Pump Discharge Pressure Calculator
Estimate discharge pressure based on flow rate using the pump curve approximation method.
Note: To calculate pressure from flow accurately, you need a reference point from your pump's performance curve (Rated Flow & Head) and the Shut-off Head (Head at zero flow).
1. Operational Inputs
2. Pump Curve Reference Data
Estimated Discharge Pressure
0 PSI
Differential Head at Target Flow:
0 ft
Differential Pressure (Boost):
0 PSI
Estimated Pump efficiency Point:
Normal
How to Calculate Pump Discharge Pressure from Flow Rate
Calculating the discharge pressure of a centrifugal pump based on a specific flow rate is a common task for process engineers and operators. Unlike positive displacement pumps, centrifugal pumps do not produce a fixed pressure; instead, they operate along a performance curve where pressure (Head) generally decreases as the flow rate increases.
To perform this calculation without complex software, we use a quadratic approximation of the pump curve. By defining the "Shut-off Head" (pressure at zero flow) and one "Rated Operating Point," we can mathematically estimate the pressure at any other flow rate.
The Formula
The calculation involves three main steps:
-
Approximating the Head: We assume the pump curve follows the equation:
H = H0 – B × Q2
Where H is Head, H0 is Shut-off Head, Q is flow, and B is a curve coefficient derived from your rated data. -
Converting Head to Pressure: Once the Head (in feet) is found for the target flow, it is converted to pressure (PSI) using the Specific Gravity (SG) of the fluid:
Pressure (PSI) = (Head (ft) × SG) / 2.31 -
Adding Suction Pressure: The pump only adds differential pressure. To get the final gauge discharge pressure, you must add the incoming suction pressure:
Discharge PSI = Suction PSI + Differential PSI
Understanding the Inputs
- Target Flow Rate (GPM): The flow rate at which you want to calculate the pressure.
- Suction Pressure (PSI): The pressure entering the pump eye. If drawing from an atmospheric tank at the same level, this might be 0 PSI (gauge).
- Specific Gravity: The density of the fluid relative to water. Cold water is 1.0. Heavier fluids (like brine) are >1.0, lighter fluids (like gasoline) are <1.0.
- Shut-off Head: This is found on the pump manufacturer's curve at 0 GPM (the Y-intercept).
- Rated Point: Pick any known operating point on the curve (e.g., Best Efficiency Point) to help the calculator define the curve slope.
Example Calculation
Imagine a water pump (SG = 1.0) with a suction pressure of 10 PSI. The pump curve shows a Shut-off Head of 150 ft. At a rated flow of 200 GPM, it produces 120 ft of Head.
If you want to know the pressure at 100 GPM:
- The calculator determines the curve coefficient based on the drop from 150 ft to 120 ft over 200 GPM.
- It calculates the new Head at 100 GPM (likely around 142.5 ft).
- It converts 142.5 ft to PSI: 142.5 / 2.31 ≈ 61.7 PSI.
- It adds Suction Pressure: 10 + 61.7 = 71.7 PSI Discharge Pressure.