Steam Dome - Development

Development

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.