Modeling a Positive Displacement Pump With A Manual Pump Curve

Modeling a positive displacement (PD) pump in PIPE-FLO is a common question we’re frequently asked. Even if you’re not a PIPE-FLO user, understanding how to model a positive displacement pump may prove useful for you. There are two methods for modeling a PD pump:

  1. Creating a pump bypass line with a component that represents the slippage in the pump.
  2. Reading the flow rate versus pressure data from the manufacturer pump curve and input this data into PIPE-FLO as a pressure gain device.

Both methods accurately model a positive displacement pump, where you have to apply realistic flow limits to the model based on the actual performance of the pump.

In this article, we’ll go over the second way to model a positive displacement pump: reading the flow rate versus pressure data to input into PIPE-FLO.

What is a Positive Displacement Pump?

First, let’s define a positive displacement pump. A positive displacement (PD) pump moves a fluid by enclosing a fixed volume and moving it through the system. The pumping action is cyclic and can be driven by pistons, screws, gears, rollers, diaphragms or vanes.

Positive Displacement Curves

All PD pump manufacturers display their pump curves differently. The key data is flow rate and pressure drop corresponding to the viscosity of the fluid and the speed of the pump. The following graphs are PD pump curve examples for gear, lobe, and vane pumps.

Manufacturer: Pulsafeeder
Type: Eclipse Gear Pump, Model 5
Graph Axes: Pressure vs Flow Rate with speed lines

Manufacturer: Viking Pumps
Type: RL Industrial Lobe Pump, Model RL 150
Graph Axes: Flow Rate vs. Pump Speed with Pressure Lines

Manufacturer: Corken
Type: Coro-Vane Pump for LPG and NH3, Model 1021, 950 RPM’s
Graph Axes: Flow Rate vs. Differential Pressure at a fixed speed

All three of these curves display the flow rate, pressure drop, and pump speed in different formats.

Adding the PD Pump Curve Data to PIPE-FLO®

Adding the PD pump curve data to PIPE-FLO® consist of four steps. The first step is to choose the PD pump curve that represents the viscosity of your fluid. Next, read the line or individual graph that represents the revolutions per minute (RPM) of the pump. For the Viking Lobe pump curve shown above, the viscosity is 150 CentiStokes and the RPM is along the horizontal axis.

For this article, flow and pressure data for 400 RPM is illustrated. If you zoom into the graph and read up the 400 rpm axis, you can read the flow rate off the vertical axes that correspond to the individual pressure lines.

The following is the PD curve data from this lobe pump.

The following is the PD curve data from this lobe pump.

The next step is to determine the pressure value when the flow is zero. This is needed because the first entry in the PIPE-FLO® Curve Data dialog box is the shut off head, or the pressure at a zero flow rate. So we will set Flow Rate (gpm) as the Y Intercept.

Although this value has no real meaning for a PD Pump, it is necessary in order for us to use the pressure gain device dialog box to model the PD Pump. An easy way to find the Y Intercept that corresponds to the Zero flow rate is through an Excel® spreadsheet and using the INTERCEPT (Y range, X range) function. Below is an example.

The Y intercept is 1240 for the above data.

The last step is adding the data to PIPE-FLO® as a pressure gain device. This is done after inserting a pressure gain device on the FLO-Sheet, right clicking on the pressure gain device, selecting Enter Curve Data and finally entering the data into the appropriate fields. Note that this may also be done through the pressure gain device’s Property Grid. An example of the entered data into the Curve Data dialog box is shown below.

PIPE-FLO® will create a pump curve based on the manual imputed data. The Viking Lobe pump curve is shown below.

The user needs to apply realistic flow limits to the model based on the actual performance of the pump. The above example shows at 400 RPM, the flow range is between 420 and 540 gpm and this is the realistic flow. Any flow values outside of this range would not be accurate because it is not practical based on the extrapolated pump data at 400 RPMs.

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