Sizing a boiler feed pump: head, NPSH and standby capacity

Boiler feed is one of the few duties where getting the pump right is not simply a matter of matching a flow rate to a head figure. The pump has to operate across a range, cope with a feed water temperature that changes the available NPSH, and still provide standby capacity.

Start with the maximum continuous rating of the boiler, add the margin the boiler manufacturer requires, and confirm the pump can deliver that at drum pressure plus the pressure drop through the economiser and pipework — not just at the design point on the curve.

Then check NPSH at the highest feed water temperature you will ever run. Deaerated feed water at 105 °C has very little NPSH margin left, and this is where marginal installations fail. A low-NPSH first stage or a lower-speed pump is often the difference between a reliable installation and one that cavitates at full fire.

Finally, decide the standby philosophy before selecting the pump. Duty and standby pumps sized at 100 % each behave very differently from two pumps at 50 % each with one spare, and the pipework and controls follow from that decision.

Multistage centrifugal ranges — Grundfos CR, Allweiler and Ritz HP series — cover most of these duties; send us the boiler data and we will return a selection with the NPSH margin stated.