Remember that trail derailment in Ohio?

[bikeman564™]

I was reading an article in the National Board Bulletin, a quarterly publication I get from the valve test lab   Anywho, the article was on the term "knock-on effects" where one action leads directly to an undesired or often surprising result.

In this article the author talked of the Nor Folk Southern derailment in Ohio. The NTSB in their report looked at the safety valves on the tank cars. And they found that there were melted aluminum covers on three cars. The NTSB didn't say the aluminum debris interfered with pressure relief, but aluminum flowed onto the discharge mechanisms of the safety valves. A noticed was sent out by the pipeline & hazardous materials safety administration that it be a good idea to replace aluminum covers w/ carbon steel.

Because the tank cars were not relieving pressure, Norfolk elected to breech the tank cars to burn off the vinyl chloride. While this was bad, if they did not do this there would of been a boiling liquid expanding vapor explosion which would been catastrophic. The breech occurred just short of this happening.  Good thinking on someone's part I say.

 

[Thaddeus Kosciuszko]

What would have been the effect if the relief valves has worked properly, and it wasn't necessary to breach the cars?  It seems to me the release by the safety valves would have been uncontrolled - in the sense that the amount released would not be under human control - but that there would have been less vinyl chloride released because the safety valves would have kept at least some of the contents in the cars.

Second question - if the safety valves had worked properly and vented some of the contents, when the fire was put out and the cars cooled off would the tank cars have collapsed like this or is there a second safety valve that allows air back in?

[bikeman564™]

22 minutes ago, Thaddeus Kosciuszko said:

What would have been the effect if the relief valves has worked properly, and it wasn't necessary to breach the cars?  It seems to me the release by the safety valves would have been uncontrolled - in the sense that the amount released would not be under human control - but that there would have been less vinyl chloride released because the safety valves would have kept at least some of the contents in the cars.

Second question - if the safety valves had worked properly and vented some of the contents, when the fire was put out and the cars cooled off would the tank cars have collapsed like this or is there a second safety valve that allows air back in?

 

question 1: Safety valves must close after pressure is reduced and remain fully functional. So you answered your own question ASME Code requires this anyway.

question 2: There would be a vacuum relief valve to prevent an implosion. Not sure if its incorporated into one valve, or separate devices. Mythbusters did this by eliminating the vacuum relief valve. Great episode.

[2Far ★]

Unintended consequences.