Vent ignition

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 [1].  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 (6.3.2.2).  It also notes the low likelihood of ignition by static, partly based on the experience of pipeline companies (6.3.4.1.4).

So I find this pretty convincing.

Having said all that, here is an extract from a Queensland government bulletin in May 2006 [2]:

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.

[1] API 521, Pressure-relieving and Depressuring Systems, 5th Ed., Jan 2007

[2]  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.

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16 Responses to Vent ignition

  1. Mark Coates says:

    Don’t forget the use of high velocity jet engines during the first Gulf war which used air velocity to blow the flames out of the oil wells – much quicker than explosives, and resulted in the fires being extinguished faster tha expected.

    On the subject of the Qld incident, the pipeline may have been at 8,000 kPag, but if the vent system was somehow fitted with an RO to control gas discharge, this could have given rise to a flame being supported. More information would be required to avoid speculation.

    Thanks

    Mark C.

    • petertuft says:

      Agree about the RO. Generally a BAD idea in many vent lines partly for this reason and partly because of the low temperature effects downstream. Better to get the pipe size right in the first place rather than oversize then choke it.

    • Victor Stanciu says:

      Do not forget about transient regimes before rising to sound speed and at the end of the blowdown where the speed of the jet goes below sound speed.
      Within the jet of gas the concentration cannot be less than LEL, it is well above UEL (too rich to burn). The gas will get diluted to LEL concentration further away from the source of gas.

  2. Warren King says:

    Peter

    I seem to remember an incident on the DBNGP many years ago. My memory is a little hazy these days but someone may be able to confirm or correct me here. On the original compressor stations there were 2 vent pipes close to each other. One was for venting the compressor and the other was for a full station blowdown. I think there was a thunderstorm at the time and I am not sure what happened but the compressor was shut down and vented via the small vent, A lightening strike ignited the small vent which caused an automatic emergency shut down of the whole station with the station being blown down. The small vent then ignited the main vent. I did not witness it but it was apparently quite spectacular. No one was injured that I am aware of.

    This may have been during commissioning – I am not sure.

    • petertuft says:

      Warren,

      I’ve heard about that incident too. However there is an important difference between a compressor station vent and a pipeline blowdown. A compressor station can trip and automatically vent regardless of weather conditions, including thunderstorms, exactly as you describe. Blowing down the linepack from a pipeline (which is what I was writing about but now realise I didn’t make that clear) is a manual operation and no operator in their right mind would discharge large volumes of gas when there is a risk of lightning.

  3. Anonymous says:

    I typically design the pipeline blowdown to have an actuated valve (with a manual bypass in case of in-operability). The actuator is energised by a pneumatic solenoid fed from a remote (20-40m) N2 bottle via flexible hose (appropriately restrained). This prevent the need for the operator to be anywhere near the vent in the (unlikely) event of a failure (fire or other) and there is a greatly reduce risk to the operator due to noise.
    It is somewhat expensive but I consider it effective and from feedback operator’s seem to like it.

    • petertuft says:

      That might work OK for quite small vents. However even for a DN 100 blowdown (such as is typical on a DN 300 pipeline) my very rough estimate is that the 4.7 kW/m2 distance is around 150 m and the 12.6 kW/m2 distance is around 90 m. And much more for larger vents. So a 20-40 m offset from the vent would still leave an operator very exposed if there was a fire. But if it helps with noise then there is value in that – after all, the noise problem occurs at every blowdown, unlike the ignition problem.

      • Anonymous says:

        Thanks for your comments regarding radiation distance, I had issues around that in my head so thanks for solidifying it. However, the separation distance may still be ok I think. In the event of a fire the operator simply closes the N2 cylinder valve and the flame goes out (fail closed actuated valve). Also, last time I blew down a line I put the N2 cylinder on the far side of a local embankment to provide natural shelter “just in case”. With the arrangement I describe the N2 cylinder could be just as easily put behind a control hut or behind a service vehicle. If all of this is too difficult then a 150m 1/8″ or1/4″ flex line will fit in the back of a car. Am I too cautious? Perhaps.

  4. Phil Venton says:

    Peter,
    There are two simple ways to assess this. The first is to attempt to light an oxy acetylene torch (or a propane equivalent) with the gas pressure set too high. The gas will usually not ignite, or if it does, it blows out. The second is a little more scientific, and that is to determine the flame velocity. Methane is a couple of feet per second ( I don’ t have the data with me). A flame cannot be sustained unless the fuel velocity is below the flame velocity (combustion science)

    Note that there a number of events where a station vent with a leaking valve has been ignited by lightning – a reason that installing a closure or blind flange on a pipeline vent is good practice.

    • petertuft says:

      Thanks – the gas torch is another good illustration of the principle.

      Laminar flame velocity for methane is apparently around 0.5 m/s. It can increase dramatically for turbulent combustion and if there are obstructions (equipment, buildings, trees) it can apparently go very fast indeed and approach or exceed sonic velocity (deflagration -> detonation). But apparently in unobstructed open space the increase is relatively modest, only a few tens of m/s which is going to be easily blown off by the sonic discharge. (At least, until the pipeline pressure and flow rate drops a lot.) There is some quite interesting information here: http://www.gexcon.com/handbook/gexhbchap5.htm

  5. Colin Bristow says:

    Peter
    Someone has been bugging the office here. Calculations have been carried out for radiation zones for the pipeline vents with safe distances between the valve and the vent stack. Fences are to be erected at a safe distance from the vent stack to stop the public wandering into the radiation zone. And don’t forget solar radiation. This and other “Good Practice” is producing so very expensive pipelines.

  6. Lynndon Harnell says:

    The other apparent reason for separation is the matter of the noise. From previous work done by myself in the 1980’s (unfortunately I did not keep a copy of this so it is from memory), and more recent field studies indicate an operator at nominal say 1 to 2m from vent will experience noise levels in excess of 140 dBa. At these levels you CANNOT wear adequate hearing protection. In the 80’s when we measured it, the operator would insert earplugs, fill the earmuff cavity with earplugs, put the muffs on and then duct tape them to the head otherwise they would vibrate off. Then they would operate the valve to start blowdown and run away to a tolerable noise level. It is a matter of bone (skull) conduction of the noise, not through the ears! Noise can be either muffled which results in quite large and expensive “silencers” for a 1 or 2 event frequency over the life of the pipeline, or the operator is separated from the noise by a suitable distance. Logically this can be by either running a vent pipe (which seems to be the current preference) or put an actuator on the valve (possibly portable), and remotely actuate it. Of course then the argument (myth) that the vent ignition will detroy all the plant around the vent kicks in, which is why I think the vent pipe is the option selected.

    The reason for RO’s is to get the flow to choke at the RO and hence avoid low temperatures before that. Otherwise if you have say a plug valve and then perhaps 100m of vent piping and then the vent, I think you will get a choke condition at the plug valve and then another at the vent tip, resulting in quite low temps in the vent piping and of course higher velocities.

  7. Adam Gifford says:

    Peter

    Although I was not there when the incident you refer to occured, I have spoken to people that were there. As I understand, the vent ignited when the valve had only been opened about 1-2 turns therefore the velocity was presumably not high enough to extinguish the flame. This highlights the potential issue with assuming that blowdown velocity will always be sufficient to ensure the flame wont be supported and probably a good reason to design the vent assuming that ignition is possible.
    From their investigations, the ignition source was thought to have most likely been static electricity from the metallic vent cap that was not earthed.

    • Anonymous says:

      Wow. Perhaps I should start specifying earthing straps on vent caps… Thanks for sharing.

    • Lynndon Harnell says:

      I refer to the following extract from API 521. Note the use of the expression “well-grounded vent stack”. In other words the API standard is recommending grounding. This in contrast to my experience where pipeline blowdowns are deliberately not earthed but float on CP potential.

      6.3.4.1.4 …….During high velocity discharges from gas wells to the atmosphere, static discharges are developed that are sufficient to cause sparks and ignition [78]. The condensate zone in the jet of well-head gas apparently tends to produce a high level of charge, although ignition does not actually occur. Another theory relating to static ignition proposes that gas flow through a piping system during venting induces a static charge on any solid or liquid particles in the pipe stream that contact the pipe wall. As the gas reaches the sharp edges of the vent outlet, static discharges can occur, either by complete electrical breakdown (spark discharge) or by partial electrical breakdown (corona discharge). There is a lack of documented information on the ignition of relief-valve vapour discharges attributed to the development of electric potential at the discharge point. The experience of pipeline companies (who customarily discharge natural gas to the atmosphere at low elevations) includes gas-gauge pressures as high as 6 200 kPa (900 psi) and discharge rates as high as 82 kg/s (650 000 lb/h) from a single vent stack. The probability of ignition by static electricity is, therefore, very low because of a relatively weak charge build-up in the jet and reasonable isolation from the well-grounded vent stack.

  8. Pingback: Vent ignition follow-up | Pipelines OZ

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