Around Australia there must be hundreds of blowdown vents on gas pipelines. With a few minor variations the standard design is a short vertical pipe, maybe 2.5 m high, with a removable closure on top and a plug valve to open the vent. The operator stands right at the vent pipe to operate the valve (and wears extra heavy duty hearing protection). Around the world I suspect that there are many thousands of similar installations. It could be described as very well-established practice.
And yet in the design phase of many (most?) new gas pipeline projects there are painful debates about the possibility of the released gas igniting. Of course, if it did ignite the operators would almost certainly die because the gas release rate is huge and the thermal radiation distance is also large (not as bad as for the full bore rupture case but substantial all the same). But can a pipeline blowdown vent ignite?
Almost all the evidence suggests not. The gas emerges vertically at sonic velocity. There are no ignition sources in the open air above a vent, other than perhaps static (or lightning, but who’d blow down in a thunderstorm?). Even if an ignition source was somehow introduced, the gas velocity (sonic) is vastly greater than the flame velocity so the flame would simply blow off and extinguish. (Sonic velocity is a few hundred m/s; my hazy recollection of a long-ago combustion engineering course is that flame front velocities are a few m/s at most.)
I have heard two separate anecdotes about deliberate attempts to ignite a (small) blowdown for test or demonstration purposes. One involved a flare pistol, the other an old-fashioned kerosene blowtorch. Both successfully demonstrated the principle outlined above – the flame is blown off and instantly extinguished. In an inverted application of the same principle, after the first Gulf War when Iraq invaded Kuwait and set fire to numerous oil wells the fires were extinguished with explosives that blew the fires out. And closer to home, every child with candles on a birthday cake finds out a little about the practical effect of a high flow velocity on a low flame velocity.
All this is hand-waving argument. API 521 takes a more serious engineering approach, particularly in Section 6.3 on atmospheric discharge . It makes clear that for high-velocity discharges the concentration is gas is reduced to below the lower flammable limit well within the jet-dominated portion of the discharge stream (18.104.22.168). It also notes the low likelihood of ignition by static, partly based on the experience of pipeline companies (22.214.171.124.4).
So I find this pretty convincing.
Having said all that, here is an extract from a Queensland government bulletin in May 2006 :
A meter station operator was ready to commence exporting gas into a pipeline when the pipeline operator had a view that the gas was still too wet and requested the meter station operator to vent gas from the line until it met with approval. The venting of gas commenced through a two-inch vent located approximately 30 metres from the pipeline infrastructure. The line being vented was at an operating pressure of 8000kPa. After venting commenced, ignition of the vented gas occurred. The local fire service was called and depressurisation of the pipeline commenced. When the pipeline pressure had fallen to 2000kPa, the vent valve was closed and the fire extinguished. The vent isolation valve was sprayed with water to preserve it, allowing it to be used to stop the venting gas. Damage was confined to a pressure gauge on the line and ignition was sufficiently distances from the pipeline to prevent damage. A project is to be established in an attempt to recreate the circumstances of this incident, which may have an impact on venting procedures throughout the industry.
A while ago I tried to find out more about this but without success. I have no idea whether the investigatory project mentioned in the last sentence proceeded.
Both the hand-waving and API 521 arguments about the safety of atmospheric discharge are valid if the gas is discharging at very high velocity. However ignition seems possible if the gas is wafting out at a relatively low velocity. There is a double whammy because under these conditions the jet-induced dilution to below LEL is not effective and the resulting combustible mixture may more readily be blown by wind to somewhere that it may find an ignition source. On the other hand, gas emerging at low velocity also has low mass flow and will result in a much smaller fire and generally reduced hazard.
It would be nice to think that in the case of the Queensland incident the vent flow was throttled so that the actual discharge velocity was low, notwithstanding an initial pipeline pressure of 8000 kPa. That would provide a satisfactory explanation of this incident despite all the other reasons that it should not have occurred.
On the other hand, if would be very alarming indeed if it ignited despite discharging unthrottled at sonic velocity. That would have serious implications for operating procedures and hardware at nearly every mainline valve in the country
Maybe someone reading this can shed more light on what actually happened.
Thanks to Lynndon Harnell for reminding me of this long-standing debate and for pointing me to the information in API 521.
 API 521, Pressure-relieving and Depressuring Systems, 5th Ed., Jan 2007
 I have a copy of this but can not find it on the internet: Queensland Petroleum and Gas Industries, Summary of Serious Accident and High Potential Incident Reports, May 2006, Qld Dept of Natural Resources, Mines & Water.