The Impact of Viscosity on Positive Displacement Pump Performance

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:

  1. Temperature Effects on Viscosity
  2. Power Requirements
  3. 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.

 

Power Requirements

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 Risk

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:

  1. Pump selection
  2. Optimizing Pump Speed
  3. Heating the Fluid
  4. Properly Sizing Suction Piping

Pump Selection

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:

  1. Identify Fluid Viscosity
  2. Determining Pump Performance at Reference Viscosity
  3. Identify the Appropriate Correction Factors
  4. 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

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

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:

  1. Pump Selection
  2. Optimize Pump Speed
  3. Pulsation Dampeners
  4. Proper System Design

Pump Selection

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.

Pulsation Dampeners

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!

Types of Positive Displacement Pumps and Their Applications

Fluid enters. Flows. Fluid leaves. 

Functionally, that’s all that a pump is.

However, with complex and unique design features, the pump variations available are countless. One of the most widely known is positive displacement pumps.

What is a Positive Displacement Pump?

A positive displacement pump takes a specific amount of fluid and pushes it to its outlet. 

How do positive displacement pumps move fluid?

A cavity in the pump expands and contracts to cause displacement, which pushes the fluid from the inlet to the outlet. 

Pump designs vary, but most pumps can be divided into two categories: reciprocating and rotary.

Reciprocating Pumps

Reciprocating pumps move fluids through displacement by using a back-and-forth motion. 

There are three common types of reciprocating pumps:

  • Piston pumps
  • Plunger pumps
  • Diaphragm pumps

Piston and plunger pumps are functionally the same with slight design differences. These pumps will typically have a piston or plunger inside that shifts back and forth in a larger cylinder. This movement creates a vacuum-like suction on one side of the pump that pulls fluid in. When the piston or plunger moves to the other side, fluid is compressed and pushed through the outlet. 

On the other hand, diaphragm pumps use a flexible membrane to draw fluid into a pumping chamber, and push it out.

Rotary Pumps

Unlike the back-and-forth motion in reciprocating pumps, rotary pumps use rotating mechanisms, such as gears to pump and transfer fluid. These pumps create a vacuum-like suction to draw in the fluid, carry it within the teeth of the gear, and transfer it to the outlet.Types of Positive Displacement Pumps and Their Applications

Although the basic functionality remains the same, the three types of rotary pumps vary in design:

  1. Gear pumps use two or more gears that drive each other within a pumping chamber to create pockets of pumped fluid.
  2. Vane pumps move its vanes in and out of a rotor to create spaces that move fluid out of the pump.
  3. Lobe pumps use multiple rounding lobes that create pockets of fluid volume that are moved from inlet to outlet. 

While these three rotary pumps have very different designs, all positive displacement pumps have the same functionality – moving fluid from the inlet to the outlet.

Positive Displacement Pump End-Use Applications

Positive displacement pumps transfer fluids accurately and without a broad operating range. As a result, these pumps are widely used: 

  • Oil and gas industry
  • Agriculture
  • Food and beverage

Oil and Gas

As a reliable, accurate way to transfer fluids, these pumps are becoming an increasingly popular solution for efficiency. Positive displacement pumps frequently are found transferring crude oil, petroleum products, and natural gas.

Agriculture 

With their ability to handle a vast range of fluids and operating conditions, positive displacement pumps are the perfect fit for agriculture. These pumps are most commonly used in irrigation systems, but also transfer chemicals and work in water supply systems.

Food and Beverage

For the food and beverage industry, control is the main focus. Positive displacement pumps transfer fluid at a controlled, measurable rate while maintaining sanitary conditions.

Rotary lobe pumps and diaphragm pumps are two of the most frequently used pumps in the industry. Rotary lobe pumps transfer fluids such as dairy, fruit juices, and sauces. Diaphragm pumps are used for things such as processing ingredients.

Limitations of Positive Displacement Pumps

While positive displacement pumps have their advantages, they do have their limitations: 

  1. Less efficient than centrifugal pumps
  2. Handling low viscosity
  3. Maintenance

1. Less Efficient Than Centrifugal Pumps

As a positive displacement pump uses mechanical energy to push each volume of fluid against a system restriction, it generally uses more energy to move a given volume of fluid than a centrifugal pump, which has much less internal mechanical resistance

The conditions of service may determine that a centrifugal pump is better suited to a certain application.

2. Handling Low Viscosity

Since positive displacement pumps are designed for moving highly viscous fluids, working with low-viscosity fluids can result in slippage and leaks.

Slippage occurs when the fluid pumped leaks back into the clearance of the pump, instead of the outlet. Most positive displacement pumps also count on the fluid to provide a fluid film that prevents contact of the meshing surfaces and supports journal bearings. As a result, the pumping process decreases efficiency and increases wear and tear on the pump.

Although positive displacement pumps can pump these fluids, to avoid slippage centrifugal pumps are the preferred alternative. 


Positive Displacement Pumps vs. Centrifugal Pumps 

Centrifugal pumps move fluid by using impellers to transfer rotational energy. Fluid enters the pump and lands on the impeller axis, then centrifugal force pushes the fluid through the impeller’s vane tips and towards the pump outlet.

While both centrifugal and positive displacement pumps work to pump fluids, there’s one major difference – the viscosity.

Centrifugal pumps are most efficient when pumping low-viscosity fluids in low-pressure environments. Choosing one pump over the other heavily depends on this key difference.


3. Maintenance

Think of a puzzle. When you have thousands of pieces, the likelihood of losing a piece while putting it together is nearly inevitable – resulting in an unfinished project.

The same concept applies to pumps. 

Typically in pumps, the internal design all fits together and relies on the other parts. If one stops working, the whole pump will break down. 

Spending extra time and money on maintenance is critical to ensure one part doesn’t affect the rest of the pump.

Positive Displacement Pumps: Simple, Yet Complex

While simple in function, positive displacement pumps have a complex design. Designed for pumping highly viscous fluids efficiently, positive displacement pumps have been and will continue to be a preferred method in many industries. 

Which Type of Positive Displacement Pump is Best?

Choosing the right positive displacement pump is a tough decision. 

Reach out to one of our technicians to simplify your decision.

Positive Displacement Pumps vs Other Common Pump Types: A Comparative Analysis

Pumps play a vital role in many industries, facilitating the transfer of fluids in a wide range of applications. While there are numerous pump types available, they can be broadly classified into two categories: positive displacement (PD) pumps and dynamic pumps.

Here, we explore the key differences between positive displacement pumps and other common pump types, focusing on their operating principles, advantages, disadvantages, and typical applications.

Operating Principles of Positive Displacement & Dynamic Pumps

First, let’s begin by examining how PD pumps and dynamic pumps work:

Positive Displacement Pumps: Operating Principle

PD pumps transfer fluid by capturing a fixed volume of fluid within a cavity and then mechanically displacing it into the discharge pipe. This process is achieved through reciprocating or rotary elements, such as pistons, diaphragms, gears, or screws. Examples of PD pumps include gear pumps, diaphragm pumps, piston pumps, and progressive cavity pumps.

Dynamic Pumps: Operating Principle

Dynamic pumps, also known as kinetic pumps, transfer fluid by imparting kinetic energy to the fluid through the action of a rotating impeller or a spinning disk. This kinetic energy is then converted into pressure energy, propelling the fluid through the discharge pipe. Examples of dynamic pumps include centrifugal pumps, axial flow pumps, and regenerative turbine pumps.

Comparing Positive Displacement & Dynamic Pumps

Here, we contrast selection criteria for PD pumps and dynamic pumps in various key areas, including flow rate and pressure, viscosity, efficiency, pulsation and shear sensitivity, self-priming capability, and maintenance and wear.

Flow Rate & Pressure

PD pumps deliver a consistent flow rate regardless of the discharge pressure, making them ideal for applications that require precise dosing or constant flow. In contrast, the flow rate of dynamic pumps is highly dependent on the system pressure, with the flow rate decreasing as the pressure increases.

Viscosity

PD pumps can handle fluids with a wide range of viscosities, including high-viscosity fluids which can be challenging for dynamic pumps. Dynamic pumps, particularly centrifugal pumps, are generally more suited for low to medium-viscosity fluids, as high-viscosity fluids can result in reduced efficiency and increased wear.

Efficiency

Positive displacement pumps typically have higher volumetric efficiency than dynamic pumps, especially when dealing with high-viscosity fluids or high-pressure applications. Dynamic pumps may offer higher overall efficiency for low-viscosity fluids and low-pressure applications.

Pulsation & Shear Sensitivity

PD pumps can generate pulsating flow, which may not be suitable for certain applications, such as those involving shear-sensitive fluids or where a smooth constant flow is required. However, there are possible pump/porting design options that can minimize the impact of pulsations. Dynamic pumps, particularly centrifugal pumps, generate a smoother flow with less pulsation.

Self-Priming Capability

Most PD pumps have self-priming capabilities, allowing them to handle fluids with entrained gases or operate under suction lift conditions. Dynamic pumps typically require a flooded suction or an external priming system to function effectively.

Maintenance & Wear

PD pumps generally have more moving parts and can be more susceptible to wear, particularly in applications involving abrasive or corrosive fluids. However, with proper pump selection and maintenance, you can minimize wear and tear. Dynamic pumps, especially centrifugal pumps, tend to have simpler designs with fewer moving parts, which can translate to lower maintenance requirements.

Which Pump Type Will You Choose?

Selecting the right pump type for a specific application requires a thorough understanding of the differences between positive displacement and dynamic pumps. By considering the aforementioned factors, you can make informed decisions that optimize system performance, efficiency, and reliability.

Ultimately, the choice between a positive displacement pump and a dynamic pump will depend on the unique requirements of each application, with each pump type offering distinct advantages and disadvantages suited for different scenarios.

Ready to learn more? Check out our resources page.

Positive Displacement Pump Cavitation: What it Is and How to Avoid It

Positive displacement (PD) pumps are essential in numerous industries, including oil and gas, food processing, and pharmaceuticals, providing consistent and efficient fluid transfer. However, one common issue that can impact the performance and lifespan of your PD pump is cavitation. 

Here, we explore the phenomenon of PD pump cavitation, its causes, consequences, and effective strategies to mitigate its occurrence.

What is Positive Displacement Pump Cavitation?

Cavitation is a phenomenon that occurs when the pressure in the pumped fluid falls below the fluid’s vapor pressure, causing the formation of vapor bubbles or cavities. As these bubbles travel through the pump, they collapse when they reach regions of higher pressure, releasing energy as shock waves. These shock waves can cause significant damage to the pump components, reduce efficiency, and increase noise and vibration levels.

What Causes Cavitation in Positive Displacement Pumps?

The main causes of cavitation in positive displacement pumps include:

  1. Insufficient Net Positive Suction Head (NPSH): NPSH is a measure of the energy available in the fluid at the pump inlet relative to the fluid’s vapor pressure. When the NPSH available (NPSHa) is less than the NPSH required by the pump (NPSHr), cavitation is likely to occur. 
  2. High Viscosity Fluids: Pumping high-viscosity fluids can lead to increased resistance to flow, resulting in a pressure drop at the pump inlet, increasing the risk of cavitation.
  3. Inadequate Suction Piping Design: Improper design or installation of the suction piping system, including undersized piping, excessive pipe length, numerous bends or elbows, and partially closed or obstructed valves, can cause pressure losses and contribute to cavitation.
  4. Blocked or Clogged Filters/Strainers: Obstructions in filters or strainers can cause a pressure drop at the pump inlet, increasing the risk of cavitation.
  5. Pump Operating at High Speeds: When positive displacement pumps operate at high speeds, the time available for fluid to fill the pump chamber decreases, which can cause a pressure drop and promote cavitation.
  6. Inappropriate Pump Selection: Using a pump unsuitable for the specific application or fluid properties can lead to inadequate performance and an increased risk of cavitation.
  7. High-Altitude Operation: Pumps operating at higher altitudes have a lower atmospheric pressure, which reduces the available NPSH and increases the likelihood of cavitation.
  8. High-Fluid Temperature: Elevated fluid temperatures can lower the fluid’s vapor pressure, making it more susceptible to cavitation.
  9. Air or Gas Entrainment: The presence of air or gas in the fluid being pumped can reduce the fluid’s effective NPSH, increasing the risk of cavitation.

Ways to Address PD Pump Cavitation

The good news is that there are steps you can take to keep your PD pump running smoothly, including:

Ensure Adequate Net Positive Suction Head 

To prevent cavitation, it is crucial to maintain an adequate NPSH. This can be achieved by reducing the suction lift, increasing the suction pipe diameter, or using a booster pump to increase the available pressure.

Select the Appropriate Pump

Choose a PD pump specifically designed for the fluid properties, system requirements, and operating conditions of your application. Consult with your pump manufacturer or engineers for proper pump selection.

Proper Pump Installation

Ensuring your PD pump is correctly installed, aligned, and connected to the piping system can help reduce the risk of cavitation.

Regular Maintenance

Regularly inspecting and maintaining your pump and its components can help identify early signs of cavitation and take corrective measures before significant damage occurs.

Monitor Pump Performance

Regularly monitoring pump performance indicators, such as flow rate, pressure, and temperature can help identify potential cavitation issues and enable proactive maintenance or system adjustments.

Determining the Speed of Pump Cavitation

Determining the speed at which a pump will begin to cavitate is a fairly complicated process. Numerous factors contribute to pump cavitation, including:

  • Vapor pressure of the fluid (water, oil, gasoline)
  • Available atmospheric pressure (sea level, higher altitude)
  • Pressure losses in the inlet lines (length, diameter, surface roughness, momentum changes, turbulent flow, laminar flow)
  • Pressure losses in the pump itself (length, diameter, surface roughness, momentum changes, turbulent flow, laminar flow, bypass circuit)
  • Rotor geometry (diameter, chamber volume as a function of rotation, carry-over volumes)
  • Pump’s operation speed

To determine the speed of pump cavitation, engineers use a combination of analytical methods, computer simulations, and experimental testing. Computational fluid dynamics (CFD) analysis can be a valuable tool to simulate fluid flow and pressure changes within the pump and the system, allowing you to identify potential cavitation risks and optimize the pump design and operating conditions to minimize cavitation. 

In addition, conducting tests on physical pump prototypes and monitoring for signs of cavitation, such as noise, vibration, and reduced pump performance, can help confirm the theoretical predictions and refine the cavitation risk assessment.

Preventing Displacement Pump Cavitation

Understanding and addressing the causes of positive displacement pump cavitation are critical to maintaining the efficiency, reliability, and longevity of your pumping equipment. By following these steps, you can mitigate the risk of cavitation and maximize the performance of your PD pump systems.

Have any other questions on PD pumps or cavitation? – Reach out to one of our experts.