Karl provides a good basic discussion. A couple of additional points can
come into play. First, the type of fuel makes a difference; oil
provides a hot fire as soon as it is lit while solid fuels such as coal
or wood take time for the fire to establish itself and begin to heat the
boiler and its contents. Second, the mechanism of drafting the fire
during startup makes a difference. Natural draft (uncommon) or house
air uses ambient temperature air which provides no heating in and of
itself, whereas steam from another boiler (house or another locomotive)
provides some warming of the boiler.
Lastly, too rapid heating, or cooling for that matter, not only shortens
the life of the boiler, it significantly increases the chances of a
catastrophic failure of steam lines or the boiler itself. This is a
major safety concern and nothing you want to be anywhere near if it happens.
As Karl says, firing a cold boiler and shutting down a hot one are tasks
best done slowly.
Mike Gardner
Karl L Rethwisch wrote:
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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