Sanitary Pump 101- Positive Displacement Pumps vs. Centrifugal. What’s the difference?  

Today, we kick off a series of posts where we get back to basics and review some of the simple topics we often take for granted. In today’s post, we’re going to take a look at sanitary PD pumps and sanitary centrifugal pumps, specifically how each works and what the differences between the two are.

Let’s start at the top and talk about what a pump is. A pump is a mechanical devised that uses suction or pressure to move fluids, compress gases, or force air into inflatable objects. More simply put, it converts mechanical energy into kinetic energy to do the work gravity won’t do on its own.

At Triplex, we focus almost exclusively the pumps that move fluids, specifically sanitary or high purity fluids. In this world, we have two broad categories of pumps- positive displacement and centrifugal pumps.

Waukesha C114 Centrifugal Pump

Centrifugal Pumps- What are they?

In a centrifugal pump, it’s all about the impeller. Fluid is directed to the impeller eye, which forces the fluid into a circular movement by the impeller vanes. There are a number of different “types” of centrifugal impellers (more on that in a future post), but for now, just know that these impeller vanes take the mechanical “work” of the motor and transfer it to the fluid. The rotating impeller creates a centrifugal force (as defined by Google as, “an apparent force that acts outward on a body moving around a center, arising from the body’s intertia”), forcing the fluid outward through casing discharge with increased pressure and velocity.

Said another way, the fluid comes in and the super-fast spinning impeller flings the fluid out. How’s that for 101?

Centrifugal pumps have 5 main parts- a pump casing, back plate, mechanical seal, a motor, and the impeller.

Centrifugal pumps operate based on the affinity laws, meaning they displace a variable amount of fluid dependent on the discharge head in the system.

Centrifugal pumps do a great job of moving a high volume of fluid with relatively low power requirements.

They are not great for high viscosity fluids and the high spinning impeller means that they aren’t very low shear.

Accordingly, centrifugal pumps are used primarily for high volume transfer of low viscosity fluids at low to medium pressures. Think applications like pumping water, CIP supply, wort transfer, and water system loop recirculation.

Waukesha Universal 3 Positive Displacement Pump

Positive Displacement Pumps- What are they?

Now that we’ve been through centrifugal pumps, let’s take a look at the other big broad category of pumps- Positive Displacement Pumps. PD pumps include technologies such as rotary lobe, external circumferential, twin screw, quaternary diaphragm, gear, plunger, and progressive cavity pumps. All PD pumps are defined by their movement of a specific volume of liquid per revolution. If sized and applied properly, they’ll move this volume regardless of the downstream line pressure.

All PD pumps do this using the same basic premise- there is an expanding cavity at the inlet that creates a low-pressure area to draw fluid in, and as the motor turns, these cavities collapse, pressurizing the fluid and forcing it out the discharge.

Because of this constant volume displaced per revolution, PD pumps are great for metering and dosing applications. They are also ideal for high pressure or stuffing applications. PD Pumps are also our go to for high viscosity fluids. We run PD pumps slower, which means that they allow more time for higher viscosity fluids to fill pump cavities and also handle these products gently.

While this all sounds great, there’s good reason we don’t use PD pumps for every application. PD pumps are almost always more expensive than their centrifugal counterparts, meaning more $/volume displaced. They also require larger motors to generate the torque to turn the pump shafts and a larger footprint.

And because there are so many more technologies within the PD pump family, we’ll want to ask lots of questions before specifying the right technology for your application. We’ll spend future Sanitary Pump 101 posts talking about how we arrive at the best technology for you.

In sum, there are two main categories of sanitary pumps- centrifugal and positive displacement. Centrifugal pumps use a high-speed impeller and centrifugal force to transfer high volumes of low viscosity fluids at a given discharge head. PD pumps use expanding and collapsing cavities to move a specific amount of fluid per revolution regardless of discharge head. The goal of this post was to highlight these differences and make it simple to understand. But as always, if you have any questions, please contact a Triplex Sales Engineer today!

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