Re: tank car domes

D. Scott Chatfield

A rule of thumb is most true domes* had a capacity of roughly 2% of the tank body (and this volume is measured above top dead center of the tank body).  Now you mention earlier tanks seeming to have taller, narrower domes.  Those cars also tended to be smaller in capacity, so they were lower in overall height.  Once you get to the 10,000 gallon tanks, the dome height crowds the height limit for cars of the time, so you have to make it shorter and fatter.

And the problem of overall height kept the maximum size of general service tankcars to about 12,000 gallons until somebody figured out that instead of an expansion dome, just don't fill the car all the way up.  It only took them 70-odd years to figure that out.

Note that at least one tankcar builder (Pennsylvania Tank Car, IIRC) used a dome that was the same diameter as the tank body.  This does gain you some dome volume but at the expense of a weaker tank body.

Also keep in mind that some commodities did not need 2% domes.  Some acids come to mind, so they had skinnier domes.

*Only tankcars that carried liquids needed domes back in the day.  Tankcars for pressurized gases, even those liquified in the process (like propane/LNG), do not have domes.  However, they do have a dome-looking "bonnet" that protects the loading/unloading valves and safety vents.

Scott Chatfield 

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