A Steam dome is a vessel fitted to the top of the boiler

Boiler

A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...

 of a steam locomotive

Steam locomotive

A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

. It contains the opening to the main steam pipe and its purpose is to allow this opening to be kept well above the water level in the boiler. This arrangement acts as a simple steam separator

Steam separator

A Steam separator, sometimes referred to as a moisture separator, is a device for separating water droplets from steam. The simplest type of steam separator is the steam dome on a steam locomotive...

 and
minimises the risk that water will be carried over to the cylinders

Cylinder (steam locomotive)

The cylinders of a steam locomotive are the components that convert the power stored in the steam into motion.Cylinders may be arranged in several different ways.-Early locomotives:...

 where it might cause a hydraulic lock

Hydrolock

Hydrolock is an abnormal condition of any device which is designed to compress a gas by mechanically restraining it; most commonly the reciprocating internal combustion engine, the case this article refers to unless otherwise noted...

 - this is also known as priming

Priming (steam engine)

Priming is a condition in the boiler of a steam engine in which water is carried over into the steam delivery. It may be caused by impurities in the water, which foams up as it boils, or simply too high a water level...

.

A steam dome should not be confused with a sand dome

Sandbox (railways)

A sandbox is a container on most locomotives and self propelled multiple units, or trams, that run on tramways and adhesion railways...

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