need picture of hopper car imploding when unloading when hatch is not open?

[Anonymous]

In '64, the company moved me from Los Alamos, NM, out to Oxnard, CA.  We owned two cars so my wife and I drove them rather than ship them.

As we neared Needles, California, the older car, a Studebaker, stopped running.  Gas gauge showed it was not out of gas but start, it wouldn't.

I pushed it into Needles and into the nearest gas station.  Explained the problem, the mechanic took off the gas cap (which I had failed to do), and a great hiss ensued.  I immediately knew what the problem HAD BEEN.  The car had been repainted and the gas cap's vent had been painted over!

The tank had been filled at Flagstaff - high altitude - with gas and high altitude air.  No other air was able to get into the tank as the gas was used driving down to near sea level; the poor gas pump wasn't designed for that type of work so couldn't pump gas into the carburetor.  A new gas cap was installed.

Art 

[jsoderq]

Airslide cars have a fabric lining in the floor. Air is blown in under the fabric to fluff up the load(like flour) so it will flow out. PD (pressure differential) cars have plumbing at the bottom which are used to vacuum the load out the bottom with air flowing in the top. Hatches on conventional cars are opened to allow the load to flow freely.

[stmtrolleyguy]

Don't they even make some covered hoppers where you pressurize the car from the top to help blow the load out the bottom?

[JSGreen]

Iwould think that the difference in pressure is more important than the rate...unless, of course, the one-way vent valves leak a little, letting higher atmospheric pressure leak back into into the tank slowly...

Remembering that Atmospheric pressure at sea level is 29.92 in of HG (14.4 psi), and the standard lapse rate is about 1in of HG  per thousand feet, if you emptied the tank at 10,000 ft, and sent it back to Houston, the pressure differential inside would be only about 10 in of HG or a little less than 5 psi.  Steam cleaning (heating the air inside), then allowing it to cool, would reduce the pressure to something on the order of 1-2 psi, creating three times the force...

I doubt that emptying a hopper with a closed vent would have the same effec.  The vent would be more for speedy unloading than prevention of collapse.  THink of a gas can you are filling the mower with, without opening the little vent.  It guggles...air going in interfers with gasoline coming out, but the pressure equalizes and the product continues to flow.  You can actually see the sides of some cheap plastic gas bottles flex inward and outward, in until the pressure equalizes with a gurggle...

emptying a hopper would probably work the same way.  With lighweight plastic pellets, it might actually stop flowing if the pressure at the top is equal to the weight of the pellets gooing through the bottom....

 

[Anonymous]

Stupid Question Time:

If that can happen during a steam cleaning, can similar results occur as the result in a change of altitude?

 

In Ed's very impressive example, it's not  just "air pressure" that imploded the tank.

 

It was the volume of steam condensing back into water, (phase change)  creating a vacuum inside the tank.  something like a 1000:1 ratio in volume delta, if I remember right.  

[edblysard]

The change in air pressure or temperature has to be fairly quick, and fairly large in values, which is why your bottle collapses.

The vents on tank cars are over pressure vent, designed to allow the car to vent out, not in.

This was a general purpose tank car; you can fill it with just about any chemical commodity.

 

If a tank car was unloaded up there, and sent here empty, the change would be gradual, and not that extreme.

Loaded here, and sent there, the vents would release any pressure build up.

 

Loaded there, and sent here, there would not be enough empty space in the tank...keep in mind you would have to compress the contents also.

Air is relatively easy to expand and compress, compared to most other things.

[tree68]

you imagine what that sucker sounded like when it went?

While I doubt it went quietly, I don't imagine it was a sudden, catastrophic collapse either.  I'd like to have seen the look on the face of the person who discovered it....

[CopCarSS]

Stupid Question Time:

If that can happen during a steam cleaning, can similar results occur as the result in a change of altitude? The reason I ask is that often after finishing a bottle of water at high altitude (say above 12,000 feet or so), and then proceeding to descend to a lower altitude, I'll find a water bottle that looks much like that tank car (though maybe not quite as extreme). So, if a tank car is unloaded up here, and then sent down your way, Ed, could it look a little thinner at the end of its trek?

[edblysard]

You can duplicate the same problem in miniature...take an empty coke can, add a teaspoon or two of water, and using tongs, hold it over a stove burner till the water boils into steam, hold the can at the bottom)...turn the can upside down, with the opening facing down, and plunge it in a bucket of cool or cold water...the can will crush down almost flat....

 

VoPak industries Deer Park, who specialize in cleaning and repairing tanks, has a protocol that requires anyone who steam cleans the interior of a tankcar to have a second person, supervisor rank or above, sign off on the finished job, by checking a work list tag that must be affixed to the car...the top item on the list is a notation that all vents are open, and the manway and loading hatch is locked open, just to prevent this very thing from happening.

 

Can you imagine what that sucker sounded like when it went?

Here is a link to a neat page about how this happens...the experiment works, I tried it when I first read the page.

http://www.delta.edu/slime/cancrush.html

and the other photos...

[Murphy Siding]

      If you just open a vent, doesn't it just re-inflate, like those big yellow rubber rafts in the movies always do at the wrong time?

[MP173]

Ed

I find that amazing that it could happen.

 

ed 

[edblysard]

How about a tankcar that was steam cleaned inside, then, without letting the interior cool off, the cleaners closed and sealed the man way hatches and vents.

The steam condensed, pulled a vacuum greater than the structural integrity of the tank, and you can see the results.

[Anonymous]

  I suppose that this  can happen? i see the warning lables all over the cars