Methane gas deflagration temperatures

[TonopahJoe]

The power plant I work at has four 170Mw combustion turbines that are supplied with 500 psig natural gas. Periodically the fuel gas pipeline and its associated heat exchangers and filter vessels need to be depressurized for maintenance purposes. Until I intervened recently, a large volume of this gas was vented off to atmosphere in a downward direction onto people and a number of potential ignition sources such as electric carts, pickups, motor starter contactors, lighting, etc., through a 2-inch line. I know this because I was one of the people who "felt the cool breeze" of the methane on my skin one night. But I had to fight an uphill battle to modify the vent lines to vent the methane vertically.

Unbelievably, workers and even management simply don't believe that a methane gas cloud will form in a relatively open area nor do they believe that it can ignite. Some even believe that the gas rises because CH4 "is lighter than air". It's not hard to find video and pictures of explosive gas accidents and disasters, but these guys can't see the connection so I'm trying to educate them with the facts so they don't blow themselves (and most importantly me) up in the process.

I know that the gas cools as its pressure drops increasing its density to where it will settle into low spots as it diffuses into the atmosphere, and that a deflagration (fire ball) can easily occur, but I don't know what temperature to expect if ignition occurs in the atmosphere. Anybody got a clue?

 

[CFDFEAGURU]

From http://en.wikipedia.org/wiki/Methane#Combustion

"Methane is only flammable in a narrow range of 5-15% percent [by volume] in air"

Thanks
Matt

 

[brewnog]

Auto ignition temperature is 580°C (obviouly in a concentration between the LEL and UEL). Adiabatic flame temperature (at constant volume) is 1950°C so in your scenario, you'll be looking at something less than that.

As far as fuels go, methane is a pretty safe gas. As your colleagues tell you, methane is indeed less dense than air, and does dissipate quite freely into the atmosphere (unlike propane and butane, which can be incredibly dangerous if allowed to collect in low areas). However, your installation must still comply with the gas regulations for your country (and it's insane for methane in any concentrations to be vented directly onto electrical components).

Report your concern to the nominated responsible person at your facility.

 

[Bob S]

Hello TonopahJoe-
Although methane can ignite in only a small range of concentration (see post #2), it can be very dangerous if it collects in an enclosed area. I have seen demonstrations by (I think Bureau of Mines) where a flame (shock wave) propagated in a long 4 inch diameter plexiglass tube filled with coal gas (methane). I have seen the damage caused by an accidental liqiud hydrogen spill which vaporized and collected under the roof of a physics laboratory, and blew the roof off. Even in an open area, like your situation, venting methane will have an ignitable concentration somewhere, and will create a shock wave when it ignites.
Bob S.

 

[TonopahJoe]

Gentlemen:

Thank you all for the information and validation. We stopped venting the CNG down onto equipment (after I brought in OSHA to convince my site management to stop this insane practice). Although our plant alone spends over $250,000,000 on CNG and has four large H2 cooled generators, we treat these gasses as if they're as harmless as air.

Not surprisingly, we had four deaths within 10 months of each other which resulted in the removal of our top management, replacing them with people who are absolutely committed to putting a stop to the fatalities and injuries. To the shock of many, my plant manager (and many others) thought of himself as a shoe-in for a new position as director of all the generating plants, but having twice pounced on me for bringing in OSHA--even after being told that I was not to be "stifled" in any way when it comes the pursuit of safety issues by the big cheese after the first time he pounced on me--the position went to a person who recently worked for him. Ouch.

That had a big impact on him and raised a lot of eyebrows, and suddenly others in leadership positions are jumping on the safety band wagon. It's about time. Top management has already acted on my request that we all be trained on explosive gasses, and I am gathering as much information as I can on the subject from enough sources to convince the non-believers that this stuff can really hurt you under certain circumstances.

I have a unique perspective: I witnessed a CNG explosion in one of our boilers during a "puff". The explosion tore the welds of 18 inch I-beam webs like tissue paper causing a hundreds of thousands to repair, plus lost revenue. I also know two people who were badly burned by natural gas in "accidents". They were the lucky ones who survived the incredibly painful burn treatments, keeping one of them out of work for over 2 years. While, I may not have much of a career path with this company, I do have a clear conscience. Thanks again.

 

[brewnog]

Good man, you seem have the right attitude.

Working (as I do) for a huge American-owned company, our work ethic now is "no job is so important that we can't take the time to do it safely". It can really slow things down at times, but it's for the right reasons and people are avoiding death as a result.