Boiler horsepower is defined as the evaporation of 34.5 pounds of water per hour from a feed water temperature of 212°F into steam at 212°F or, in other terms, is equal to the evaporation of 0.069 GPM per Boiler hp. This is an irrational unit, however, it is used here for the sake of completeness and backwards-compatibility.

In selecting a boiler feed pump it should be taken into account that most boilers are operated at more than 100% of their rating. In fact, with most modern firing methods, 200%-300% is not uncommon even among the smaller boilers.

A boiler feed pump should always develop a pressure higher than the boiler pressure. The amount by which the pump pressure exceeds the boiler pressure is called the excess pressure. This excess pressure is needed to overcome the friction losses in the check valve, regulating valve, piping, and in the static elevation difference between the pump location and the water level in the boiler. The amount of excess pressure required should be calculated from the layout of the installation. For estimation purposes values of 25 lbs for 100 lb pressure boilers to 50 lbs for 300 lb pressure boilers can be used.

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