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  • Writer's pictureChris Hoskin

Creating a vacuum

Latest in the mailbox is this picture of the ejector and its pipework being set up.

Ejector being set up on County 1014 County of Glamorgan

What's an ejector I hear you ask. Well, it is the means by which vacuum is created in the train pipe (brake pipe) of the train to keep the brakes off. When the vacuum is destroyed in the brake pipe, either by the driver using the brake control, or by the train pipe splitting, the brakes automatically come on.

An ejector works by utilising what is known as the Venturi effect which is a way to change the velocity and pressure of a flow of gas by using a cone or constrictor. The effect is also used in injectors for putting water into the boiler.

In the picture above you can clearly see the cone shape of the pipe to right of the flanged end. The diagram below gives some idea of what is being looked at. The open end is where the nozzle will go, and the cone in the picture is the converging nozzle. On a steam loco the diverging diffuser is in the smokebox and is often not much more than a flaring of the end of the pipe.

Steam ejector diagram

Below is a diagram of the GWR braking system. The ejector is at the top just left of middle. Rather than go into it here in detail, it is well worth visiting the South Devon Railway website here for a description of how braking works.


GWR braking system diagram

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