Sunday, March 15, 2015

McKenzie Valves and Oil Train Fires

A question has been bothering me since I wrote the post earlier this week about the Federal Railroad Administration’s (FRA) railworthiness directive about McKenzie UNNR valves; could leaks from these valves have contributed to recent derailment fires in non-DOT 111 railcars?


Now, before I proceed any further with this let me make two things perfectly clear. First, I am not a railroad accident investigator and I have not gotten any closer to the recent wrecks than some photos that I have seen on the web. Second, the FRA directive only mentioned the McKenzie UNNR valves being seen on DOT 111 railcars; there is no mention of CPC 1232 railcars; the type cars that were involved in the West Virginia derailment.

Crude Oil Explosions

The reason that I ask this question is that the one good picture of the cars involved in the WV derailment that I have seen shows the cars being intact. There does not seem to be any of the damage that we would associate with exploding railcars like has been seen in some of the earlier incidents. There were certainly reports of explosions associated with this derailment, but I think that they could be explained by leaking valves.

First you have to understand that there are two different types of ‘explosions’ that can be involved in a crude oil train (or any flammable liquid train) derailment. The first is not really an explosion in the military sense of the term; a sealed railcar is involved in an exterior fire. The heat of the fire outside the railcar increases the temperature of the liquid within and this causes a build-up of pressure inside. With enough heat the pressure builds up to the point where the railcar can no longer contain the pressure and the car catastrophically fails. This can be aggravated if the flames of the exterior fire impinge on the car above the liquid level and weaken the exposed metal.

The second type of explosion is an explosion exterior to the railcar caused by the ignition of a flammable gas cloud that has escaped from the railcar. This will usually accompany the first type of explosion, but it may also happen when a less intense fire is involved. As the interior contents of the railcar increase in temperature and pressure begins to build safety systems on the railcar known as pressure release valves will open and allow some of the flammable gas in the headspace to be released to the atmosphere. You can actually get multiple explosions of this type from a single railcar if the fire continues long enough.

The main difference in the two types of railcar explosions is the extent of the damage. The explosion caused by the catastrophic failure of containment is typically much more damaging especially since it typically includes a very large secondary explosion of the second type. With the non-catastrophic explosions of the second type, less fuel is actually involved so the amount of energy released is significantly smaller. Now you don’t want to be around either one, but you can safely be closer to the second than the first.

Cause of the Fires

One of the reasons for the surprise at the fires involved in the West Virginia derailment was that they involved the CPC 1232 railcars not DOT 111 railcars. The newer CPC 1232 are designed to withstand derailments better than the DOT 111 railcars. They are supposed to remain intact better and thus prevent oil spills and subsequent fires. As I have mentioned a number of times, you cannot guarantee that they will be 100% failure free (and improvements to their design are probably included in the upcoming PHMSA rule), but we had hoped for better performance than was seen in WV.

If McKenzie valves, however, were installed on one or more of the involved railcars, the situation changes substantially. You could have had crude oil on the outside of the railcars, even before the accident. That material would have been on the top of the cars. This is critical because that is the last place that you want a fire to start if you want to stop the venting of flammable gasses as it takes less heat to heat up the gasses in the head space than it does to heat up the liquid to produce more flammable gas.

Additionally, the FRA notes that the McKenzie valves that they tested were unable to hold 50 psi of pressure. This is significantly less than the pressure at which the pressure relief valves are set to function at so that there would be flammable gas releases from these valves at much lower temperatures than normally expected.

Finally, if a derailed railcar ended up on its side (as at least one clearly did in the West Virginia wreck photo) that we would expect to see crude oil leaking out of that valve throughout the time the railcar was on its side and it would be leaking faster while the fire was burning because of internal pressure in the car. This longer burning fire would contribute to more type two explosions from the other cars.

Investigations Continue

I am sure that the FRA is taking these possibilities into account as they continue to investigate these recent derailments. But it is also something that we need to consider as we continue to debate the problems associated with crude oil train derailments.

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