Following the release of natural gas, it can be ignited resulting in fire which in turn can potentially result in an explosion. So how likely will a gas release ignite?If you are interested in the major failure modes for natural gas transmission pipelines reported, please refer to an earlier post – External Damage: Number One Cause Natural Gas Pipeline Releases.The ignition probability is related to leak size. Let us consider three hole sizes consistent with European Gas Pipeline Incident Data Group (EGIG) database – small (pin-hole leak, 2-cm or less), medium (2-cm < Leak < pipe diameter) and large (full bore rupture, FBR). The table below summarizes ignition probabilities from EGIG report.
Ignition Probabilities following Natural Gas Release
|Leak Size||Ignition Probabilities|
|Pin-hole leak (<2-cm)||3%|
|Medium Leak (2-cm < Leak < Pipe Dia.)||2%|
|Rupture (Pipe Dia. < 16-in)||10%|
|Rupture (Pipe Dia. > 16-in)||25%|
For leaks resulting from guillotine-rupture of a natural gas pipelines 16″ or higher, every fourth release will ignite. Surprisingly, ignition probability for medium releases is lower than that for small releases based on this data. There is a wide variation in ignition probabilities depending on pipe size. So to say that the average ignition probability for natural gas release is 4% (average of ignition probabilities in the above table) is significantly under-predicting ignition probability for releases from larger pipelines.