Railway Pressure Vessel Cars

[Sharik]

Does anyone know why railway pressure vessels cars (propane, ethylene, etc.) are wasp-waisted at the top, in the middle, right where the top manway is?

I was just noticing this on the weekend and couldn't get a good answer from anyone.

Thanks

 

[codeeng]

Probably for drainage.

 

[JStephen]

I'm not sure exactly what you mean. I know some of the tank cars appear to be intentionally sway-backed, which I assume has some structural advantage.

Could the wasp-waisting be something to just lower the manway for clearance or better access? Or maybe to exempt people from fall-protection requirements?

 

[Sharik]

If you look at these railway cars from the side, the top is angled downwards at a slight angle from the ends, to the middle. The sides and bottom of the cars are straight.

I can't imagine this being for drainage and I can't figure out any structural advantage to this design.

 

[JStephen]

Okay, been doing a bit of websearching, for what it was worth.

All I can find is reference to cars where the whole car (top and bottom surface) slopes down to the center. And that is so the product can drain out better. Are you SURE that the bottoms of the cars you're seeing are straight?

I find this called a "funnel flow" design by the way, and not limited to pressurized cars.

Quote 1:
"The silhouette of some modern tank cars depart from the strictly barrel-like horizontal cylinder by the ends turning upward somewhat and the middle sagging slightly downward. This shape allows better drainage of the contents and may help to accommodate sloshing when in motion."

Quote 2:
"An interesting variant is the "Funnel Flow" tanker which is slightly sloped toward the middle from each end, giving it a bent appearance, but allowing it to be drained by gravity."

Note that these quotes are from people dealing with models, not from carmakers, so are not necessarily authoritative.

 

[GenB]

What about elasticity of the metal!,
I see those tanks every day and observing the foll:
there are no external supports for the load and what that tells you,
that huge tank will a little.
GB

 

[codeeng]

If its only slopped on the top and the bottom is flat then perhaps this type of tank has been sized for maximum inventory while staying within the rail gauge clearance. e.g. the manway in the center is at the same elevation as the top of shell. Just a wild guess.