Inside a Centrifugal Pump

Many of us look at a pump and think, ‘that’s cool’ but how does it work? What is it that drives your fluid to the process? It’s not a mystical box that works with the wave of a magic wand. Let’s start with the components of a pump. The biggest piece is the casing. The casing, which is also known as the volute, is the main housing. It is what the fluid flows through when it is being pumped. So the fluid (typically water or a glycol mix) enters through the pump inlet into the casing (or volute) where it is spun rapidly by the impeller.

Closed Impeller

Scot Pump Closed Impeller

What is the impeller? The impeller is what makes all of this work. There are two kinds of impellers: Open and Closed. Open impellers are found on many cast iron end suction pumps. They can handle some debris without clogging which makes them a great cooling tower pump. The closed impeller is a favorite for hydroformed stainless steel pumps. The closed impeller basically looks like two frisbees stacked on top of each other with vanes in between (think of a brake rotor). There is a picture of one shown to the left. This particular image comes from Scot Pump, a brand we use on many of our packages. Closed impellers are great for applications where the fluid is relatively clean and free of larger particulates.

How it works: The fluid enters the through the eye of the impeller. As the impeller spins, it throws the fluid away from the center. This movement through the impeller increases the angular velocity and centrifugal acceleration of the fluid. The fluid is forced radially outward into the pump volute, where it continues to spin around until it exits the pump discharge.

chemical pump

Frame mounted Pump

While the impeller plays a very important part in this, the motor is the driving force of all of it. The electric motor can be close coupled or, in other words, the pump casing and impeller are directly attached to the motor (as shown in the featured image of this post). The other option is frame mounted. The frame mounted pump package features (typically) a T frame motor with a flexible coupling and a frame mounted pump. With the frame mounted setup, the pump and motor rest on their own mountings. Many maintenance and service techs tend to prefer frame mounted pumps because it is easier to replace a seal or impeller when compared to a close coupled pump. We always recommend installing suction and discharge valving in your piping to allow for pump removal if necessary. If the pump is installed correctly, any seal should be able to be changed fairly easily.

The last component of this mix is the pump seal. No matter how your pump is attached to your motor, a seal is required to keep fluid from leaking out of the pump volute. The pump seal rides along the shaft of the motor (or pump) and prevents your process fluid from wicking down the shaft and creating a leak. Here is a great video from Ebara Europe showing how to install a mechanical seal in their pump: https://youtu.be/CiPm3gKVUOs. If you ever wondered how to change a pump seal, this will prove very informative.

There are other technical details to a pump that make it a little more difficult to decipher. Terms like cavitation, NPSH, and suction lift can all be a little confusing. In the case of NPSH (Net Positive Suction Head), several very informed engineers actually differ on their thoughts for this subject. I will go only as far as to give you the industry definition per the Hydraulic Institute: “The net positive suction head in feet of liquid absolute determined at the suction nozzle and referred to datum less the vapor pressure of the liquid in feet absolute.” One could write an entire article on NPSH and you still may not walk away feeling like you understand it completely. It’s probably a topic left for another day.

You are probably wondering about your author. I am not a pump expert by any means, but I spent several years working with both a pump distributor and Ebara International early in my career. After that, I moved into industrial process cooling systems. Between my cooling system experience and years in the pump industry, I know just enough to be dangerous. 🙂 Hope you all enjoyed this informative write-up on pumps. Check back soon for another entry.