Centrifugal Pumps at Work
I'm going to use the Wikipedia.org website again, to offer some Historical background for pumps. If we think about it, we would probably feel that pumps have been around "forever"! Maybe that's a little stretch but not too far off. Wikipedia gives us the following information on the "beginning" of centrifugal pumps.
"According to Reti, the first machine that could be characterized as a centrifugal pump was a mud lifting machine which appeared as early as 1475 in a treatise by the Italian Renaissance engineer Francesco di Giorgio Martini. True centrifugal pumps were not developed until the late 17th century, when Denis Papin built one using straight vanes. The curved vane was introduced by British inventor John Appold in 1851."
So we can say, with some certainty, that this device goes back to the late 1400's. That's a LONG TIME to be using something as extraordinarily simple and amazing as the centrifugal pump. Maybe the 1400's is a stretch, but the development of the "True centrifugal pump..." (as stated above), in the 1600's is pretty much a fact. And think about this... it took another 200 years, until 1851, to come up with the curved vane improvement. Now, here we are, another 160 years later, and the Centrifugal Pump has not changed significantly. I'd say that's a rather strong testimony to the original concept and design, wouldn't you?
Any pump needs a "prime mover" (like an electric motor or diesel engine), to make it work. The shaft of the pump is connected to the prime mover either directly or through a gearbox, transmission, belts and sheaves, or some other device. As the prime mover operates, the rotational energy it produces, is converted to energy within the pump, in the fluid it is moving. Some of the energy produced in the pump, is converted into kinetic energy within the fluid. Fluid is "sucked" into the center of the impeller through an opening in the casing. It is caught up in the impeller blades, and is thrown "tangentially" and "radially" outward, until it leaves the impeller into the diffuser part of the casing. Both velocity and pressure are increased while passing through the impeller. The diffuser section of the casing decelerates the flow but further increases the pressure. It is important to note that the fluid is not pushed radially outward by "centrifugal force", but rather by inertia, which is the natural tendency of an object to continue in a straight line (tangent to the radius) when traveling around a circle.
A centrifugal pump, as stated before, is simple in design and a significant solution to the moving of fluids. While this is a "plus", the device certainly has it's good points and bad. So let's talk about a few from each side.
On the Plus Side... they are relatively inexpensive to purchase. In the design shown above, there are relatively few "maintenance" components. It consists of a "shaft", "impeller", 2 bearings, and (in most cases) some type of fluid seal, either packing or mechanical lip seal. So simplicity is on the plus side of the equation. Another plus is it's normally "rugged" construction. The majority of the pump castings are "cast iron" which makes them somewhat "indestructible". Maybe not totally, but rather "robust", to say the least. And they are reasonably efficient when used within their designed parameters; i.e. flow rate, suction lift, vertical head, and material being pumped.
Now we've got to address the Negative Side... One such point is material leaking along the rotating shaft. This is handled as a maintenance item through the packing gland or lip seal. Either (or both) of these solutions have the ability to "wear" the area of the shaft upon which they ride. A second item is the "wearing of the impeller". This can be worsened by improper fluid transmission, such as abrasive or corrosive fluids being pumped. This is more of an application issue than a true negative aspect of the pump, itself. A third, and important topic, is the "priming" of the pump. By nature, a centrifugal pump must be "primed" before it will work properly. Priming simply means that the cavity of the pump must be filled with fluid in order for the impeller to produce a suction effect and draw in additional fluid in the pumping action. One solution is to have the inlet of the pump at a physical position, LOWER than the incoming flow line. In this way, gravity assists the pump by the fluid flowing toward the LOWER position, thus filling the pump cavity. Even after the pump is "primed" and will pump the liquid, additional fixtures such as check valves must be added to the system to keep the fluid from "siphoning" out of the pumping chamber, causing the pump to "loose prime". But manufacturers have modified the pump design, and created a "self-priming" centrifugal pump. One of these designs includes an internal suction stage that is capable of evacuating the air from the chamber allowing the pump to fill with fluid.
A final discussion on negative points of the centrifugal pump is "Cavitation". Cavitation is a phenomenon in which rapid changes of pressure in a liquid lead to the formation of small vapor-filled cavities, in places where the pressure is relatively low. Cavitation is, under most conditions, considered to be an "undesirable" phenomenon and one that is to be avoided or solved, should it occur. These small "vapor-filled cavities" (or bubbles), when they burst, can do so with a force sufficient to cause "shock waves". With millions of the bubble bursting on a continual basis, they can actually cause "pitting" or "cavities" within the metal impeller material. Such wear is detrimental to the efficiency of the pump and can also cause rebuilding or replacing of the impeller if the condition is allowed to continue for extended periods.
But the other side of the Centrifugal Pump is that with the extensive list of specialty designs, these pumps can handle LARGE solids, like MUD PUMPS in an oil field atmosphere. In food processing plants, in addition to the ability to fabricate the pump from Stainless Steel, they can be made from plastics and other man-made material compatible with the food industry.
Other designs include "Multi-stage" centrifugal pumps. These pumps, instead of having ONE impeller, will have 2 or 3, or more impellers (or "stages"). Through special configuration, the impellers can be connected in "series", whereby the "outlet pressure" would be increased before discharge, or in "parallel" that would increase the "out-flow" from the pump. A specialty type of pump, but they have their place. Additionally, we'll look at the "multi-stage" pump in our topic on Vertical Turbines on a different page. You can reach that page by clicking this link.
If you have additional interest in reading about Centrifugal Pumps, you can download this brochure from Crane Industries. It's a great read on the different types of Centrifugal Pumps, their application, advantages and disadvantages. You'll need Adobe Acrobat Reader to view the file, so if you don't have the latest version, you can go to our "Read Me" page and use the Adobe Reader Update link to download it for free.
So if you're getting ready to begin your next project and it requires the pumping of a liquid... give us a call and we'll see if we can't point you in the right direction.