Boiler Feed Pump Calculator
Accurately determine Total Dynamic Head, Net Positive Suction Head available (NPSHa), and recommended motor HP for boiler feed pumps.
Includes 10% safety factor and elevation head.
How It Works & Formula
TDH is calculated by converting all pressure components to feet of head. BHP uses the flow rate, TDH, and an assumed 75% pump efficiency.
Calculation Steps:
- 1Convert boiler pressure and friction losses from psi to feet of head by multiplying by 2.31.
- 2Add elevation head and apply the safety margin percentage.
- 3Calculate NPSHa using atmospheric pressure and vapor pressure at the given temperature.
- 4Determine Brake Horsepower (BHP) using (Flow × TDH) / (3960 × Efficiency).
Example: 150 psi Boiler, 100 GPM
Given Parameters:
Step-by-Step Calculation:
- 1Boiler head: 150 psi × 2.31 = 346.5 ft
- 2Friction head: 10 psi × 2.31 = 23.1 ft
- 3Base TDH: 346.5 + 23.1 + 15 = 384.6 ft
- 4Add 10% safety margin: 384.6 × 1.1 = 423.1 ft TDH
- 5BHP: (100 × 423.1) / (3960 × 0.75) = 14.2 HP
For this system, the pump must overcome 423 feet of head. A calculation of 14.2 BHP means you should select the next standard motor size, which is a 15 HP motor.
Standard Motor HP Sizes
NEMA standard horsepower ratings for electric motors.
| Standard Size (HP) | Next Size Up (HP) | Industrial Standard (HP) |
|---|---|---|
| 1, 1.5, 2, 3 | 5, 7.5, 10 | 15, 20, 25 |
| 30, 40, 50 | 60, 75, 100 | 125, 150, 200 |
Frequently Asked Questions
What is NPSH?
Net Positive Suction Head (NPSH) is the total suction head in feet of liquid absolute, determined at the suction nozzle, less the vapor pressure of the liquid. It is critical for preventing cavitation.
Why do we multiply psi by 2.31?
The factor 2.31 is the conversion constant to change pressure in psi to feet of head for water at standard temperatures. One psi equals 2.31 feet of water column.
How do I choose pump efficiency?
If you don't know the exact pump curve, 75% (0.75) is a standard conservative estimate for centrifugal boiler feed pumps. Actual efficiencies can range from 60% to 85%.
Why is a safety margin necessary?
A safety margin (typically 10-20%) accounts for wear over time, fouling in pipes that increases friction, and fluctuations in boiler pressure that require extra head to overcome.
What happens if feedwater is too hot?
Higher temperatures increase the vapor pressure of water, which decreases NPSHa. If NPSHa falls below NPSHr (required by the pump), the pump will cavitate and fail.
How do you calculate required flow rate?
Flow rate (GPM) is typically determined by the boiler's maximum evaporation rate. A rule of thumb is 1 GPM for every 10 Boiler HP, plus a 1.5x continuous operation factor.
Can I oversize a boiler feed pump?
Oversizing can lead to operation too far to the left on the pump curve, causing excessive vibration, overheating, and premature seal failure. Sizing should be as accurate as possible.
What is TDH?
Total Dynamic Head (TDH) is the total equivalent height that a fluid is to be pumped, taking into account friction losses in the pipe and the pressure needed to enter the boiler.
CalcHub Editorial Team
Engineering & Education Content Specialists
Standards reviewed & updated: