Arguably the most important ingredient in steam operation is water. All steam engines require water to generate steam. This water is heated in a pressure container called a boiler. As the water is heated (usually by some fire source, eg. Coal, wood, gas) it expands and vaporises into steam. Dry steam is 1600 times greater in volume than the water it came from. This is why boilers must be able to hold great pressure within them selves and why even small steam engines are so powerful. This pressure makes it difficult to put new water into the boiler to keep the cycle going. The new water has to be forced into the boiler under greater pressure than the water/steam already there.
If we tried to simply gravity feed water into the boiler from a tank connected with a pipe it would not flow. The high pressure steam/water would flow out of the boiler through the pipe into the tank. John Byers explained to me that Gravity can be used to fill a boiler under pressure. Although he didn't know of any in use today, large stationary systems were designed with tanks that could withstand boiler pressure above the boiler that were equalized by a steam connection. While making up water in the tank required a pump, this system had the advantage of holding a large reservoir of warm water once the unit had been under steam for a while.
Initially operators used hand or steam operated pumps to keep the boiler level up. The hand pump is hard continuous work. The most common steam pump is driven directly from the cross head as the engine runs. The cross head steam pump can only be used to keep the water level constant once the engine is running. It cannot be used to fill a cold boiler. John Byers commented that when water was low in older railway locomotives which only had a cross head pump that the crews sometimes had to uncouple the train and run the engine back and forth to bring up the boiler level. The History of the American Locomotive by John H. White, p.124 describes this and other problems associated with pumps in locomotive service.
In 1858 Henri Gifford a French Engineer made a breakthrough when he invented a device which solved the problem. His device known as the Water Injector seemingly works by magic as it uses steam from within the boiler at boiler pressure to overcome that same pressure and force cold water into the boiler. When first examined it appears that "extra" energy is coming from nowhere.
The injector is a very simple device with no moving parts. In its simplest form the injector is made up of three cones which work together. The cones are inline as per the diagram below:
The steam enters the first cone and is compressed into a tight stream, the pressure energy of the steam is converted into velocity (movement) energy.
The tight stream enters the second cone called the combining cone where it mixes with the cold water and condenses. As the steam condenses it greatly reduces in volume creating a vacuum which draws more feed water into the combining cone.
The velocity energy present in the combined steam and water stream carries the water across the gap between the second and third cones. The third cone is called the delivery cone. Here the water expands changing the velocity energy into pressure energy. The pressure energy forces the water through a one way valve called a "clack valve" into the boiler. The clack valve is so named because of the noise it makes when it opens and closes. The job of the clack valve is to prevent reverse flow through the injector.
The injector includes an overflow which is present to allow excess water to escape back into the water tank or be released from the system onto the ground. The injector depends on steam and water flow. If the steam pressure is not high enough then the water will not cross the gap and will flow from the overflow. Water escaping from the overflow in quantity tells the operator that the injector is not working. Steam escaping from the overflow in quantity also tells the operator that the injector is not working as it is not receiving water.
One common mistake made by operators is to pre-heat the feed water before attempting to inject it. While it is acceptable (and desirable from a heat energy point of view) to make the feed water tepid, if it becomes too hot the steam will no longer condense when it mixes with the water and the injector will stop working because it will stop "sucking" feed water. The net result is that there will be a trickle or no flow through the overflow and nothing going into the boiler. An injector in this situation with a low water level in the boiler can quickly make a bad situation disasterous as the boiler vents steam through the injector making the level lower and lower until the crown sheet or fire tubes are exposed likely leading to an explosion.
There are two principal types of injector, lifting and non-lifting. Lifting injectors can suck water from a source lower than the injector such as a tank or nearby stream. The lifting injector is usually present on steam tractors and trucks. A non-lifting injector is gravity fed from a tank at the same level as or higher than the injector.
How can you tell when an injector is not working? The body of the injector is all metal so you cannot see the action. Indications of a working injector are a dry overflow pipe and rising water level in the boiler as indicated by the glass level gauge and/or try cocks. The overflow outlet is usually above the foot plate to allow the operator can see any water flow. The operator can check for steam by closing the feed water valve and watching for steam at the overflow. For a non-lifting injector the steam valve can be closed with the water valve open and then watching for water flow at the outlet. Operating an injector is more art than science.
Injectors are known as temperamental and even in the hands of an experienced operator the can stop working without notice. This is why steam boilers usually have at least two injectors and one other method (eg. Cross head pump) of getting water into the boiler. Each injector is capable of filling the boiler by itself so if one fails the other(s) can be used to keep operations going while the failed unit is isolated, removed, and repaired. Usually the injector is plumbed in such a way that it can be completely isolated and removed for inspection. If both injectors fail, operations can keep going using the cross head pump. The cross head pump will only keep up as long as the steam being used matches the water inflow.
This is the Back Head of a Huber Traction Engine. If you look to the left of the fire box door over near the left hand bunker you can see a brass coloured object piped into the boiler. This shows a fairly typical arrangement of injector plumbing. It is at a reasonably convenient height for the operator to use with the overflow where it can be seen. The pipe going upwards the highest is the steam pipe and it comes from what serves as the steam dome on this model. The pipe closest to back, and left most is the water inflow which interestingly is coming from the water heater. The overflow pipe while present is not easily visible in this picture.
Thanks to John Byers for the comments and additional information he sent me to correct some aspects of this article. I am not a steam expert and always strive to make my articles as accurate as possible and always appreciate constructive criticism.