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.