US & Canada 1-800-786-8545 / International 1-360-292-4050

Modeling a Variable Speed Pump in PIPE-FLO

  • Print this page
    Print this page
  • Remove Highlighting
    Remove Highlighting
  • Edit this Article
    Edit this Article
  • Export to PDF
    Export to PDF

When designing systems in which the system demand varies (for example, HVAC systems with varying loads throughout the day or year), variable speed pumps may be an effective cost savings option.  In these systems, the pump speed can be adjusted to meet the varying demands of the system.

Figure 1 below shows a pump curve superimposed on a system resistance curve for a simple system with one pump:

Figure 1

In this example, there are two different operating points:  The design point which the pump was originally sized for is at 600 gpm.  There is also a secondary operating point which is at 400 gpm.  When the system is required to operate at 400 gpm, there are two options:

1)  Run the pump at a fixed speed and introduce more resistance into the system (for example, by using a control valve).  The dP shown in Figure 1 is the pressure drop required across the valve to limit the flow rate to 400 gpm.  The energy dissipated by the valve is wasted energy.

2)  Use a variable frequency drive to adjust the pump curve down to meet the 400 gpm operating point (see Figure 2 below).  This option saves energy, but the savings in operating cost must be weighed against the cost of implementing a variable speed drive.

Figure 2

Once you have determined that a variable speed drive is feasible, and you have selected the pump from a manufacturer's electronic catalog, it can be modeled in PIPE-FLO by setting the pump to run at a variable speed setting.  When a calculation is performed, PIPE-FLO applies the Pump Affinity Rules to determine the speed necessary to meet the new operating point.  The speed is calculated using the following equation:

(H1/H0) = (N1/N0)2

where:  H = pump head in feet
            N = impeller speed in rpm

In the above example, you would set the pump to run at a Variable speed flow rate of 400 gpm and then run the calculations.  When you point your mouse at the pump, the fly-by viewer at the bottom of the window will indicate the pump speed has been adjusted to 2920 rpm.  Also, the pump graph window will display the adjusted pump curve.

Article Details

Last Updated
15th of September, 2009

2005, 2007, 2009

Would you like to...

Print this page Print this page Email this page Email this page Remove Highlighting Remove Highlighting Edit this Article Export to PDF

User Opinions (5 votes)

100% thumbs up 0% thumbs down

How would you rate this answer?

Thank you for rating this answer.

Visitor Comments

  1. Comment 1 Posted by: Zartosht Oveisi

    Would you please advise me about the software which can help us to model the variable speed pump in a thermal power plant feedwater line? Best Regards

  2. Comment 2 Posted by: Engineered Software Inc.

    Our PIPE-FLO Professional software would be suitable for this application. You may want to download the demo to explore the capabilities of the program. The PIPE-FLO Professional demo is available for free on our website. There is a download link on the product information page:

Post a comment

To post a comment for this article, simply complete the form below. Fields marked with an asterisk are required.

Entering your email address is optional. It will not be displayed on this page, but may be used by our staff to contact you regarding your comment.

For technical support, please do not use this form. Use our Web Support Form or email, instead.

* Comment:
* Enter the code below: