The course of steam from the dome of a locomotive to the atmosphere is illustrated in plate I. which shows in section, the dome, smoke box, steam chest, one cylinder and part of the boiler of a locomotive.

Steam enters the main pipe through the regulator or throttle valve (which is operated by a rod not shown) and passes along the main steam pipe until it reaches two branch pipes which lead to a steam chest on either side of the engine (see plate 2) and after passing the valves, and doing its work in moving the pistons as shown by the arrows, it passes out to the exhaust or blast pipe and out of the funnel.

In the case of the regulating valve being placed in the dome, the pipes are empty when the valve is closed, the pipe in the dome, also the long straight pipe, having the full boiler pressure pressing on their outer surfaces. If the joints marked “A” are defective, steam can blow through into the pipes and to the cylinders all the time the boiler is under steam.

If the joint in the top of the dome is defective or the valve (as indicated by the arrow marked A1) the steam issuing from the cylinder taps is dry; if, however, the joint at the bottom of the dome is defective, the steam from the cylinder taps is usually wet.

Owing to the shocks and strains which the pipes have to stand every time the regulator is opened and closed, the long pipe is apt to become weakened and cause trouble.

The joints marked B in the smokebox can blow only if defective when the regulator valve is opened. The blast pipe can show a defect only when the engine is being moved with steam on.

A large number of engines have their regulating valves in the smoke box. This is an advantage as the pipes in the boiler are not strained as are the pipes in other types of engines, for the pipes of the former always have the same inside pressure as the boiler.

The long stay nuts (a few of which are shown), the tubes and the large steam pipe joint next the tube plate, can blow, if they page 31 are defective, all the time the boiler is under steam. In an engine fitted with a super-heater the element joints can blow only when steam is turned on.