Could some one here offer a brief tutorial on the size of tank car domes?  This query comes from a recent comment about wanting to 'cut down' domes to shorter size for specific cars, related to car capacity and ventilation.  Earlier cars had more narrow and taller domes?  Later larger cars had larger but shorter domes (until when)?  Other factors relating to size of the dome?  Venting?

I suspect this information might be of interest to others on the list.

Ray Hutchison

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Tim O'Connor
Sterling, Massachusetts

As Tim O'Connor has already noted, tank car dome capacity provides a space for thermal expansion of the lading in order to prevent over-pressurization and/or lading leakage. Most cars are based around petroleum products which uses a factor of 2% – so an 8,000 gallon car requires a dome capacity of 160 gallons, a 10,000 gallon car requires a 200-gallon dome and so on. Traditionally the major use for tank cars is petroleum (and related products), and most cars utilize this 2% number.

Other commodities have very different requirements: for example, acid has much lower expansion numbers, so acid cars tend to have very small domes (which are also frequently much narrower than standard domes, although limited by the need to provide access to the car for workers). There are some other cars with very large domes, which were provided for a very specific purpose.

The expansion requirement has not gone away, but today the understanding is that this expansion capacity is to be provided within the tank body itself – the cars are simply not filled to the top. The reason for this change is simplified construction and a stronger carbody.

Ian Cranstone
Osgoode, Ontario, Canada
lamontc@...

Ian's message contains several persistent misconceptions about the "2% rule" (which didn't apply to MCB Spec II cars built before May, 1917), the (in)adequacy of a 2% dome for volatile and inflammable products, and the notion that "most" Spec III cars were built with domes that just met the 2% minimum requirement (they weren't).  The provision of "extra" expansion capacity by not filling the cars to shell-full is not a modern practice at all, but also dates back to the teens.  We discussed this at length in August of 2019 starting with message 166255

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Dave Parker
Swall Meadows, CA

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 

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).

The rule specified 2% as the _minimum_ size. If you look at the 1955 tank car tariff 300-H, for example, you will see that most tank cars had well in excess of the 2% capacity. That was one of the many choices made by the buyer, not the builder.

Tony Thompson
tony@...