How Do You Size a Boiler Feed Pump

Properly sizing a boiler feed pump is essential for ensuring optimal boiler operation and efficiency. The boiler feed pump is responsible for supplying water to the boiler at the required pressure and flow rate. Incorrectly sized pumps can lead to issues such as inadequate water supply, excessive energy consumption, and premature pump failure. To size a boiler feed pump accurately, follow these steps:

Step 1: Determine the Boiler’s Steam Output

The first step in sizing a boiler feed pump is to determine the boiler’s maximum steam output. This information can be found in the boiler manufacturer’s specifications or calculated based on the boiler’s heating surface area and steam pressure.

To calculate the steam output, use the following formula:

Steam Output (lb/hr) = Heating Surface Area (ft²) × Evaporation Rate (lb/ft²/hr)

The evaporation rate depends on the boiler type and operating pressure. For example, a watertube boiler operating at 150 psig may have an evaporation rate of 5 lb/ft²/hr.

Step 2: Calculate the Feedwater Flow Rate

Once you know the boiler’s steam output, you can calculate the required feedwater flow rate. The feedwater flow rate must be sufficient to replace the water evaporated as steam and account for any losses due to blowdown or leaks.

To calculate the feedwater flow rate, use this formula:

Feedwater Flow Rate (gpm) = Steam Output (lb/hr) ÷ (500 × [1 - Blowdown Rate])

The constant 500 represents the approximate number of pounds of water per hour that equals 1 gallon per minute (gpm). The blowdown rate is the percentage of water deliberately discharged from the boiler to control dissolved solids concentration. A typical blowdown rate ranges from 1% to 5%.

Step 3: Determine the Required Pump Discharge Pressure

The boiler feed pump must overcome the boiler’s operating pressure and any pressure losses in the feedwater system. To determine the required pump discharge pressure, add the boiler operating pressure to the total head loss in the feedwater piping, valves, and fittings.

Pump Discharge Pressure (psig) = Boiler Operating Pressure (psig) + Total Head Loss (ft) ÷ 2.31

The constant 2.31 is used to convert feet of head to psi.

Step 4: Select a Pump Based on Flow Rate and Discharge Pressure

With the required feedwater flow rate and pump discharge pressure known, you can select a boiler feed pump that meets these specifications. Consult pump manufacturer catalogs or work with a pump supplier to choose a pump with a performance curve that meets your requirements.

Consider factors such as pump efficiency, materials of construction, and motor size when making your selection. Opt for a pump with a high efficiency to minimize energy consumption and operating costs.

Step 5: Consider Redundancy and Future Expansion

When sizing a boiler feed pump, it’s important to consider system redundancy and potential future expansion. Installing multiple pumps in parallel can provide backup capacity in case of pump failure and allow for increased flow rates if the boiler steam output is expanded in the future.

Size each pump to handle at least 50% of the total required flow rate to ensure adequate redundancy. This approach also allows the pumps to operate at their best efficiency points, reducing energy consumption and extending pump life.

Boiler Feed Pump Calculation Example

Let’s walk through an example of sizing a boiler feed pump for a watertube boiler with the following specifications:

• Heating Surface Area: 5,000 ft²
• Operating Pressure: 150 psig
• Evaporation Rate: 5 lb/ft²/hr
• Blowdown Rate: 3%
• Total Head Loss: 50 ft

Step 1: Calculate the Steam Output

Steam Output = 5,000 ft² × 5 lb/ft²/hr = 25,000 lb/hr

Step 2: Calculate the Feedwater Flow Rate

Feedwater Flow Rate = 25,000 lb/hr ÷ (500 × [1 - 0.03]) = 51.5 gpm

Step 3: Determine the Required Pump Discharge Pressure

Pump Discharge Pressure = 150 psig + 50 ft ÷ 2.31 = 171.6 psig

Step 4: Select a Pump
Based on the calculated flow rate of 51.5 gpm and discharge pressure of 171.6 psig, select a pump with a performance curve that meets or exceeds these requirements.

Step 5: Consider Redundancy
For redundancy, install two pumps in parallel, each sized for at least 50% of the total flow rate (25.75 gpm).

By following these steps and using the provided formulas, you can accurately size a boiler feed pump to ensure optimal boiler performance, energy efficiency, and reliability. Remember to consider factors such as redundancy and future expansion when making your final pump selection.