Imagine trying to sip a milkshake through a tiny straw – it takes much more effort than sipping water through the same straw, right?
This simple example illustrates the challenges of handling fluids with different viscosities, a factor that plays a critical role in the performance of positive displacement (PD) pumps.
Join us while we delve into the fascinating world of fluid dynamics, exploring how viscosity affects pump performance and providing tips for optimizing pump selection and operation based on fluid properties. So grab your favorite milkshake (or any viscous fluid), and let’s start!
Effects of Viscosity on Positive Displacement Pump Performance
In this section, we’ll unravel the intriguing effects of viscosity on pump performance, from temperature effects to power requirements and cavitation risks. Viscosity can affect positive displacement pumps in the following ways:
- Temperature Effects on Viscosity
- Power Requirements
- Cavitation Risk
Temperature Effects on Viscosity
The viscosity of liquids decreases rapidly as they heat up, and vice versa. Some commonly pumped fluids, such as gear oil, have viscosities that increase exponentially with temperature.
It’s essential to understand the operating temperature range of the pumped fluid so that the best method of handling the viscosity changes can be discussed. For example, cold temperature additives are used for fuels like diesel. Newer oils, such as those used in electric vehicle lubrication, are formulated with viscosity enhancers.
Higher viscosity fluids require more energy to pump due to increased resistance to flow. As a result, the power requirements for the pump will increase, leading to higher energy consumption and operational costs. It’s crucial to accurately estimate the power requirements when selecting the pump motor to avoid overloading and premature motor failure.
Cavitation occurs when the fluid pressure drops below its vapor pressure, leading to the formation and subsequent collapse of vapor bubbles. High-viscosity fluids can increase the risk of cavitation in positive displacement pumps, as they require more suction head to overcome the resistance to flow. Insufficient suction head can lead to pressure drops, resulting in cavitation, which can cause pump damage and reduced performance.
Strategies for Handling High-Viscosity Fluids
Now that we’ve delved into the challenges of pumping high-viscosity fluids, it’s time to discover the secrets to successfully navigating the complex world of high-viscosity fluid handling. From selecting the perfect pump type to optimizing pump speed and exploring innovative solutions like fluid heating, we’ve got you covered.
Strategies for handling high-viscosity fluids include:
- Pump selection
- Optimizing Pump Speed
- Heating the Fluid
- Properly Sizing Suction Piping
Choose a positive displacement pump designed specifically for high-viscosity fluids, such as gear pumps, lobe pumps, or progressing cavity pumps. These pumps typically have larger clearances and are designed to handle the increased resistance to flow.
Optimizing Pump Speed
Reducing the pump speed can help minimize slip and improve volumetric efficiency when handling high-viscosity fluids. Operating at lower speeds also reduces wear on pump components and lowers the risk of cavitation.
Heating the Fluid
For temperature-sensitive fluids, heating the fluid can lower its viscosity, making it easier to pump. This can improve the pump’s efficiency and reduce energy consumption. However, care must be taken not to exceed the fluid’s thermal limits or damage its properties.
Properly Sizing Suction Piping
Design the suction piping system to minimize pressure losses, ensuring adequate suction head is available to prevent cavitation. This may include using larger diameter pipes, minimizing bends, and avoiding obstructions.
Viscosity Correction for Positive Displacement Pumps
When dealing with fluids of varying viscosities, understanding how to apply viscosity corrections to pd pump performance data is crucial for accurate pump sizing and selection. Here, we explore the concept of viscosity correction, its importance, and the steps to apply these adjustments to PD pump performance data.
Steps for Viscosity Correction
Positive displacement pump performance characteristics, such as flow rate, pressure, and efficiency, can be significantly affected by fluid viscosity. Applying viscosity corrections to pump performance data ensures that the pump is accurately sized and selected for the specific fluid properties and operating conditions, ultimately leading to optimal pump performance and reliability.
Steps for viscosity correction include:
- Identify Fluid Viscosity
- Determining Pump Performance at Reference Viscosity
- Identify the Appropriate Correction Factors
- Applying the Correction Factors
Identify Fluid Viscosity
The first step is determining the fluid’s viscosity at the operating temperature. This information can typically be found in the fluid’s data sheet or through laboratory testing.
Determining Pump Performance at Reference Viscosity
Obtain the PD pump performance data at a reference viscosity (usually for water, with a viscosity of 1 centipoise) from the pump manufacturer. This data typically includes flow rate, pressure, and efficiency values.
Identify the Appropriate Correction Factors
Consult the pump manufacturer’s guidelines or charts to identify the correction factors for your specific pump type and fluid viscosity. These factors may include flow rate correction, pressure drop correction, and efficiency correction.
Apply the Correction Factors
Adjust the pump performance data at the reference viscosity using the identified correction factors. This will provide the pump performance data for the specific fluid viscosity:
- Flow Rate Correction: Multiply the reference flow rate by the flow rate correction factor.
- Pressure Drop Correction: Multiply the reference pressure drop by the pressure drop correction factor.
- Efficiency Correction: Multiply the reference efficiency by the efficiency correction factor.
Re-Evaluate the Pump Selection
With the corrected pump performance data, re-evaluate the pump selection to ensure it meets the system requirements for the specific fluid properties and operating conditions.
Thixotropic & Shear-Thinning Fluids: Navigating the World of Non-Newtonian Fluids
Thixotropic and shear-thinning fluids exhibit unique viscosity characteristics that can have significant implications for PD pump performance. Let’s explore the behavior of these fluids and reveal strategies for pumping them effectively and efficiently.
Thixotropic fluids experience a decrease in viscosity over time when subjected to constant shear stress, and their viscosity returns to its original state once the stress is removed. Common examples of thixotropic fluids include certain paints, gels, and ketchup. When pumping thixotropic fluids, it’s essential to consider the time-dependent viscosity changes, as this can affect the pump’s efficiency and the ability to maintain consistent flow rates.
Shear-thinning fluids, known as pseudoplastic fluids, decrease viscosity as the shear stress increases. This behavior can be advantageous in PD pump applications because it allows the fluid to flow more easily through the pump under high shear conditions, reducing the energy required for pumping. Examples of shear-thinning fluids include certain polymer solutions, food products like honey, and blood.
Strategies for Pumping Thixotropic & Shear-Thinning Fluids
When pumping thixotropic and shear-thinning fluids, keep the following principles in mind:
- Pump Selection
- Optimize Pump Speed
- Pulsation Dampeners
- Proper System Design
Choose a PD pump type suitable for handling non-Newtonian fluids, such as progressing cavity pumps, peristaltic pumps, or lobe pumps. These pumps can provide gentle handling and minimize the potential for fluid degradation due to shear forces.
Optimize Pump Speed
Selecting the appropriate pump speed is crucial for maintaining consistent flow rates and ensuring the proper handling of thixotropic and shear-thinning fluids. Too high of a pump speed can cause excessive shear forces, while too low can lead to insufficient flow rates and potential settling or separation issues.
Adding pulsation dampeners to the pump system can help minimize pressure fluctuations and ensure a smoother flow, especially when handling thixotropic fluids with time-dependent viscosity changes.
Proper System Design
Designing the piping system and other components to minimize pressure losses and shear forces can enhance the performance of PD pumps handling non-Newtonian fluids. This may include larger diameter pipes, minimizing bends, and avoiding obstructions.
Conquering PD Pump Performance Challenges
Understanding the impact of viscosity on positive displacement pump performance is essential for efficient and reliable operation. By considering the fluid properties and selecting the appropriate pump type, speed, and system design, you can optimize the pump’s performance and extend its service life, even when handling high-viscosity fluids.
Have more questions? Contact one of our experts!