HVAC Pump Net Positive Suction Head (NPSH) Calculations in Excel

Net Positive Suction Head (NPSH) is a concept that describes the amount of energy available in the liquid at the suction side of a pump to prevent boiling or cavitation. Boiling or cavitation occurs when the pressure of the liquid falls below its vapor pressure, which depends on the temperature of the liquid. To avoid this, the pump needs a certain minimum amount of NPSH, which is called NPSH required (NPSHr). This value is determined by the pump design and characteristics.

The NPSH available (NPSHa) is the amount of NPSH that the system provides to the pump. This value depends on the properties of the liquid, the ambient conditions, and the suction system design and geometry. The NPSHa can be calculated by measuring or estimating the following parameters:

The static suction head, which is the difference in elevation between the liquid surface and the pump centerline. This can be positive or negative, depending on whether the pump is above or below the liquid level.
The suction line losses, which are the friction losses and other losses due to the entrance and fittings in the suction pipe. These losses reduce the pressure and energy of the liquid as it flows to the pump.
The absolute pressure at the liquid surface, which is the atmospheric pressure plus or minus any additional pressure or vacuum applied to the tank or vessel containing the liquid.
The vapor pressure of the liquid at the pumping temperature, which is the pressure at which the liquid starts to boil. This value can be obtained from tables or charts for different liquids and temperatures.

The NPSHa can be calculated by adding the static suction head and the absolute pressure, and subtracting the suction line losses and the vapor pressure. The units of NPSH are usually feet or meters of liquid. The NPSHa must be greater than or equal to the NPSHr for the pump to operate properly and avoid cavitation. If the NPSHa is less than the NPSHr, the pump may experience reduced performance, increased noise and vibration, and damage to the impeller and other parts.

Basic Theory of NPSH

The NPSH is calculated using the formula:

$NPSH = \dfrac{P_{\text{atm}} - P_{\text{vapor}}}{\rho \cdot g} - h_{\text{f}} - h_{\text{L}}$

Where:

$P_{\text{atm}}$ is the atmospheric pressure (Pa),
$P_{\text{vapor}}$ is the vapor pressure of the liquid (Pa),
$\rho$ is the density of the liquid (kg/m³),
$g$ is the acceleration due to gravity (m/s²),
$h_{\text{f}}$ is the fluid’s elevation head (m),
$h_{\text{L}}$ is the losses head (m).

Procedures for NPSH Calculations in Excel

Gather Information:
- Obtain the atmospheric pressure ( $P_{\text{atm}}$ ) from local weather data.
- Determine the vapor pressure ( $P_{\text{vapor}}$ ) of the liquid at the pumping temperature.
- Find the density ( $\rho$ ) of the liquid from its properties.
- Define the elevation head ( $h_{\text{f}}$ ) and losses head ( $h_{\text{L}}$ ).
Enter Data into Excel:
- Create an Excel table with columns for atmospheric pressure, vapor pressure, liquid density, elevation head, losses head, and NPSH.
- Enter the gathered data into the respective columns.
Excel Formulas:
- In the NPSH column, use the formula:
```
            =((A2-B2)/(C2*D2))-E2-F2
```
  Adjust cell references based on your Excel table structure.

Scenario: Detailed Example

Let’s consider a scenario with the following data:

Atmospheric pressure ( $P_{\text{atm}}$ ): 101325 Pa
Vapor pressure ( $P_{\text{vapor}}$ ): 1000 Pa
Liquid density ( $\rho$ ): 1000 kg/m³
Elevation head ( $h_{\text{f}}$ ): 5 m
Losses head ( $h_{\text{L}}$ ): 2 m

Excel Calculation and Result

Enter the scenario data into the Excel table.
Apply the NPSH formula to calculate NPSH for each row.

$P_{\text{atm}}$	$P_{\text{vapor}}$	$\rho$	$h_{\text{f}}$	$h_{\text{L}}$	NPSH
101325	1000	1000	5	2	Result

MATLAB Comparison

For comparison, let’s implement the same calculation in MATLAB:


      P_atm = 101325;
      P_vapor = 1000;
      rho = 1000;
      g = 9.81;
      h_f = 5;
      h_L = 2;

      NPSH = ((P_atm - P_vapor) / (rho * g)) - h_f - h_L;
      disp(['NPSH: ', num2str(NPSH), ' meters']);

This MATLAB code will provide the NPSH result in meters.