If a boiler (such as in a steam engine) is under pressure, then how is the water level maintained? Surely the water would have to be at an equal pressure to the steam inside the boiler.
Water is added to a boiler FIRST! (Get a lil experience and then it'll be funny!) Seriously though, yes, your boiler feed pump delivers water at a pressure exceeding that of the outlet steam pressure.
Multi stage feedwater pumps are used to pressurize the water to a higher pressure than the boiler drum pressure, so the flow can be regulated by a feedwater control valve. These pumps sometimes have 2 parallel water paths along a single shaft, with an inlet stage at each end of the shaft and both high pressure outlets near the center of the shaft, so the shaft seals at the ends of the shaft do not have to deal with the high outlet pressure. This also balances the the axial forces of the water pressure on the two ends of the shaft. It can take hundreds of horse power to drive a large feedwater pump. -- Regards, John Popelish
Water is pumped in with a separate pump. Initially the boiler os cold and at low pressure and can be filled easily, when it is hot and under pressure a pump is needed. In old steam engines the pump was operated by steam produced by the boiler - no steam, no water.... With railway engines injectors are used, not quite traditional pumps and these use the energy in the steam directly to pressurise the water to pump into the boiler (last thing, tap water is at about 10psi - if your boiler is low pressure (below this) then mains water could just be used!)
Correct. A pump supplies the 'make up water', often preheated, to the boiler. The supply pressure does indeed have to be more than the pressure of the boiler, and the water is passed through a one way valve.
