, so there is no excuse for such error. Mistakes are more frequent than you think. It is that our inspections help caught them prior to any damage occurring.

Why Boiler Failures can be Dangerous

Water boils at atmospheric pressure at 100o C (212o F). When placed under pressure, the water can be heated to extremes without flashing to steam. The typical expansion rate for 1 cubic foot of water is 1,700 cubic feet of steam. This amazing expansion rate is the reason that the boiler explosions are so powerful. Imagine this amount of steam to exit from a small pipe break under tremendous pressures. The routinely inspected water heater holds 119 gallons (16 cubic feet) of water. It is really a small bomb due to this massive energy release; in the past (during the industrial revolution between the 1800s and early 1900s) thousands of people died from such explosions. See below the results from a failed relief valve.

Boiler Inspection and Maintenance

The following provide important recommendations for heating boiler inspection, maintenance, and operation prior to the heating season. Low pressure boilers are inspected annually while in operation and internally every 3 years, if construction permits:

1. Perform an external exam for leaks. Many times we find that the leakage is caused by failure to remove hardness from inside the boiler; then the hardness expands during boiler heating, pushing apart the boiler sections and causing leakage.

2. Perform an evaluation of all safety device function.

3. Test all high temperature and high pressure switches.

4. Insulate water lines that are exposed to freezing temperatures to avoid pipe freeze-ups. Quite a few of these freeze ups occurred this past winter and another round of equally severe winter is forecast. We have our fingers crossed.

5. The safety relief valves or devices are crucial devices that must be tested very frequently, ideally every month. A malfunctioning safety valve will cause the boiler fail. If the relief valve does not properly function, then the temperature control will fail to properly control temperature. The water heater will heat unchecked until catastrophic failure occurs.

6. All other valves, gauges, pressure and temperature control instruments must be inspected for proper functioning.

7. Due to constituents found in the water, we get sediment built up pretty much everywhere in the boiler, including the sight (or gage) glass used to observe the water level. The water level sight glass should be cleaned from the accumulated water impurities and scale.

8. Disassemble the low water fuel cutoff and make-up water feeding devices. All parts should be thoroughly flushed, cleaned and reconditioned as required, then tested and evaluated before the boiler is put into regular service. While in service the devices should be tested once a week for steam boilers and once a month for hot water boilers.

9. All leaking pipes, valves, connections, etc. must be repaired or replaced to prevent water loss and property damage.

10. The boiler burner should be cleaned and adjusted to provide maximum burning efficiency.

11. All the internal surfaces of the boiler should be cleaned of all the water impurities that accumulate over time. These impurities inside the boiler are the cause of corrosion, cracking, deterioration and even explosion of the boiler.

12. A chemical water treatment should be used to minimize built-up of scale and other deposits and to slow down or minimize corrosion.

13. Ensure that all other equipment associated with the boiling system work properly.

14. Maintain a record book of boiler operation, maintenance or repairs. Some boilers have failed because technicians did not know what another tech had done to the boiler.

Regarding pressure vessels, most mechanical codes require that are inspected once every three years. This inspection requires a test of:

· Safety relief devices

· Temperature and pressure controls

· External exam to check for leaks

· Testing to verify thickness of the vessel body

We commonly see leaking and corroded boilers during many of our inspections, especially for older boilers. Unfortunately, even with a regular inspection program, people just ignore proper maintenance and operation. Just like our cars, some people are good at maintaining and some people are simply bad. We also observe leaking steam from the spindle, and many steam and water connections are also found leaking. The mud drums are another area where we observe corrosion and leakage. See figure below.

We have seen situations where there is very little monitoring of boilers in schools and municipalities on a regular basis, other than the inspector doing his annual inspection. This has led to leaks and other problems with the boiler efficiency and operation. As we wrote in the beginning, the most frequently found deficiencies are associated with the low water fuel cut offs, safety and relief valves are stuck, and high pressure and temperature switches are not often installed.

 It appears that the boiler owners believe that the service technician who may maintain their boiler will also test for the operation of the safety devices. We have found that this is not true, as the technician will only perform the service of the combustion and burner fuel systems, change the fuel filters, clean the fuel pumps and nozzles, check the igniter, vacuum the gas passes of the boiler, perform an efficiency test of the unit, etc.

Metropolitan Engineering, Consulting & Forensics (MECF)

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