Difference between rotodynamic pumps and positive displacement pumps

Centrifugal pump: this involves the transfer of kinetic energy from the motor to the liquid by a spinning impeller. As the impeller rotates it draws in fluid, increasing the velocity which moves the fluid to the discharge point. A centrifugal pump is categorised as a non-positive displacement pump. Working principles differ for centrifugal pumps and positive displacement pumps Major differences between the two pump types Factor Positive displacement Mechanics Impellers pass on velocity from the motor to the liquid which helps move the fluid to the discharge port Traps a certain amount of liquid and forces it from the suction to the discharge port.

Performance The flow rate varies with a change in pressure Flow rate remains constant as change in pressure. Viscosity The flow rate decreases as the viscosity increases, even moderate thickness, due to frictional losses inside the pump The internal clearances allow higher viscosity handling. Flow rate increases with increasing viscosity. Efficiency Pump efficiency peaks at a specific pressure - any variations decrease the efficiency dramatically. Efficiency is less affected by pressure.

Positive displacement pumps can be run at any point on their curve without damage or efficiency loss. However, positive displacement pumps have a complex design. Also, a positive displacement pump can handle variations in pressure, flow, and viscosity to remain efficient. As the flow rate remains constant in this pump proportional to the speed of operation , smooth and low pulsating is still achieved despite the change in pressure.

They are perfect solutions for dosing applications because of their accurate metering. Diagram of positive displacement pump: Watch the video below to learn the working of positive displacement pump: Difference between Dynamic and Positive Displacement Pumps in tabular form: Factor Positive Displacement Pump Mechanics Impellers pass on velocity from the motor to the liquid which helps move the fluid to the discharge port produces flow by creating pressure.

Traps confined amounts of liquid and force it from the suction to the discharge port produces pressure by creating flow. Performance The flow rate varies with a change in pressure. Flow rate remains constant with a change in pressure. Viscosity Flow rate rapidly decreases with increasing viscosity, even any moderate thickness, due to frictional losses inside the pump.

Due to the internal clearances, high viscosities are handled easily and the flow rate increases with increasing viscosity. Efficiency Efficiency peaks at a specific pressure; any variations decrease efficiency dramatically. Does not operate well when run off the middle of the curve; can cause damage and cavitation. Efficiency is less affected by pressure, but if anything tends to increase as pressure increases.

Can be run at any point on their curve without damage or efficiency loss. Suction Lift Standard models cannot create suction lift, although self-priming designs are available and manometric suction lift is possible through a non-return valve on the suction line.

Create a vacuum on the inlet side, making them capable of creating a suction lift.

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As the flow rate remains constant in this pump proportional to the speed of operation , smooth and low pulsating is still achieved despite the change in pressure. They are perfect solutions for dosing applications because of their accurate metering. Diagram of positive displacement pump: Watch the video below to learn the working of positive displacement pump: Difference between Dynamic and Positive Displacement Pumps in tabular form: Factor Positive Displacement Pump Mechanics Impellers pass on velocity from the motor to the liquid which helps move the fluid to the discharge port produces flow by creating pressure.

Traps confined amounts of liquid and force it from the suction to the discharge port produces pressure by creating flow. Performance The flow rate varies with a change in pressure. Flow rate remains constant with a change in pressure. Viscosity Flow rate rapidly decreases with increasing viscosity, even any moderate thickness, due to frictional losses inside the pump. Due to the internal clearances, high viscosities are handled easily and the flow rate increases with increasing viscosity.

Efficiency Efficiency peaks at a specific pressure; any variations decrease efficiency dramatically. Does not operate well when run off the middle of the curve; can cause damage and cavitation. Efficiency is less affected by pressure, but if anything tends to increase as pressure increases. Can be run at any point on their curve without damage or efficiency loss. Suction Lift Standard models cannot create suction lift, although self-priming designs are available and manometric suction lift is possible through a non-return valve on the suction line.

Create a vacuum on the inlet side, making them capable of creating a suction lift. Shearing A high-speed motor leads to the shearing of liquids. Not good for shear-sensitive mediums. Table 1 shows a benefits and limitations comparison chart for rotodynamic pumps and a specific PD pump type rotary. Table 1. Benefits and limitations relative comparison chart for rotodynamic pumps and a specific PD pump type rotary Graphics courtesy of Hydraulic Institute The type of pump selected for a given application depends on many factors, including rate of flow, total system head, system pressure, net positive suction head and the installation environment.

Table 2 shows a requirements matrix based on market applications. These certificates are suited for those who engineer, design, maintain, install, manufacture, specify, sell, distribute or consult on pumps. What implementations have been sought in the pumping system industry to accommodate for energy savings demand?

The U. Department of Energy DOE have recently implemented regulations that focus on energy efficiency. According to the DOE Technical Support Document for Electric Motors Final Rule , nearly 70 percent of industrial electricity demand is used by industrial motor systems, with a majority of these being rotodynamic pumping systems. DOE regulations for the electrical motor industry and pumping industry strongly indicate that increased energy efficiency will have a major impact on our economic growth for the foreseeable future.

The Energy Conservation Standard for Pumps 10 CFR Subpart Y published in January recognizes that pumps and motors equipped with variable speed drives can save significant energy and the DOE metrics for the regulation measures these savings. When properly applied, variable frequency drives VFDs play a major role in energy conservation by reducing the electricity demand and increasing the life of pumping systems.

A VFD is an electronic device used primarily for controlling the rotational speed of an alternating-current AC electric motor by controlling the frequency of the electrical power supplied to the motor. VFDs have been used to control the speed of electric motors for more than 40 years. However, many pump end users were initially slow to adopt this technology.

In recent years, innovative steps have been taken to improve the efficiency and reliability and reduce the cost of VFDs, making them a more attractive solution to motor speed control.

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Viscosity Flow rate rapidly decreases with increasing viscosity, even any moderate thickness, due to frictional losses inside the pump. Due to the internal clearances, high viscosities are handled easily and the flow rate increases with increasing viscosity. Efficiency Efficiency peaks at a specific pressure; any variations decrease efficiency dramatically.

Does not operate well when run off the middle of the curve; can cause damage and cavitation. Efficiency is less affected by pressure, but if anything tends to increase as pressure increases. Can be run at any point on their curve without damage or efficiency loss. Suction Lift Standard models cannot create suction lift, although self-priming designs are available and manometric suction lift is possible through a non-return valve on the suction line.

Create a vacuum on the inlet side, making them capable of creating a suction lift. Shearing A high-speed motor leads to the shearing of liquids. Not good for shear-sensitive mediums. Low internal velocity means little shear is applied to the pumped medium. Ideal for shear-sensitive fluids. Read more: Understanding hydraulic pump Conclusion That is all for this article, the difference between dynamic and positive displacement pumps. The difference between dynamic and centrifugal pumps is also presented in tabular form.

I hope you got a lot from this article, if so, kindly share it with other students. Thanks for reading! The Energy Conservation Standard for Pumps 10 CFR Subpart Y published in January recognizes that pumps and motors equipped with variable speed drives can save significant energy and the DOE metrics for the regulation measures these savings.

When properly applied, variable frequency drives VFDs play a major role in energy conservation by reducing the electricity demand and increasing the life of pumping systems. A VFD is an electronic device used primarily for controlling the rotational speed of an alternating-current AC electric motor by controlling the frequency of the electrical power supplied to the motor. VFDs have been used to control the speed of electric motors for more than 40 years.

However, many pump end users were initially slow to adopt this technology. In recent years, innovative steps have been taken to improve the efficiency and reliability and reduce the cost of VFDs, making them a more attractive solution to motor speed control. These innovations have been met with a rapid adoption of VFD systems across the pumping industry.

As with any rapid technological surge, there is a steep associated learning curve. VFDs have a special set of requirements that must be considered during system and application selection, installation, operation and troubleshooting. It is difficult to find objective literature that has information directed toward these special requirements with an emphasis on the pumping industry. Throughout the years, various white papers regarding VFDs have been circulated through the pump industry. As a result, HI developed a comprehensive guidebook that combined these considerations with recommendations from industry experts.

Variable Frequency Drives: Guidelines for Application, Installation, and Troubleshooting assists VFD end users and system integrators in the proper selection, specification, installation and operation of VFD-driven equipment. This guide details basic troubleshooting information to help diagnose common issues with VFD-driven systems.

The goal of this guidebook is to educate the pumping industry and to ensure the safe, reliable and efficient operation of the pumping equipment we all depend on every day. HI guidebooks are written by industry experts on topics vital to those who design, operate, or maintain pumps and pumping systems.