Snuffers for PV valves on Chemical tankers
Feedback from some PV valves Makers that their valves act as both flame arrestors and impossible for backflash to occur.Appreciate to receive comments from fellow chemical tanker operators on the use of N2 snuffing system on PV valves fires. Would it be an overkill?
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In the absense of inerted tanks I would personally prefer to see N2 snuffers in place.
The tragic incident on the Bunga Alpinia in July 2012 may have been averted if N2 snuffers had been used.
After lightening struck vent stack and caught fire, loading was stopped and tank pressure started to drop to point when Hijets would close. A flame path was created for a fraction of a second during Hijet closing when pressure was sufficiently low. Result was explosion and loss of life.
Initial explosion ignited another vent stack with a repeated explosion. The second explosion was captured and posted on youtube. Frightening: http://www.youtube.com/watch?v=zE_i5lcgdy0
If they are High Velocity Vent Valves (HVVV’s) then the exit velocity is far greater than the open air flame speed and so flashback cannot occur (if they are working properly). They will have to be ‘type tested’ to prove themselves. There will be a flame arrester on the vacuum side as there will be a lower level influx velocity if the tank goes into vacuum. Not sure that a snuffer system would be warranted, although they are fitted to mast risers on some oil tankers.
John Onslow on LinkedIn
I agree, a N2 snuffer may not be the better option. As explained by Admin above, flashback should not occur through a HVVV if it is suitable for the product being handled. The maximum experimental safe gap (MESG) of the product is the criteria. Mast risers on oil tankers are different as they are fitted with a valve to isolate the tanks.
I’m not too sure of the flame arrester (usually a wire mesh) on the vacuum side is actually effective. It might arrest sparks but I have doubt on arresting flame/explosion. Is anyone aware of any incident with conclusive proof ?
Thank you for all your valued comments. I am from Engineering Solutions of MISC and hopefully with your feedback to provide solutions to the Operations team.
Personally, I would never refer to fitting a second safety device as ‘overkill’.
Mr. Onslow is perfectly correct in saying that HVV Valves are ‘type approved’ and should work but it has to be remembered that all tests are carried out under ‘test conditions’ and that normal operations can often be far removed from ‘test conditions’. Even when an acceptable maintenance programme is followed, it has been known for venting systems to fail to operate as they were designed to and it should also be understood that any source of ignition will also not be ‘controlled’ and could itself affect the operation of the HVVV. All of these issues give rise to the possibility that the HVVV may not work correctly at the exact time that it needs to. If that happens, irrespective of where it sits on the ‘risk to probability’ matrix, it is the wrong time to be thinking ‘I should have fitted a back-up system’. So to me, fitting a snuffer system would be ‘prudent’ not ‘overkill’. Remember also that tanker safety has improved tremendously over the last few decades and most of this is due to the industry responding to ‘lessons learned’ and applying relevant changes to operating procedures and equipment (of which a secondary venting system was one). So again we come back to if you fit an extra system, even if you only use it once, it will have been a very good decision to have made.
Andy Timms on LinkedIn