MBB FINAL: UNC 78, UCLA 56 – CLICK HERE FOR RECAP
By Jeff Danner Jeff has worked in both the chemical and biotech industries and is the veteran of thousands of science debates at cocktail parties and holiday dinners across the nation. In his Common Science blog, Jeff aims to make technological and scientific concepts accessible to all.

Brazil and BBQ

By Jeff Danner Posted June 29, 2014 at 9:19 pm

I did not publish a column for the last two weeks because my family and I were in Brazil attending a few World Cup Soccer games. For this week’s column, I want to share a few non-science thoughts about our Brazil experience, and also tell you about a safety incident that occurred at my house just before we left town.

Much of news coverage here in the U.S. about the World Cup has focused on protests by Brazilian citizens regarding the expenditure of vast sums of money on a sports tournament in a country with so many other needs, as well as safety risks for tourists during the tournament. I’d like to provide you some first-hand insights of experiences in these areas.

There was a protest march in Fortaleza while we were there. What we observed was exactly what one would expect here at home. The protesters marched, held signs, and chanted slogans. Their rights to free speech and free assembly were respected and their interactions with the police were orderly and respectful. Except for it being in Portuguese rather than English, it could have been a Moral Monday march, except the Brazilian police didn’t arrest any of the protesters.

As for safety, the generous citizens of Brazil as well as the police officers were looking out for everyone and helped to create a secure and welcoming environment. For example, we had to catch a bus from a soccer stadium back to our hotel and were a bit confused. Two Brazilian women took notice of us and asked where we were trying to go. A couple of minutes later we saw one of them running through the crowds, risking missing her own bus, to find us and make sure we got on the right bus. I could list many other similar examples. It was a great trip and we felt secure and welcome the entire time.

With the travelogue completed, let me tell you about the first time I had to call the fire department since moving to Chapel Hill 14 years ago. It was the evening of the Belmont Stakes, and in-laws were over for a BBQ. I lit the propane grill on our back deck to let it warm up and went back inside to watch the horse race. When I checked on the grill about 15 minutes later, I noticed that something was wrong – really wrong.

The black insulation on the side of the grill cover had developed large blister-like bubbles. I checked the temperature gauge and it read 700ºF! I grabbed one of the valves on the front of the grill to try to start turning off the propane, and found that it had melted and no longer functioned. At this point, I decided it was time to call for professional help.

I quickly went inside, grabbed my phone and told everyone, calmly but firmly, that they needed to get up and head out the front door. Once we were out of the house and away from the overheating grill, I called 911. The dispatcher was calm and clear and stayed on the line with me until I could see the fire trucks coming. The firemen were there in under five minutes, donned their gear and stopped the fire by shutting the valve on the top of the propane tank, which was located in the cabinet below the grill. I had considered attempting this myself before calling 911 but (correctly, I think) decided that getting everyone out of the house and letting the firemen assess the situation was the right way to go.

I want to thank both the 911 dispatcher and the fire department for taking care of us when we needed them. If ever you wonder why we pay taxes, this is why.

So what went wrong with the grill? I can’t be 100% sure, but let me lay out the most likely scenario. The pressure inside of a propane tank is between 100 and 200 pounds per square inch (psi). When you connect the tank below the grill there is a small circular device in the hose. This is the regulator. Inside the regulator are a plastic diaphragm and a spring. The spring is adjusted such that the outlet pressure from the regulator is approximately 0.4 psi, significantly lower than the pressure inside the tank. Once the pressure is lowered to 0.4 psi by the regulator, the propane gas flows to two or three valves which are controlled by the dials on the front of the grill and then directed to the burners. Along the burners there are many little holes and out of each comes a small flame. The temperature inside of the grill is a function of the temperature of the small flames, the flow rate of propane from the tank to the burners, and the heat loss from the grill to outside air.

Depending on the purity of the propane – it contains some other hydrocarbons such as ethane and butane – and the fuel-to-air ratio, the temperature of each of the little flames can be as high as 1900°F. Even though the flames are very, very hot, since they are small, the heat given off by one of them is limited. When everything is functioning properly, the heat given off from the flames in conjunction with the heat loss from the grill results in a temperature within the grill of approximately 300 to 450°F.

The most likely scenario which resulted in my grill overheating was that after 14 years of reliable service, the regulator in the feed line failed, probably due to a broken plastic diaphragm. Therefore, the pressure of propane being supplied to the burners was much higher than the intended 0.4 psi. As a result, the flow of gas to the burners was much higher, resulting in the dramatic increase in temperature.

At the point I called 911, there were two key risks on my mind: a house fire and an explosion. My grill is on a wooden deck and near to some vinyl siding, so there was a realistic risk that the overheated grill could have started a fire on the deck itself. More than 1,000 home fires a year in the U.S. are started by similar circumstances. There are also occasional reports of propane grill explosions. Although I was concerned about this possibility at the time, my circumstances were not likely to lead to an explosion. The propane tank, supply line, and regulator housing are all made of steel which can easily withstand a temperature of 700 ºF, so there was no reason to expect a leak to develop. Without a leak in the supply line, it is hard to construct a scenario which could lead to an explosion.

The root cause for this safety incident is clear: I operated my grill for 14 years, well past its intended service life. The manufacturer supplied a gas regulator which functioned perfectly for a long time, so I cannot ascribe any blame to them. I have since learned that you should not operate a propane grill for more than 10 years, so I recommend you think about how old yours is and join me at the grill store if it is too old. As I am in the market for a new grill now, please email me at commonscience@chapelboro.com if you have a recommendation. It’s good to be back.

Have a comment or question? Use the interface below or send me an email to commonscience@chapelboro.com. Think that this column includes important points that others should consider? Send out a link on Facebook or Twitter.

Comments box goes here.

Leave a Comment

Recent Articles

More Lifestyle