How the Injector Works
The injector is an appliance for delivering feed
water to the boiler. In its simplest form it embodies three essential
cones; the "steam" cone, the "combining" cone and
the "delivery" cone. The steam cone admits the steam from
the boiler to the injector, guides it in the direction in which it should
flow and limits by its bore, the amount of steam passing through.
Steam leaving this cone comes into contact with
the water from the tank/tender, is condensed by it and passes into the
combining cone. when steam is allowed to expand in the steam cone from
a higher to a lower pressure, a certain amount of heat is available
for conversion into work and this is spent in giving velocity of the
steam itself in the direction of its flow. The first point to remember
is that the change from pressure energy to velocity energy is brought
about in the steam cone.
In the combining cone, the slowly moving water
combines with the swiftly moving steam and the function of this cone
is to ensure that the steam jet is condensed by the water. The cooler
the feed water the better the condensation of the steam. The combining
cone is convergent in shape, the bore of the cone decreasing with the
result that the jet consists, at its inlet end, of a mixture of steam
and water, and at the outlet of a solid jet of hot water flowing with
high velocity into the delivery cone. Between the combining cone and
delivery cone, is a gap known as the overflow gap, through which excess
steam and water are passed in the starting and finishing operation.
The second point to remember is that the combining
cone effects the complete combination of the steam and water into the
solid jet by the condensation of the steam and the transference of its
energy to the water.
The delivery cone is so constructed that the
change from velocity to pressure energy takes place as uniformly as
possible. The momentum of the jet, which is greatest at the smallest
diameter of the delivery cone, is gradually reduced in velocity and
increased in pressure sufficient to overcome the boiler pressure on
top of the clack valve. The temperature of the feed water is usually
increased by about lOOoF in passing through the injector.
The third point to remember is that the function
of the delivery cone is to convert the velocity energy of the combined
jet into pressure energy.
When using an injector, loco crew should pre-warn
any staff standing or working near to the overflow pipe, before starting
an injector. Further caution should be taken when approaching signalmen
on the ground that are collecting/exchanging the train staff or token.
Prior to using the injector, the loco crew should
ensure that the footplate watering hose tap is off and that the hose
is stored clear from the footplate area.
Causes for Injector failure
1. Dirt or scale on injector cones or excessive
wear or distortion of cones.
2. Air leaks in the water supply.
3. Feed water supply blocked. i.e. rags or foreign
matter in the tank sieves.
4. Feed water too hot.
5. Clack valve not seating properly.
