Steam dome

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, also known as priming.

The dome is placed forward of the firebox, in an area of less vigorous boiling and thus fewer suspended water droplets.

The square-topped Belpaire firebox allows steam to be conveniently collected at its top corners and therefore locomotives with Belpaire fireboxes often dispensed with a dome, for example express engines such as the GWR Castle Class (the large brass boiler fitting on a Castle is the distinctive GWR safety valve cover, not a dome).

The regulator could also conveniently be built into the casting, immediately between the dome passage and the valve chest, without requiring long steam pipes.

Water-tube designs had a suitable steam drum mounted high above their evaporating surface and this performed a similar function.

Where Cornish boilers were unusually fitted with a dome, in Cornwall this was known as a 'Dolly Varden', from the stovepipe hats forming part of the local women's traditional costume.