Why we need injectors
Clan Line is powered by steam, which is produced in the boiler by heating water. Over time, the water gets used up and needs to be replenished. We use steam, at boiler pressure, to get the water from the tender into the boiler. However, we need to supply it at more than boiler pressure, otherwise it won't be able to overcome the back pressure which is exerted on the non-return clack valve.
Instead of using a feedwater pump, most steam steam locomotives use two injectors. They have the advantage of having no moving parts when they are running, and are, therefore, less susceptible to wear.
The injectors that Clan Line, and other Bulleid Pacifics, use are Davies & Metcalfe Monitor injectors. Ours are size 11, each with a maximum capacity of 2,850 gallons per hour.
If a fluid passes through a restriction, the theory of Fluid Dynamics says that its speed must increase. The Venturi Effect means that this is associated with a drop in pressure. Conservation of Energy, the First Law of Thermodynamics, is what causes this. As the fluid speeds up, its Kinetic Energy increases, and this energy has to come from somewhere. Pressure is a form of Potential Energy, and this is where the energy comes from, causing the pressure to drop. This principle is what allows an aeroplane to fly.
The other main principle is that of Latent Heat. An easy to understand example of this is evaporating water. If you make your hand wet, as the water evaporates, it takes in energy, making your hand feel cold. As steam condenses into water, the opposite happens, and energy is released.
When steam first enters the top of the injector, through the steam valve, it encounters the steam cone. This is a converging cone, which causes the steam to speed up, and, therefore, the pressure to drop. The steam cone is actually made of two concentric cones. The steam goes through a ring of holes and down the outside of the inner cone (an annulus), where it is joined by the steam that has come through the main cone. This steam through the main cone is known as the "forcing jet". It is this design of steam cone that allows the injector to operate over a large range of boiler pressures. We have had ours working with the boiler pressure as low as 20 psi.
The lower part of the steam cone is located inside the top part of another cone - the combining cone. The combining cone is another converging cone, so the steam speeds up more, and the pressure drops further. The water supply from the tender enters at this point, via the water valve, through the gap between the steam cone and the combining cone. Because of the lowered pressure, the water is "sucked" in, and combines with the steam.
The final cone, which is part of the same assembly as the combining cone, is the delivery cone. Unlike the other two cones, this is a diverging cone, which means that the steam and water mix slows down, and the pressure increases.
However, even though the pressure has increased, it is still not enough to overcome the pressure within the boiler. The extra pressure required comes from the energy released by condensing the steam into water. This brings the pressure in the delivery pipe up to about 50-80 psi more than the boiler pressure, so that the non-return clack valve will open, and the feed water will enter the boiler. It is not unknown for injectors to fail to work if the water in the tender is too warm to condense the steam.
Over time, the cones do wear. The part that is most susceptible is the transition from the combination cone and the delivery cone. In Davies & Metcalfe Monitor injectors, this is a separate piece, the delivery cone renewable end that can be changed without replacing the entire cone assembly. You can just see it in the above photograph. It is the shiny piece of metal visible through the right hand hole in the cone.
Within the injector body, when the injector is working, there are chambers at three different pressures. To stop leakage between these chambers, there are seals between them. These are traditionally made from string, which is wrapped round a groove in the cone, and seals against the body of the injector.
Operating the injectors
The first thing that the fireman does to operate an injector is to turn on the water supply. This water will flow out of the overflow pipe, by way of the overflow pot on the side of the injector. There are holes in the cab floor immediately over the two overflows, and a light under the floor, so that the fireman can see this. He then turns on the steam supply. If necessary, he trims the water back until the injector "picks up". When it does, the lowered pressure inside the injector will "suck" the overflow valves shut, and there should be nothing appearing at the overflow pipe. An experienced fireman will also know when the injector has "picked up" by the sound that it makes. An operating injector is said to "sing".
To turn off an injector, the fireman does the opposite - he shuts off the steam supply, first, and then shuts off the water supply.
It is good practice to use the injectors alternately, so that the fireman knows that they both work.