How to Calculate Boiler Feed Pump Capacity and Head
Properly sizing a boiler feed pump is critical for maintaining safe water levels inside a steam boiler. The pump must be capable of delivering water at a rate slightly higher than the maximum evaporation rate of the boiler, and it must produce enough pressure to overcome the boiler's internal pressure, piping friction, and safety valve settings.
- lbs/hr = Maximum steam capacity of the boiler.
- SG = Specific Gravity of the feedwater (water density changes with temperature).
- 1.15 = Standard 15% safety margin added to the required flow rate.
- 1.20 = Standard 20% pressure margin to overcome system resistance.
Note: The 500 constant in the flow formula represents the weight of 1 GPM of water over an hour (8.33 lbs/gal × 60 mins).
Because boiler feedwater is typically heated in a deaerator before entering the pump, its density is lower than cold water. Therefore, the Specific Gravity (SG) must be factored in to accurately determine the Total Dynamic Head (TDH) in feet.
| Water Temperature (°F) | Specific Gravity (SG) |
|---|---|
| 60 °F (Standard Cold) | 1.000 |
| 150 °F | 0.980 |
| 200 °F | 0.963 |
| 220 °F (Typical Deaerator) | 0.954 |
| 250 °F | 0.942 |
Frequently Asked Questions
Why add a 15% margin to the flow rate?
A boiler feed pump must provide water faster than the boiler can evaporate it to quickly restore safe water levels during periods of high demand. Furthermore, the pump must be able to keep up during boiler blowdown procedures, and the 15% margin accounts for standard wear and tear over the pump's lifespan.
What is Total Dynamic Head (TDH)?
TDH is the total equivalent height that a fluid is to be pumped, taking into account friction losses in the pipe. Pump curves are generally rated in feet of head rather than pounds per square inch (psig), making this a crucial calculation for selecting the right pump impeller.
How does temperature affect pump sizing?
As water heats up, it expands and becomes less dense, lowering its Specific Gravity. Because centrifugal pumps move a constant volume (not a constant mass), a pump pushing hot water will physically move less "weight" of water per minute. The SG factor corrects the formula to ensure the pump moves enough actual mass to keep the boiler filled.