Vent ignition follow-up

The previous post certainly generated discussion – a total of 10 useful comments (so far, and not counting my responses).  Thanks to all those who contributed.  I started replying to individual comments but it became a bit overwhelming so this post is an overall response.

The most gratifying feedback was that the Queensland vent ignition incident occurred when the vent valve was only just opening and hence the flow was not a full bore sonic discharge.  So the arguments that high-velocity venting cannot sustain a fire are not refuted by this incident.  I had been just slightly concerned that, despite the strong evidence, some bizarre combination of events might have allowed ignition of a sonic discharge.

However there is still the problem of possible ignition during reduced flow before the valve is fully open and also in the later stages of the blowdown when pipeline pressure has dropped.  Some comments mentioned static as the source of ignition, and the fact that mainline valves are generally not earthed because they float at the CP potential.  There is a simple solution to static – before opening the blowdown valve provide a temporary connection from the vent to the site earth grid.  It would be trivial to include provision for that in a new design and it shouldn’t take much effort for an existing installation either.

Other comments talked about valves operated semi-remotely with the aid of pneumatic actuators, nitrogen bottles and long hoses.  I remain doubtful that this is necessary as to protect operators against ignition but it may be justified to protect them against the serious issue of bone-shuddering noise.  And a fail-closed actuator might provide you with an extra level of comfort if you remain concerned about ignition.  Fitting every blowdown valve with all this kit would be expensive but a single portable unit might be a reasonable approach.

There was also some discussion, drifting away from the main topic, of restriction orifices in vent lines.  I don’t like them partly because they may reduce the velocity at the vent opening to the point where a flame may be sustained and partly because they result in severe chilling within the vent piping which then needs more exotic materials.  A comment was made that an RO will fix the position at which choked flow occurs and hence control where low temperatures are generated.  However a vent line that is essentially unrestricted will choke at only one position – the opening at the end.  All the “cold” will be generated in the open air beyond the vent and there is no need to select piping materials for low temperatures.  A valve with a straight-through flow path (such as a ball or plug valve) is not sufficient restriction to create a second choke within the piping.

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10 Responses to Vent ignition follow-up

  1. Tom Prezwanski says:

    Hi Peter,
    The RO at the end of vent stack looks like a very good idea.
    I am currently working on the pipeline, which cresses a number of the European countries and found out that despite the common EU standards, each country has its own statutory requirements for design of the pipelines. In each of the four countries there are different requirements for venting of the gas. The most common is a limit of volume (mass) of the gas vented per hour.

    There are specific requirements for the sizes, distances and locations from vent valve due to noise and heat radiation if the gas ignites. Some countries require temporary vents other permanent. One country requires flaring of the vented gas.

    The use of a mobile compressor to transfer some of the gas to another section of the pipeline instead of venting has a very strong support from the environmental groups. If the pipeline pressure is reduced by 50% the venting velocities would be significantly reduced. I understand that the size of the vent can be reduced, but this is not allowed, as sometime the compressor may not be available.

    Regards,
    Tom Prezwanski

    • petertuft says:

      Tom,
      The best solution is no RO at all, just size the vent pipe so it gives you about the right vent rate. I realise that might not be possible if you have tight limits on the allowed discharge rate or variable conditions.

  2. Anonymous says:

    “Fitting every blowdown valve with all this kit would be expensive but a single portable unit might be a reasonable approach”
    I like this idea, thanks. A portable actuator able to be bolted to the blowdown valves would be cost effective on long, multi-segment pipelines, particularly if the blowdown valve is large in diameter . As my recent projects have not had more than two pipeline segments this cost reduction method had not occurred to me. Thanks again.

  3. Nathan Biggins says:

    Peter,

    Would it be possible if you could clarify the requirement for third party design verification with respect to Pressure Equipment.

    Regards, Nathan

    • petertuft says:

      Nathan,
      Sorry, not really my area. AS 2885 says nothing about verification of piping and equipment design but I think that’s not what you are asking about, and I’m not going to pretend to have in-depth expertise in the piping and pressure equipment standards.

  4. Chris Hughes says:

    When we were designing the SEAGas pipeline we considered ways to remotely actuate the blowdown valves (for noise, not ignition) and ended up with a portable hydraulic unit which attached to the valve and allowed the operator to be sitting behind his ute 20m or so away. I never actually saw these units, either in store or in operation, and it would be good to hear from someone at SEAGas as to whether they are effective.

    When I started in this industry it was common practice to attach a temporary earth wire to a vent stack before blowing down to drain off any static build-up, and vent stacks were provided with a welded on lug for clipping the earth wire to, together with an adjacent earth stake.

    • petertuft says:

      Thanks Chris. It would be interesting to know whether other pipeline operators include earthing the vent stack as part of the blowdown procedures. I must admit that despite having designed a few I don’t think any included an earthing lug, so my suggestion about that seems to have been reinventing the wheel.

      Interesting the way the experience and history gets lost (but we know all about that, hence the APIA Pipeline Engineer Training project).

  5. Mike Peoples says:

    Chris/Peter

    SEA Gas did use the remote actuation method for a short mainline blowdown (around 10km). The actuator was a pneumatic torque wrench with custom mounting plates, clamped to the vent stack , and with a machined adaptor made to fit over the centre hub of the hand wheel on the vent stack plug valve. The air regulator was set to a low pressure to make sure the wrench couldn’t wreck the gearbox at the end of the stroke. We were able to operate the setup from about 40m away, but with longer air lines this distance could be much longer. The only drawback was that the torque wrench took around a minute to get the valve fully open.
    The noise was still bone shaking but probably significantly less harmful than standing right underneath the stack.
    Also from my observation of a few blow downs the only low temperature area appears to be at the vent tip, backing up your comment that choke occurs only in this area.

    • Lynndon Harnell says:

      I also worked up a design for remote portable actuator in the 80’s when at TPA. It comprised an air motor driven by gas from the pipeline (through regs and lubricator), with solenoid actuated shuttle valve. We could then retreat to a suitable distance, unreeling an electrical cable that triggered the solenoid. This day and age you could probably do this with Wi-Fi or Bluetooth.

      Btw, there is actually (as measured) a “cone of (relative) silence” directly under the blowdown (still pretty noisy), and as you move away, at a approximate 45 degree angle of incidence (e.g. ~3m), the noise increases, and then reduces with distance as per the calcs.

      A compressor to transfer some of the gas to another section of the pipeline instead of venting at a reasonable rate I think is going to be very expensive due to the constantly changing dP it has to accomodate.

  6. Iain McG says:

    Worthwhile discussion filled with useful facts and experience.

    I’ve made reference to the API STD 521 data myself but feedback from Safety & Risk types that I’ve come across over the last year or so is that there are other references to consider. The International Association of Oil & Gas Producers publish some relevant references that are well regarded, quantitative and are based on recent field data and relevant theory.

    http://www.ogp.org.uk/publications/safety-committee/qra/risk-assessment-data-directory-ignition-probabilities/
    http://www.ogp.org.uk/publications/safety-committee/qra/risk-assessment-data-directory-process-release-frequencies/

    Note the influence of location and vent rate on the ignition probability in OGP Report No. 434 – 6.1, Data Sheet 1, Scenarios 3 & 4. Worthwhile being aware of this data source to provide an alternate perspective to the discussion.

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