Often a pump needing to provide a fixed pressure at the pump discharge does so by adjusting the pump speed with a variable frequency drive (VFD).  This is not a native feature of the PIPE-FLO program.  This knowledge base article describes how to model a variable speed pump to control the discharge pressure to a set value.

To model a pump with a VFD you must have pump curve data entered, either by selecting a pump through PIPE-FLO or by entering the pump performance data in the pump's dialog box.This information shows you how the pump operates at a fixed speed, and allow the pump curve to be adjusted for speed.

This system has a pump supplying Demineralized Water to thirteen sprayers.Ideally, each sprayer will receive around 20 gpm, and the pump will be discharging at 28 psig.

Let's calculate the system.

Notice the spray flow rates are all over 20 gpm and the discharge pressure is 35.26 psig.Also, the total head required for the system is 72.95 ft.We can model a pump with a VFD to control the flow rate and the discharge pressure.

To simulate the operation of a pump with VFD maintaining a set pressure requires the addition of two demand pressures and the pump set to a fixed flow rate.  The figure below shows how to model the system in PIPE-FLO when the objective is having the pump deliver a fixed pressure of 28 psi at the pump discharge.

In PIPE-FLO, when simulating a pump with a VFD you always select the pump to run as a set flow rate, but the flow rate must be calculated based on the needs of the system.  PIPE-FLO then calculates the Total Head needed to meet the system conditions.  Our objective is to supply a fixed pressure of 28 psi at the pump discharge regardless of the system conditions.  

To simulate 28 psi at the pump discharge:

Insert a demand pressure (Set in) and set the pressure to 28 psi

Install a second demand pressure further downstream of the pump (Set out) and set that to 28 psi

Close the pipeline between the two pressure sources.

This drawing shows how to simulate a pump with a variable speed drive controlling on the pumps discharge pressure.

The Set in demand pressure provides the value the discharge of the pump must produce.  The Set out demand pressure simulates the pressure acting on the remainder of the piping system.  Closing the pipeline between the demand pressures isolates the operation of the centrifugal pump from the system.  

When performing a calculation PIPE-FLO calculates the flow rate from the Set out pressure source to the rest of the system (259.6 gpm for this example).We then set the pump for a flow rate of 259.6 gpm, and PIPE-FLO calculates the total head required of the pump.

To determine the pump speed, with the results calculated, we will look at the pump curve.

PIPE-FLO displays the pump curve at the manufacturers test speed.It also shows the pump operating point (the red bracket).The operating point is the head needed to deliver the 259.6 gpm when the pump discharge is set to 28 psi.To determine the speed, we'll click on the down elevator button next to the pump speed box until the pump curve intersects the calculated operating point.

Notice from the new pump curve that the pump speed is 1630 rpm.  By using this method, we were able to determine the pump speed and total head required to supply about 260 gpm with a discharge pressure of 28 psi.