Calculating Net Positive Suction Head (NPSH)

  Product(s): WaterGEMS, WaterCAD
  Version(s): CONNECT Edition, V8i
  Area: Modeling

Problem

Is there a way of calculate the net positive suction head (NPSH) directly in WaterGEMS and WaterCAD?

Solution

Starting with the CONNECT Edition Update 1 release of WaterGEMS and WaterCAD, you can now model Net Positive Suction Head (NPSH) directly in the program. 

Cavitation occurs in a fluid when the absolute pressure drops below the vapor pressure of the fluid, resulting in cavitation (formation and collapse of small vapor pockets). The most common place where this occurs in water and wastewater systems is at the suction side of a pump. The head needed to prevent cavitation is called the Net Positive Suction Head required (NPSHr). This value is compared with the Net Positive Suction Head Available (NPSHa) at the pump. If NPSHa > NPSHr, then cavitation damage is unlikely to occur.

The software calculates the NPSHa and can compare it with the NPSHr which the user obtains from the pump specifications. Available NPSH is calculated at every time step for every pump using the equation below:

NPSHa = Hb + HGL - Elev - Hvap - Loss

Where: Hb = barometric pressure, ft
HGL = hydraulic grade at pump suction, ft
Elev = elevation of pump impeller
Hvap = vapor pressure (absolute) of water at temperature, ft
Loss = head loss between inlet of pump and impeller (Usually negligible), ft

Barometric pressure and vapor pressure can be specified in the calculation options.

Starting with CONNECT Edition Update 1, a NPSH Required field is available in the  Pump Definition for a pump. To include NPSH analysis in the calculation, check the box beside “Define NPSH Required Curve for pump cavitation check?” You can then include flow and head values that represent the required head and flow to satisfy this.

New results fields “NPSH (Available)” and “NPSH (Required)” are available in the pump properties and pump FlexTable. “NPSH (Available)” is the NPSH calculated by the program; “NPSH (Required)” is the NPSH derived from the table you entered in the pump definition.

After computing the model, the program will check if the available NPSH calculated in the model is greater than the required NPSH entered in the curve. New property fields “NPSH Fails (Now)” and “NPSH Fails (Ever)” are also included to report if the NPSH fails for current time step or any time in the simulation. If the available NPSH is less than the required NPSH, this will return a value of True. If the available NPSH is greater than the required NPSH, a value of False is returned. This can be useful with queries and color coding.

User notifications are generated when the NPSH requirement fails, enabling you to see which time steps may be impacted. It is also possible to graph the available and required NPSH to see a graphical representation of the results over time.

For older versions below CONNECT Edition Update 1

Previously (before 10.01.XX.XX), you needed to use WaterObjects.NET to code in a way to calculate the net positive suction head And starting with V8i Select Series 2, formula-based user data extensions (UDX) can be used.

This UDX allows the user to create their own results field that can be based on constants as well as calculated results already in the model. With this, you can use the formula to calculate NPSH as a formula-based UDX to display this directly in the program without needing to use WaterObjects.NET to implement it, or having to export data to Excel. Note that if changes are made to the model, you will need to recompute to see the UDX value update as well. 

See Also

Creating Custom Calculated Results with Formula User Data Extensions 

What's New with WaterGEMS and WaterCAD CONNECT Edition Update 1 

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