For approximately twenty years, home-use propane cylinders have been equipped with valves that contain six very important safety features:

1. Gas service valve to manually open and close the flow path from cylinder to appliance.
2. Pressure relief valve which vents a small amount of gaseous propane if the internal cylinder pressure exceeds a safe value (to prevent catastrophic cylinder rupture).
3. Bleeder valve (+ dip tube) to provide visual indication (white mist) during filling when liquid propane level inside the cylinder reaches its safe upper limit (i.e., overfilling is imminent).
4. Overfill protection device (OPD) which functions to halt liquid inflow to a cylinder when liquid propane level inside the cylinder reaches its safe upper limit.
5. Automatic shutoff valve which prevents gas flow out of the cylinder when the hose to the appliance is not fully connected to the cylinder nozzle.
6. Excess flow valve which halts propane flow out of the cylinder when the flow rate is excessive (e.g., after a catastrophic hose failure).

This author has investigated multiple cases where propane releases and flash fires were caused when the safeguards failed to function as they were intended.

The underlying hazards that create the need for the safeguards are:
1. Gas service valve. The user should be able to exercise control over the flow of propane to the appliance.
2. Pressure relief valve. The cylinder may be subjected to excessive internal pressure for either of two reasons: (a) if the cylinder is overfilled at room temperature and is later exposed to mildly higher temperatures (+25 degrees F or higher), the propane liquid can expand and create unsafe internal pressures; or (b) if the cylinder is filled to a safe level (i.e., approximately 20% vapor space above the liquid) and is later subjected to excessive heating from a fire, the liquid+vapor mixture can reach an unsafe pressure. In either case, the pressure relief valve’s job is to reduce the internal pressure by venting a small amount of the propane vapor until the internal pressure declines to a safer level. This safeguard helps prevent internal pressures that can cause the cylinder to rupture catastrophically.
3. Bleeder valve (+ dip tube). The bleeder valve and dip tube help the filling attendant visibly see when the liquid propane level in the cylinder has reached its maximum safe level (see 2 above). When the tiny stream of propane exiting the bleeder valve during filling changes from transparent vapor to a white mist, the maximum safe liquid level has been reached and the attendant is warned to stop the filling operation immediately.
4. Overfill protection device (OPD). The OPD is an automatically-actuated valve that is intended to achieve the same outcome as the bleeder valve (i.e., prevention of overfilling the cylinder with liquid), albeit without human intervention. The OPD comprises a toilet-tank style float inside the cylinder that closes off the flow path for liquid to enter the cylinder from the filling pump when the desired safe liquid level (see 2 above) has been reached.
5. Automatic shutoff valve. The Automatic shutoff valve (ASV) is a poppet-style valve that is intended to prevent the flow of propane vapor out of the cylinder until the connector nut has been fully tightened to a gas-tight sealing condition. In principle, an engagement tab inside the connector doesn’t engage the ASV’s spring-loaded isolation component until a satisfactory seal is made between the cylinder nozzle and the threaded connector nut. This is a feature that prevents gas flow out of the cylinder if the service valve is accidentally opened during storage or at any time before the appliance connection is made.
6. Excess flow valve. The Excess flow valve prevents or minimizes propane flow under circumstances where the connector is properly sealed but a downstream component (e.g., hose) fails in a way that fuel may be released to the environment instead of the appliance burner. The feature that carries out this function is a spring-loaded sphere that is normally positioned to permit a normal flow of propane vapor but is relocated to a position where the flow orifice is obstructed by the sphere when the drag force on the sphere exceeds the spring force (i.e., when flow velocity is high).

The subject safeguards (applicable to 20 lb propane cylinders) are described and specified in the CGA V-1 document “Standard for Compressed Gas Cylinder Valve Outlet and Inlet Connections”, specifically under connection style CGA 791.

While there is no doubt that the CGA 791 design has prevented numerous propane releases and injuries since it was first promulgated in 2000, this investigator believes it is inferior in one important way to the CGA 600 design that is currently approved only for small (1 lb) propane cylinders. The inferior feature involves the sealing geometry (radial versus axial) in the two designs, and the 791 design inherently provides a lower level of sealing certainty than the 600 design.

This investigator has opined that the 791 design’s inferior feature constitutes a safety defect if the resultant sealing inadequacy leads to a dangerous release of propane that causes personal injury or property damage.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information.

The California Mechanical Code (CMC) is one of thirteen parts of the California Building Standards Code that is adopted into law every three years by the California Legislature. The other 49 U.S. states adopt similar safety codes, which generally include the following titles (preceded by ): Building Code, Electrical Code, Fire Code, Mechanical Code, Plumbing Code, Residential Code, etc.

One of the primary objectives of the mechanical code is to help ensure that heating, ventilating, and air conditioning equipment installed in buildings are designed, operated, and maintained safely. Many HVAC systems utilize natural gas, which is highly flammable and can cause explosions.
This author investigated a natural gas explosion (also called deflagration, or subsonic combustion wave) that was caused by a series of maintenance errors, a heater malfunction, and a major code violation.

The maintenance errors caused natural gas to be released into a heater room, the code violation permitted the natural gas to accumulate in the room instead of being safely vented outdoors, and the malfunction permitted the heater to re-start automatically in an unsafe state, which ignited the explosion. Two workers received serious burn injuries from the incident, but the explosion wasn’t strong enough to damage the building.

It was difficult to rank the errors and defects according to their level of egregiousness, but the worst one was undoubtedly the combined design defect and construction defect associated with the building that housed the gas-fired heater. The heater was located at ground level inside a 23-foot tall enclosure construction from concrete masonry units (i.e., cinder blocks). The architect was responsible for the defective design, which contemplated heating equipment inside the room but didn’t incorporate the code-required ventilation area. The general contractor and appliance installer were responsible for allowing the heating equipment to be installed in the room without the proper ventilation.

The CMC requires ventilation at the top of any enclosure that houses gas-fueled appliances. The purpose is to vent natural gas (which is lighter than air) in the event of a substantial release of gas into the indoor space. Allowing flammable gas to accumulate in an enclosure is the first step in the process of forming an explosive device that lacks only an ignition source to turn into a horrific fireball or a destructive blast wave. The subject room was well sealed along the upper 75% of its height but was equipped with a louvered door at the bottom that effectively allowed combustion air into the room to supply oxygen for the heating appliance. Combustion products from the heater were vented directly to the outdoors by an electric blower, and the replacement air entered through the door louvers.

When the gas pipe developed a leak (the facts weren’t entirely clear about the size of the leak was or how it began), the gas rose to the ceiling and accumulated there, displacing the air below it to the outside environment through the door louvers.

This author performed a Large Eddy Simulation (LES) of the gas accumulation phase which showed the steady-state fuel gas concentration in the upper three-quarters of the room to be substantially greater than the Upper Flammable Limit for natural gas (approximately 15 percent by volume). This fact turned out to be the sole reason the building didn’t explode – a large fraction of the fuel gas present in the room had accumulated in zones that were too rich to burn (not enough oxygen present).
Nevertheless, when the employees were instructed to enter the room and shut off the gas to the heater, their motion created a flammable zone in some portion of the lower 25% of the room’s volume. When the defective heater ignited the flammable mixture as they were exiting the room, the fireball that was created pushed flames out through the open door and burned them badly as they tried to escape. Thankfully both survived.

Posted below are two videos showing the LES simulations for Case 1 – as installed without any venting at the ceiling, and Case 2 – as required by code, with a code-compliant opening of only 150 square inches of flow area at the top of the heater room. Case 1 shows high gas concentration (red) from ceiling down to the top louver of the entry door when gas is flowing and no significant dissipation after the gas source is shut off. Case 2 shows a temporary accumulation of moderate gas concentration (green) until the gas source is shut off, after which full dissipation occurs through the upper vent. The simulation runs approximately 24x faster than real time.

This gas accumulation simulation (along with testing of the defective heater, timeline analysis of witness testimony, and plumber standard of care analyses) helped the parties reach a resolution in this case.

Some products/activities are inherently dangerous (e.g., cigarette smoking, skydiving) and yet some people enjoy them and pay good money to partake of them. In most cases, the more dangerous the activity, the fewer people choose to engage in it. One notable exception is the hot beverage – a potentially hazardous product that is enjoyed by millions.

Numerous studies have shown that customers will reject a cup of coffee or tea when it is served at a temperature that won’t scald them if accidentally spilled. Scalds can occur at temperatures as low as 140°F if the contact time is long enough and temperatures above 150°F will almost certainly cause 1st degree burns or worse if a sufficient quantity contacts the skin. Such details notwithstanding, a large percentage of consumers will reject hot beverages for being “lukewarm” if the temperature doesn’t exceed 155°F. In fact, the average coffee temperature preferred in these studies is approximately 185°F and the average preferred tea temperature is above 200°F.

Furthermore, the chemical process associated with brewing coffee or steeping tea is more effective when the water is as hot as possible. More essential flavors and fragrances are extracted from the beans and leaves when the water is close to boiling. This is why tea kettles “whistle” (to ensure the water has reached a rolling boil before pouring into the teacups) and espresso machines “hiss” (to announce the conversion of pressurized liquid water to steam as it migrates through the coffee grounds).

To be fair, these studies also show that the optimum temperature for a beverage to be “consumed” is considerably below 212°F, but customers invariably realize that if they “receive” the beverage at a temperature that is well above optimum, it will reach its “ideal” temperature within a few minutes. On the other hand, the 2nd Law of Thermodynamics ensures that if a beverage is “received” below its optimum temperature, no amount of time will prevail upon it to absorb heat from its surroundings and approach a more desirable (i.e., hotter) temperature.

The other factor that makes hot beverages a dangerous product that consumers willingly enjoy is their ability to mitigate the risk of drinking them. They can slowly sip or aerate the liquid, thus preventing an injurious amount of heat from being transferred into the mouth tissue, even though the temperature is high.

It is worth noting that the term “safe” should never be construed to mean “without risk”. Most people consider driving a car to be a “safe” activity even though they are fully aware that tens of thousands of motorists die each year in collisions. On the contrary, “safe” simply means that the level of risk is “acceptable” to the population who engages in that activity or uses that product. Consumers believe that drinking coffee is “safe” because the benefits of consuming a great cup outweigh the risk of serious injury if they happen to spill it.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information. Copyright Martin Thermal Engineering, Inc. (2016)

When a consumer product fails thermally, customers may get “steamed” and demand their money back.  When the failures are frequent enough that the Consumer Product Safety Commission receives dozens of complaints about “melted plastic” and “first degree burns” a few weeks after the initial launch of the product, they may require the seller to pull the offending product from retail shelves and issue a “safety recall” notice to all consumers.  If you consider a product that is being sold at a rate of 100,000 units per month, it is easy to see how quickly the recall costs could add up.

However, the matter could become even more problematic if the supply chain involves multiple entities (e.g. a product designer, a contract manufacturer, and a marketing entity).  When the recall costs are tallied up, the manufacturer and designer could find themselves in a legal battle to determine whether the thermal failures were caused by “design defects” or “manufacturing defects”.

One particularly challenging aspect of an engineering failure investigation is to understand why only a small percentage of all the shipped products fails prematurely.  By carefully examining the failed units, an engineer may be able to identify the correct failure mode(s), but inspection alone likely will not be sufficient to determine whether the root cause was a bad design or low quality manufacturing.

After the failure mechanism is identified (e.g., loose connection or excessive current draw) the engineer should examine and test a large number of “new-in-box” units to see if there is a correlation between parts that are “out-of-spec” and parts that fail when used normally.   If brand-new parts meet the dimensional and functional requirement of the design, it’s pretty obvious that the design wasn’t adequate to prevent the overheating.  On the other hand, a finding that many of the parts don’t conform to the design dimensions (and other requirements) doesn’t definitively prove that manufacturing defects were the cause of the safety problems.

In a recent investigation of a recalled consumer electronic product, this author discovered that 80% of “new-in-box” samples did not meet the design specification…but less than 3% of the samples failed thermally when first used.  Tellingly, we also found that 9% of the samples were not only “out-of-spec”, but “grossly-out-of-spec” and that each of the samples that failed thermally fell into the “grossly-out-of-spec” category.  (Conversely, none of the 20% of the “in-spec” parts failed when used normally, which provided validation that the design was adequate.)

Using “statistical inference” we concluded that it was virtually impossible (48 chances in a billion) for all of the failed samples to come from the “grossly-out-of-spec” population if only random forces were at play – hence there must be a “causal link” between the “grossly-out-of-spec” condition and the thermal overheating result.  Statistical methods proved extremely helpful in illustrating that the manufacturing “nonconformances” were indeed the “defects” that caused the safety recall!

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations.  We hope you are instructed by this content, and we encourage you to contact us if you seek additional information.

Portable cookstoves are used by caterers and campers to bring the convenience of a “kitchen appliance” to places where such appliances are normally not available. Some cookstoves are fueled with liquefied propane that is supplied in 1-lb canisters (first photo) or 20-lb containers (second photo). Others utilize liquefied butane that is supplied in aerosol-like cans (third photo). While all these systems are equipped with unique safeguards, certain types have failed catastrophically, injuring workers and guests.

Cookstove Safeguards. Butane cookstoves (example in fourth photo) are generally equipped with a multi-function gas safety valve. When the fuel can is installed, a lever must be pressed to latch the can into place so that the gas will begin flowing through the valve to the burner. The valve also has a pressure-safety pin that is designed to trip the latch and disengage the can when the fuel pressure exceeds a safety threshold. However, in some models, the retracting mechanism has been observed to fail intermittently and the can fails to disengage even though the pressure-safety pin performed as it should.

Can Safeguards. Small butane fuel cans are also equipped with certain safety features that are designed to prevent or mitigate catastrophic releases of flammable gas. Because the fuel cans are installed horizontally into the cook-stove chassis, the fuel withdrawal tube is equipped with a right-angle extension to ensure that butane vapor is withdrawn from the headspace, rather than butane liquid from the lower portion of the can. The neck of the can contains a feature that is intended to prevent improper rotation in the stove, but users can unknowingly defeat this safeguard and install the can in the wrong orientation, permitting butane liquid to be withdrawn instead of butane vapor, which is undesirable.

This author has investigated several flash fires involving portable cookstoves and has identified product defects as well as certain ways the products can be misused. In the worst-case scenario, the fuel canister overheats and ruptures, releasing a fireball that causes personal injury and property damage.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information. Copyright Martin Thermal Engineering, Inc. (2013)

Most people know that water boils at 212°F (100°C) at sea level. They also know that if the pot is open (i.e., not a pressure cooker), the bubbling, steam-water mixture will not exceed 212°F. While this observation is generally true, there is another aspect of boiling that many people are not familiar with – “superheating”. In order to form a steam bubble in a pool of liquid water, the water temperature must actually exceed 212°F in a thin film near the heated pot bottom. This “superheating” phenomenon is usually limited to a few degrees at most, and generally diminishes to 0.1°F or less when vigorous boiling begins.

This superheating is the driving force for the rapid phase-change that is called boiling. Without excess thermal energy available in the liquid molecules, their conversion to gas is a slow process – evaporation. Evaporation occurs only at the upper surface of a hot liquid when individual molecules are sufficiently energetic to break the weak intermolecular bonds they share with their neighbors and travel into the air space above.

In contrast, bubble formation involves a huge number of molecules simultaneously. Because of the excess energy of the molecules in the superheated film near the heated pot bottom, they can expand and change phase (from liquid to gas) very rapidly. Thus, the rate of vapor formation in bubbles is many times higher than the rate of vapor formation by evaporation alone. When the vigorous motion of the rising bubbles begins, the superheated liquid in the film and the balance of the 212°F liquid above it in the pot are very effectively stirred together and we measure an average temperature of 212°F essentially everywhere in the pot.

By contrast, when a container of water is heated in a microwave oven instead of through the wall of a pot, bubble formation in the film next to the pot bottom does not occur. Accordingly, dangerous amounts of energy can be deposited into the interior of the water mass creating a large body of superheated liquid water. If superheated liquid water accumulates in the vessel, it can expand explosively when a “nucleation site” (e.g., a fork or dry food particle) makes contact with the energetic liquid.

This author investigated one such injury-causing explosion involving an 8-cup container of homemade chicken soup heated in a microwave oven. Unfortunately, the water was heated about twice as long as it should have been, and it vaporized explosively when a fork was inserted, scalding the woman who was simply trying to spear a piece of chicken. Recommendation: Don’t heat water for long periods of time in a microwave oven and do check it frequently to be sure it doesn’t approach the boiling temperature.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information. Copyright Martin Thermal Engineering, Inc. (2013)