The Union Pacific ran a test on a cold 4000 engine. They achieved operating
steam pressure in approximately 40 minutes. The Union Pacific also mentioned
that this was only a test and would be considered destructive if done on a
regular basis.
To heat a cold boiler too rapidly can cause severe damage to the assembly.
There are many parts that heat at vastly different rates and, as a result,
expand at vastly different rates. The rate at which parts expand directly
impact the overall integrity of the boiler assembly. The combustion chamber, on
locomotives so equipped, the firebox and the wrapper sheet all depend on each
other for support. Excessive expansion rates of sheets can lead to staybolt
fractures, metal fatigue, leaks in joints and other harmful effects of thermal
and mechanical changes in the components.
A large boiler can "grow" by nearly one inch in length between its cold state
and the point when the entire assembly is at operating temperature and
pressure. A period of at least 24 hours should be devoted to warming the boiler
before any serious attempt is made at getting up steam pressure. All this
precaution, of course, is only warranted if one wants to avoid some extremely
expensive repair work later on.
Karl
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