Steam dome

The steam dome is a vessel fitted to the top of the boiler of a steam locomotive. 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 and minimises the risk that water will be carried over to the cylinders where it might cause a hydraulic lock - this is also known as priming.

A steam dome should not be confused with a sand dome.

The first locomotive with a deliberate dome added to the boiler barrel was Stephenson's Rocket, although this was so small as to have little effect against priming. Many other locomotives built shortly after this date instead used either the 'haycock' boiler, where the firebox outer casing was raised high above the main part of the boiler, forming a steam dome, or Gooch's development of this where the semi-cylindrical firebox wrapper was raised above the boiler barrel.

The most vigorous boiling in a locomotive boiler takes place around the hottest part, the firebox. This was a drawback to the haycock arrangement, and led to the general adoption of the separate dome instead. The dome is placed forward of the firebox, in an area of less vigorous boiling and thus fewer suspended water droplets.

From the 1840s, boiler barrels were constructed from hoops of rolled iron, or later steel, sheet. As boilers were longer than the width of the available iron sheet, two or three hoops were required. For strength, the dome was always placed in the centre of a hoop, rather than spanning a joint. Early boilers used narrow plates and thus had a centrally located dome, in the centre of the middle hoop. Later boilers could use the wider plates then available and used two hoops, so as to reduce the number of riveted joints. These domes were thus placed at a quarter of the barrel's length (from the front of the firebox wrapper).

Boiler made of three hoops, with the dome placed centrally

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Wikipedia