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% |
Source: EGIG Pipeline Incident Database. Failure data collected from six European gas distributors for the period 1970-2001.
For leaks resulting from guillotine-rupture of a natural gas pipelines 16″ or higher, every forth 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 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.
