When the user changes the fluid, why doesn't the calculated pump head change?

  Applies To 
  Product(s): WaterCAD, WaterGEMS, HAMMER
  Version(s): V8i, CONNECT Edition
  Area:  N/A
  Original Author: Mark Pachlhofer, Bentley Technical Support Group

Problem

When changing the liquid kinematic viscosity and liquid specific gravity in the calculation options (which may be done by way of the temperature) to model another fluid the results at a pump do not change.

Background

The pump characteristic curve is assumed to be representative of the pump for the liquid that is being modeled. As such, changes to the liquid properties will have no direct effect on pump characteristics. It will have an effect on the reported water power, however, because this is calculated based on pump head, discharge, and fluid density.

There will also be an effect on the system headlosses (and therefore the pump operating point), however, if the kinematic viscosity of the fluid is changed when using the Darcy-Weisbach friction method. Depending on how significant the viscosity change is, even this difference may be very small (depending on flow rate).

Also, keep in mind that empirical friction methods such as Hazen William's and Manning's are only applicable to water at room temperature (20 deg. C / 68 deg. F)."

Solution

You will need to change the pump curve to represent the flow and head for the fluid you are modelling. For effects on friction losses, you would want to use the Darcy-Weisbach fiction method as mentioned above. This is selected in the calculation options, and the roughness height factor is entered in the pipe properties.

Anonymous
Recommended
Related