Introduction
Positive Displacement (PD) pumps operate by trapping a fixed amount of fluid and forcing it from the inlet to the outlet. Unlike centrifugal pumps that generate flow by increasing the fluid's kinetic energy through impeller rotation, PD pumps create flow by physically displacing the fluid. This fundamental difference means PD pumps:
Deliver nearly constant flow regardless of discharge pressure
Provide more precise flow control for applications requiring accuracy
Are self-priming and can handle higher viscosity fluids
Generate higher pressures at lower flow rates compared to centrifugal pumps
FluidFlow software enables engineers to model PD pumps with high accuracy, accounting for critical factors like slip, efficiency corrections, and performance degradation that are essential for realistic system modeling and performance prediction.
PD Pump Options in FluidFlow
FluidFlow offers two primary options for modeling positive displacement pumps, depending on the information available:
1. Positive Displacement Pump or Compressor
This option requires input for performance correction or volumetric efficiency factor.
By default, volumetric efficiency factor is set to 1 unless specified otherwise in the pump database
Duty flow calculation: Operating Speed × Displaced Volume per Revolution × Performance Factor
The performance factor accounts for pump slippage or performance degradation as a function of pump speed
Required vendor information for database entry:
Max Operating Pressure
Max Operating Speed
Pump Suction and Discharge pipe size
Pump Efficiency
Displaced Volume per Revolution
Performance Factor
2. Rotating Positive Displacement Pump
This option requires slip capacity data, which is dependent on fluid viscosity.
Slip refers to the amount of fluid that remains in the pump cavity, reducing the actual fluid delivered
Screw pumps are a typical example of rotating PD pumps
Flow remains nearly constant regardless of pressure, but is sensitive to viscosity effects
Required vendor information for database entry:
Max Operating Pressure
Max Operating Speed
Pump Suction and Discharge pipe size
Pump Efficiency
Displaced Volume per Revolution
Max Viscosity Limit
Slip Data
Accessing the PD Pump Library in FluidFlow
FluidFlow includes a comprehensive library of PD Pumps and Rotating PD Pumps that users can:
Select from existing models in the library for immediate use
Add new pump models as needed — once added, they are stored in the database and available for all future projects
Defining a PD Pump in the Database
When adding a Positive Displacement Pump to the database, the key setting to define is "Use Performance Correction", which can be set to Yes or No:
Choosing "No" → FluidFlow automatically applies a correction factor of 1
Choosing "Yes" → You must define the curve relationship for Performance Factor (a measure of performance degradation as a function of pump operating speed)
Understanding the Performance Correction Factor
The Performance Factor:
Represents performance degradation due to operational conditions
Typically ranges between 0 and 1
Accounts for pump slippage and performance decline at different pump speeds
Defining a Rotating PD Pump in the Database
When adding a Rotating Positive Displacement Pump to the database, slip data or information on slip capacity vs. viscosity is required.
Slip and Its Impact on PD Pump Performance
Slip is a major factor affecting PD pump performance:
In ideal conditions, the expanding inlet cavity is filled with fluid from the inlet pipeline
When slip occurs, some fluid from the discharge side flows back to the inlet side through pump clearances
Slip reduces effective flow and overall pump performance
Key Insight: The degree of slip directly influences PD pump performance and is affected by:
Fluid viscosity
Internal clearances
Pump geometry
Differential pressure
Calculating Pump Slip When Data is Unavailable
If slip data is not available, you can still use the rotating positive displacement pump by calculating slip using flow rate vs. head data for specific viscosities:
Get the pump output at no slip condition (QNo Slip). This is the pump flow at zero differential head and is not affected by changes in viscosity
Subtract from QTheoretical the pump output (QActual) at varying differential head and viscosity to determine how much flow rate is lost via slip
Calculate Slip Capacity
Note: The value of slip capacity is constant for a given liquid viscosity.
Using the Auto-size Feature
Alternatively, you can use the Auto-size feature of FluidFlow and specify the design flow requirement for your pump:
This is commonly used during the initial design stage when limited information is available
Helps determine the required booster differential pressure to meet the system design flow
Best Practices
Always verify the Performance Factor curve when "Use Performance Correction" is set to Yes
Use manufacturer data when available to define slip-related performance loss
Avoid assuming zero slip — this can lead to overly optimistic performance predictions
Consider viscosity effects on slip, especially for rotating PD pumps like screw pumps
For high-viscosity applications, pay special attention to slip data accuracy
Remember that PD pump flow is less affected by pressure changes but more by viscosity compared to centrifugal pumps
FAQs
Q: Do I need to re-add the same PD Pump for each project?
A: No, once added to the database, the pump is available for all future projects.
Q: What is the typical range for Performance Factor?
A: Between 0 and 1, depending on speed, slip, and other operational factors.
Q: How do I choose between a standard PD pump and a Rotating PD pump node in FluidFlow?
A: Choose based on available data: use the standard PD pump when you have performance factor data, and the Rotating PD pump when you have slip capacity data related to viscosity.
Q: Can I model a screw pump in FluidFlow?
A: Yes, screw pumps are typically modeled using the Rotating PD Pump option as they are particularly sensitive to slip caused by viscosity, internal clearances, and pump geometry.
Q: What should I do if I don't have manufacturer slip data?
A: You can calculate slip using flow rate vs. head data at different viscosities, or use the Auto-size feature during preliminary design stages.
Conclusion
Modeling PD Pumps in FluidFlow is straightforward when efficiency corrections and performance factors are defined correctly. Understanding slip behavior ensures accurate flow predictions and reliable system design. The software offers flexible options based on available data, whether using the standard PD Pump model, the Rotating PD Pump model, or the Auto-size feature for preliminary design.
By correctly modeling PD Pumps in FluidFlow, engineers can accurately predict system performance, account for slip losses, and ensure optimal equipment selection, ultimately leading to more efficient designs and reduced operational issues in real-world applications.