Steam Boiler Water Feed Principles
As the steam engine runs, the water in the boiler evaporates as it turns to steam that is used by the engine. There are a few ways to keep your steam boiler topped up with water.
1) Hand pump.
The popular type of hand pump is simply a lever-operated ram in a cylinder, which via a pair of inlet & outlet ball valves admits and releases the water as the ram is moved in & out of the cylinder. A hand pump can be used as the primary boiler feed, but whichever type of boiler feed pump is fitted to your steam plant, it is highly recommended to always have a reliable hand pump there, just in case your other water feed system fails.
2) Steam engine crankshaft driven pump - or axle pump on a steam locomotive.
A crankshaft driven pump is basically the same as a hand pump, but instead of hand power, it uses the steam engine to do the work. Engine driven pumps need a water bypass valve that allows adjustment of the amount of water going into the boiler, excess water is returned to the water tank.
Disadvantages of engine driven pumps is that they rob power from the engine & can slow the engine on the pumping stroke, which also exerts some pressure on the engine's bearings. Locomotives can be a little "jerky" on the pump stroke when the axle pump is used.
3) Weir Type steam pumps.
Sometimes referred to as "Donkey pumps"
Although not always strictly to the Weir pump design, these ingenious devices are a self-contained steam powered water pumping unit. They usually comprise of a single cylinder steam engine, which is self-starting owing to having a "shuttle piston" which is really a "dummy" piston in a cylinder. The purpose of the "shuttle piston is to operate a slide (or piston) valve to admit steam to either end of the main steam cylinder. A "Duplex" steam pump is a twin cylinder steam pump, having two cylinders & two water pumps. There is no need for a "shuttle" piston on a "Duplex pump" as each cylinder admits steam to one another.