Centrifugal Pumps vs. Positive Displacement Pumps

If you need to move liquid material from one place to another, you’re going to need a pump. Industrial pumps come in two different types: centrifugal pumps and positive displacement pumps. Each type has unique properties concerning efficiency, flow rate, viscosity, and pressure. Here’s what you need to know about centrifugal and positive displacement pumps so you can choose the right application for your needs. 

What Is a Centrifugal Pump?

ansi-centrifugal-pumpA centrifugal pump is used to move fluids by using rotational energy created by a motor or engine. The fluid enters the pump’s impeller across the rotating axis, where the spinning impeller pushes the fluid toward a scroll or diffuser. The fluids gain both velocity and pressure when being passed through the impeller. 

What Are the Benefits of Centrifugal Pumps?

Centrifugal pumps are some of the most widely-used pumps in industrial settings. And they have many benefits that make them popular options. These include the following.

  • Simplistic design — Because centrifugal pumps are simple in design, this can make them a more affordable option than other types of pumps. 
  • Fewer moving parts — Simpler design means fewer moving parts to break and maintain over the life of your pump. 
  • Various shapes and sizes — Centrifugal pumps come in a variety of sizes, meaning they are suitable for different applications and can work when you have small space constraints.

What Is a Positive Displacement Pump?

A positive displacement pump transports fluids by trapping a portion of the fluid and forcing it into a discharge pipe. Two or three spindles moving in opposite directions create the function of pumping, trapping, and displacing the liquid. There are several types of positive displacement pumps, including the following.

Rotary positive displacement pumps use a rotating mechanism to create a vacuum that draws in and captures fluid. Common examples include gear, screw, vane, peristaltic, rotary lobe, and progressive cavity pumps. Progressive cavity pumps are used in challenging applications to transfer highly viscous fluids or those containing solids, such as dirt, grit, or sludge.

Reciprocating positive displacement pumps use one or more oscillating pistons, plungers, or diaphragms. Valves restrict the fluid’s motion to the correct direction. Reciprocating positive displacement pumps are typically used for applications that need to maintain low flow rates against high resistance. Some may be used to pump highly viscous and heavy fluids, like sludge and slurry. Diaphragm valves are often used when pumping toxic and hazardous fluids.

Linear positive displacement pumps have been used for centuries all over the world to draw up water. Examples include rope pumps and chain pumps.

What Are the Benefits of Positive Displacement Pumps?

Here are some of the benefits of using positive displacement pumps.

  • Different viscosities — Positive displacement pumps can handle various levels of viscosity. 
  • Various pressures — This type of pump has the ability to operate at higher levels of pressure and flow without an impact on capacity. 
  • Consistent flow — A positive displacement pump can maintain a constant flow and speed. 
  • Don’t require sealing — These types of pumps do not have to be completely sealed. 

Differences Between Centrifugal Pumps and Positive Displacement Pumps

Both centrifugal and positive displacement pumps move fluids from one point to another. But there are quite a few differences between the two. These include:

Mechanics

The primary difference between centrifugal and positive displacement pumps is in their mechanics or how they operate. As previously described, centrifugal pumps transmit velocity to the liquid, creating pressure at the outlet. 

PD pumps trap a certain amount of liquid within the pump and transfer it to the discharge port. To sum it up, centrifugal pumps create pressure, which results in flow. And positive displacement pumps create a flow that results in pressure. 

Performance

Because the flow results from pressure with centrifugal pumps, you can change the flow rate by varying the pressure. Since positive displacement pumps work the opposite of this, you’ll get consistent flow from them even with varying pressure. 

You can regulate the flow in both types of pumps by changing the speed. The flow rate is proportional to the pump’s speed with positive displacement pumps. With centrifugal pumps, you can control the pump’s operation without pump throttling or liquid treatment. 

Viscosity

Centrifugal pumps become less efficient as the material being pumped’s viscosity increases. This is caused by frictional losses inside the pump. Because of this, these pumps are generally not appropriate for highly viscous fluids (over 850 cSt). However, positive displacement pumps become more efficient with increases. 

Inlet Conditions

Liquids must be inside a centrifugal pump to produce a pressure differential. This is also needed because the pump can’t self-prime and can’t deliver a Gas Volume Fraction (GVF) higher than 15%. But negative pressure is created on the inlet port of positive displacement pumps. The pump needs to be filled with liquid one time, but it is self-priming and can handle large quantities of gas. 

Shearing

The high-speed motor used in centrifugal pumps can cause shearing of liquids, which may not be desirable for some applications. Because positive displacement pumps produce low internal velocity, little shear is created, making them more suitable for shear-sensitive materials. 

Suction Lift

Standard centrifugal pumps cannot produce suction lift. But self-priming designs are available where this is possible. With positive displacement pumps, a vacuum is created on the inlet side, so a suction lift is possible. 

Efficiency

Centrifugal pumps achieve efficiency peaks at a particular pressure, and any variations will drastically impact pump efficiency. Operation when off the middle of the efficiency curve can cause pump cavitation and damage. The efficiency of positive displacement pumps is less affected by pressure. These pumps can be run at any point on their curve without efficiency loss or damage. 

When to Use Centrifugal Pumps

Centrifugal pumps are excellent for pumping thin liquids with low viscosity levels. This includes thin fuels and oils, many chemicals, and water. They are commonly used in high-volume pump applications that require high flow rates at low pressures. Some frequent uses for these pumps include the following.

  • Municipal water systems
  • Irrigation
  • Firefighting
  • Air conditioners and water circulators
  • Boiler feeds
  • Light fuel transfer stations
  • Cooling towers

When to Use Positive Displacement Pumps

Positive displacement pumps are excellent for pumping viscous fluids, such as oil, paint, and resin, at high pressure and low flow rates. You’ll often find positive displacement pumps in the following environments.

  • Oil processing centers
  • Municipal sewage systems
  • Manufacturing operations that process or produce thick or viscous materials
  • Food and beverage production facilities
  • Pharmaceutical plants

Each type of positive displacement pump has unique characteristics that make them ideal for particular applications. For example, lobe pumps have low shear and are easy to clean and sterilize, which makes them ideal for pharmaceutical and food processing. On the other hand, diaphragm pumps are self-priming and designed for low flows and high pressures, which makes them great for metering or dispensing oil and corrosive liquids.

Complex facilities such as manufacturing and food processing plants often benefit from using a mix of both pump types. For example, a food processing plant may need a centrifugal pump to add water to its systems and a positive displacement pump to move thicker materials. 

Quality Industrial Pumps and Services from C&B Equipment

Whether your business needs a centrifugal pump or a positive displacement pump, find out how C&B Equipment can help. We provide quality industrial pumps and services to clients throughout Oklahoma, Kansas, Missouri, the southern half of Illinois, and the Texas Panhandle. 

C&B Equipment sells a wide variety of industrial pumps to meet your operation’s specific needs and goals. We also keep you running stronger and longer with our diagnostic, maintenance, and repair services. Contact us today to learn more about our products and services.


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