The water in the boiler needs regular
replenishing and two methods can be used to introduce it when the boiler is
under pressure. The introduction of cold water into the boiler though
necessary, will also lower the steam pressure for a while. A great deal of
skill is needed in operating the injector while still maintaining sufficient
power for operation.
1. Mechanical feed pump. Some locomotives
preheat the water prior to pumping.
The principle of the injector is based on the
fact that steam escaping from a nozzle has a greater velocity than that of a
jet of water issuing under the same pressure from a boiler. If cold water is
added to the jet of steam, it begins to condense and the velocity of the
steam will increase sufficiently to overcome the pressure of water in the
boiler. By this means, water can be introduced into a boiler against its
Some injectors used a combination of exhaust steam and live steam. A
connection at the base of the blast pipe was run to the exhaust part of the
injector where it heated the feed water before it passes to an auxiliary
injector. The auxiliary injector used live steam to force the water to the
boiler. This type was patented by JJC and RD Metcalfe in 1908 and was claimed
to save up to 15% on fuel and water.
There was a type of injector, with features patented by J Gresham in 1884 and
1887, which was a "vertical restarting injector". Steam supply and feed water
passed through the flange by which it was attached to the boiler. There was
also a Davies and Metcalfe type patented in 1899 and 1907 which was designed
to operate with feed water too hot for an ordinary injector.
Injectors are tricky instruments and require a degree of skill to "prime"
them and get them working. This is normally the task of the fireman. Once the
steam is turn on, the right balance of water being applied has to be found.
This will only work if the steam and the water are at the correct pressure. A
balance also has to be found between too little and too much water being in
the boiler. Too little risks melting the fusible plug, too much risks boiler
water rising to reach the regulator, known as "priming", and getting into the
steam pipe leading to the cylinders.