Centrifugal Pumps

What is a Centrifugal Pump?

A centrifugal pump is a machine that uses rotation to impart velocity to a liquid, it then converts that velocity into flow.

In that definition you can see that there are two things going on:

• First, energy is being imparted to a liquid in the form of velocity.
• Second, that energy is converted into a steady flow.

This two-part definition mirrors the two-part nature of centrifugal pumps. Centrifugal pumps are made up of two primary components: an impeller and a casing.

A centrifugal pump is a machine which converts the kinetic energy of the water into pressure energy before the water leaves its casing. The flow of water leaving the impeller is free vortex. The impeller of a centrifugal pump may have volute casing, vortex casing and volute casing with guide blades.
blades.The following important points may be noted for centrifugal pumps:(a) The manometric head is the actual head of water against which a centrifugal pump has to
work. It may be obtained by using the following relations, i.e.,

Manometric head = Workdone per kg of water – Losses within the impeller

= Energy per kg at outlet of impeller – Energy per kg at inlet of impeller
= Suction lift + Loss of head in suction pipe due to friction + Delivery lift + Loss of head in delivery pipe due to friction + Velocity head in the delivery pipe.

The Impeller and Centrifugal Force

The impeller rotates and creates velocity.

The impeller spins rapidly in the liquid being pumped. Every impeller has 1 or more vanes that extend from the center of the impeller out towards the outer diameter. As the impeller turns, centrifugal force causes the liquid to move rapidly out of the center of the impeller, along the vanes and then exit the impeller at the outermost diameter. The result is a spray of liquid exiting the impeller at all points around the periphery of the spinning impeller.

One way to visualize how an impeller works is to consider what would happen if the rotational speed of a Ferris wheel increased rapidly.

Imagine that you’re on a Ferris wheel and suddenly it begins to spin rapidly out of control. Eventually, if the speed of the wheel was great enough, the buckets would no longer keep your feet towards the ground as the wheel rotated. Instead, the buckets would remain extended fully away from the center of the wheel regardless of their position relative to the ground. So one second you’d be right-side-up, but once your bucket reached the top of the Ferris wheel, you’d be upside down.

This is the principle of centrifugal force.

Centrifugal force is the force that will hold water in a bucket if you spin the bucket rapidly up over your head and then back down again.

To make our visualization a little safer, no imagine that no one is on the berserk Ferris wheel, but that you’re watching it from the ground. If for some reason all of the bolts that were holding the buckets onto the wheel were to fail, imagine the result. If all the bolts failed at once the buckets would go shooting off of the wheel in all different directions. Some would go straight up, some would head for the parking lot, and some would slam straight down into the ground.

This is how an impeller works, it turns rapidly creating centrifugal force. Centrifugal force pushes liquid rapidly out towards the outer edge of the impeller where it shoots out in all directions, the movement of water creates a pressure vacuum at the middle of the impeller which then draws in more liquid and the cycle repeats itself until someone shuts it down, the water runs out, or until the vacuum being created by the water moving through the impeller is no longer great enough to cause more liquid to flow into the impeller.